USGS

Water-Resources Investigations Report 03-4044

Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances, 1969 Through 2000

By Michael R. Stevens

The PDF for the report is 2,976 kb


Table of Contents

Abstract

Introduction

Hydrology

Water Quality

Trend Analysis

Summary and Conclusions

References Cited

Hydrologic and Water-Quality Data

Figures

Figure 1. Location of study area and sampling sites (modified from Mueller,...

Figure 2. Schematic diagram of CBT Project facilities (from Mueller, 1990)....

Figure 3. Percentage of Colorado–Big Thompson (CBT) water supplied for muni...

Figure 4. Land uses in watershed upstream from Lake Granby.

Figure 5. Land uses in watershed upstream from the mouth of Big Thompson Ca...

Figure 6. Land uses in watershed draining directly to Horsetooth Reservoir.

Figure 7. Maximum, mean, and minimum daily streamflows for the period of re...

Figure 8. Time-series plots of operation of selected Colorado–Big Thompson ...

Figure 9. Time-series plots of operation of selected Colorado–Big Thompson ...

Figure 10. Monthly average discharge characteristics of selected Colorado–B...

Figure 11. Monthly average reservoir contents of selected Colorado–Big Thom...

Figure 12. Boxplots of selected water-quality constituents.

Figure 13. Piper diagram showing major-ion composition of samples at select...

Figure 14. May and October field-measurement profiles at Lake Granby (dam).

Figure 15. May and October field-measurement profiles at Shadow Mountain La...

Figure 16. May and October field-measurement profiles at Carter Lake.

Figure 17. May and October field-measurement profiles at Horsetooth Reservo...

Figure 18. Time-series plots of annual areal hypolimnetic oxygen deficit in...

Figure 19. Annual trophic status index (TSI) computations for Secchi depth,...

Figure 20. Nonmonotonic distribution and seasonal Kendall test results for ...

Figure 21. Monotonic distribution and seasonal Kendall test results for all...

Figure 22. Time-series plots of reservoir reciprocal hydraulic residence ti...

Figure 23. Temporal variations in periodic measurements and concentrations ...

Figure 24. Temporal variations in periodic measurements and concentrations ...

Figure 25. Temporal variations in periodic measurements and concentrations ...

Figure 26. Temporal variations in periodic measurements and concentrations ...

Figure 27. Temporal variations in periodic measurements and concentrations ...

Figure 28. Temporal variations in periodic measurements and concentrations ...

Figure 29. Temporal variations in periodic measurements and concentrations ...

Figure 30. Temporal variations in periodic measurements and concentrations ...

Figure 31. Temporal variations in periodic measurements and concentrations ...

Figure 32. Temporal variations in periodic measurements and concentrations ...

Figure 33. Temporal variations in periodic measurements and concentrations ...

Figure 34. Temporal variations in periodic measurements and concentrations ...

Figure 35. Temporal variations in periodic measurements and concentrations ...

Figure 36. Temporal variations in periodic measurements and concentrations ...

Figure 37. Temporal variations in periodic measurements and concentrations ...

Tables

Conversion Factors, Vertical Datum, and Abbreviations

Table 1. Water-quality sampling sites.

Table 2. Seasonal periods and period of record tested by seasonal Kendall t...

Table 3. Summary of significant upward and downward temporal trends at Colo...

Table 4. Summary statistics for site 1, Lake Granby, near spillway, near su...

Table 5. Summary statistics for site 1, Lake Granby, near spillway, near bo...

Table 6. Summary statistics for site 2, Lake Granby (Rainbow Bay), sampled ...

Table 7. Summary statistics for site 2, Lake Granby (Rainbow Bay), sampled ...

Table 8. Summary statistics for site 3, Granby Pump Canal.

Table 9. Summary statistics for site 4, Shadow Mountain Lake, sampled at th...

Table 10. Summary statistics for site 4, Shadow Mountain Lake, sampled near...

Table 11. Summary statistics for site 5, Alva B. Adams Tunnel east portal.

Table 12. Summary statistics for site 6, Lake Estes, sampled at surface.

Table 13. Summary statistics for site 6, Lake Estes, sampled near bottom.

Table 14. Summary statistics for site 7, Olympus Tunnel.

Table 15. Summary statistics for site 8, Carter Lake, sampled near surface.

Table 16. Summary statistics for site 8, Carter Lake, sampled near bottom.

Table 17. Summary statistics for site 9, Horsetooth Reservoir (Soldier Cany...

Table 18. Summary statistics for site 9, Horsetooth Reservoir (Soldier Cany...

Table 19. Summary statistics for site 10, Horsetooth Reservoir (Spring Cany...

Table 20. Summary statistics for site 10, Horsetooth Reservoir (Spring Cany...

Table 21. Trend analysis results for site 1, Granby Reservoir, near dam, su...

Table 22. Trend analysis results for site 1, Granby Reservoir, near dam, bo...

Table 23. Trend analysis results for site 2, Granby Reservoir (Rainbow Bay)...

Table 24. Trend analysis results for site 2, Granby Reservoir (Rainbow Bay)...

Table 25. Trend analysis results for site 3, Granby Pump Canal, samples.

Table 26. Trend analysis results for site 4, Shadow Mountain Reservoir, sur...

Table 27. Trend analysis results for site 4, Shadow Mountain Reservoir, bot...

Table 28. Trend analysis results for site 5, Adams Tunnel east portal.

Table 29. Trend analysis results for site 7, Olympus Tunnel, samples.

Table 30. Trend analysis results for site 8, Carter Reservoir, surface samp...

Table 31. Trend analysis results for site 8, Carter Reservoir, bottom sampl...

Table 32. Trend analysis results for site 9, Horsetooth Reservoir (Soldier ...

Table 33. Trend analysis results for site 9, Horsetooth Reservoir (Soldier ...

Table 34. Trend analysis results for site 10, Horsetooth Reservoir (Spring ...

Table 35. Trend analysis results for site 10, Horsetooth Reservoir (Spring ...


Prepared in cooperation with the NORTHERN COLORADO WATER CONSERVANCY DISTRICT, BUREAU OF RECLAMATION, and the CITY OF FORT COLLINS

Conversion Factors, Vertical Datum, and Abbreviations

MultiplyByTo obtain
acre0.00156square mile
acre-foot (acre-ft)1,233cubic meter
cubic foot per second ( ft3/s)0.02832cubic meter per second
foot (ft)0.3048meter
inch2.54centimeter
mile1.609kilometer
millimeter0.03937inch
square centimeter (cm2)0.1550square inch (in2)
square foot (ft2)0.0929square meter
square mile (mi2)2.590square kilometer (km2)

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F = 1.8(°C) + 32

Vertical coordinate information is referenced to the National Geodectic Vertical Datum of 1929 (NGVD 29); horizontal coordinate information is referenced to the North American Datum of 1927 (NAD 27).

Additional Abbreviations

MRLminimum reporting level

mgmilligrams

mg/Lmilligrams per liter

mmmillimeters

mLmilliliters

NTUnephelometric turbidity units

ROEresidue on evaporation

µg/Lmicrograms per liter

µS/cmmicrosiemens per centimeter at 25 degrees Celsius

WYwater year, begins on October 1 and ends on September 30 of the following year. Identified by year in which it ends (water year 2000 ended on September 30, 2000).


Abstract

The U.S. Geological Survey, in an ongoing cooperative monitoring program with the Northern Colorado Water Conservancy District, Bureau of Reclamation, and City of Fort Collins, has collected water-quality data in north-central Colorado since 1969 in reservoirs and conveyances, such as canals and tunnels, related to the Colorado–Big Thompson Project, a water-storage, collection, and distribution system. Ongoing changes in water use among agricultural and municipal users on the eastern slope of the Rocky Mountains in Colorado, changing land use in reservoir watersheds, and other water-quality issues among Northern Colorado Water Conservancy District customers necessitated a reexamination of water-quality trends in the Colorado–Big Thompson system reservoirs and related conveyances. The sampling sites are on reservoirs, canals, and tunnels in the headwaters of the Colorado River (on the western side of the transcontinental diversion operations) and the headwaters of the Big Thompson River (on the eastern side of the transcontinental diversion operations). Carter Lake Reservoir and Horsetooth Reservoir are off-channel water-storage facilities, located in the foothills of the northern Colorado Front Range, for water supplied from the Colorado–Big Thompson Project. The length of water-quality record ranges from approximately 3 to 30 years depending on the site and the type of measurement or constituent. Changes in sampling frequency, analytical methods, and minimum reporting limits have occurred repeatedly over the period of record.

The objective of this report was to complete a retrospective water-quality and trend analysis of reservoir profiles, nutrients, major ions, selected trace elements, chlorophyll-a, and hypolimnetic oxygen data from 1969 through 2000 in Lake Granby, Shadow Mountain Lake, and the Granby Pump Canal in Grand County, Colorado, and Horsetooth Reservoir, Carter Lake, Lake Estes, Alva B. Adams Tunnel, and Olympus Tunnel in Larimer County, Colorado.

This report summarizes and assesses:

  1. Water-quality and field-measurement profile data collected by the U.S. Geological Survey and stored in the U.S. Geological Survey National Water Information System,
  2. Time-series trends of chemical constituents and physical properties,
  3. Trends in oxygen deficits in the hypolimnion of the reservoirs in the late summer season by the seasonal Kendall trend test method,
  4. Nutrient limitation and trophic status indicators, and
  5. Water-quality data in terms of Colorado water-quality standards.

Water quality was generally acceptable for primary uses throughout the Colorado–Big Thompson system over the site periods of record, which are all within the span of 1969 to 2000. Dissolved solids and nutrient concentrations were low and typical of a forested/mountainous/crystalline bedrock hydrologic setting. Most of the more toxic trace elements were rarely detected or were found in low concentrations, due at least in part to a relative lack of ore-mineral deposits within the drainage areas of the Colorado–Big Thompson Project.

Constituent concentrations consistently met water-quality standard thresholds set by the State of Colorado. Trophic-State Index Values indicated mesotrophic conditions generally prevailed at reservoirs, based on available Secchi depth, total phosphorus concentrations, and chlorophyll-a concentrations.

Based on plots of time-series values and concentrations and seasonal Kendall nonparametric trends testing, dissolved solids and most major ions are decreasing at most sites. Many of the nutrient data did not meet the minimum criteria for time-series testing; but for those that did, nutrient concentrations were generally stable (no statistical trend) or decreasing (ammonia plus organic nitrogen and total phosphorus). Iron and manganese concentrations were stable or decreasing at most sites that met testing criteria. Chlorophyll-a data were only collected for 11 years but generally indicated quasi-stable or downward temporal trends.

Introduction

The U.S. Geological Survey (USGS), in an ongoing cooperative monitoring program with the Northern Colorado Water Conservancy District (NCWCD), Bureau of Reclamation (BOR), and City of Fort Collins, has collected water-quality data in north-central Colorado since 1969 in reservoirs and conveyances such as canals and tunnels related to the Colorado–Big Thompson (CBT) Project, a water-storage, collection, and distribution system. The sampling sites are on reservoirs, canals, and tunnels in the headwaters of the Colorado River (on the western side of the transcontinental diversion operations) and the headwaters of the Big Thompson River (on the eastern side of the transcontinental diversion operations). Carter Lake and Horsetooth Reservoir are off-channel water-storage facilities, located in the foothills of the northern Colorado Front Range, for water supplied from the CBT Project (table 1, fig. 1). The length of water-quality record ranges from approximately 3 to 30 years depending on the site and the type of measurement or constituent. Sampling frequency, analytical methods, and minimum reporting limits have changed repeatedly over the period of record.

Table 1. Water-quality sampling sites.

[15-digit U.S. Geological Survey (USGS) identification numbers are latitude and longitude of site location with a 2-digit code at the end; 8-digit identification numbers are USGS downstream order numbering]

Site number (fig. 1)U.S. Geological Survey identification numberSite nameNumber of samplesPeriod of record
109018500Lake Granby near Granby, Colo. (near spillway)1541973–75, 1979–present
2400844105530800Lake Granby near Granby, Colo. (near dam in Rainbow Bay)1231989–present
309018300Granby Pump Canal near Grand Lake, Colo.1791970–present
409014500Shadow Mountain Lake near Grand Lake, Colo.1251989–present
509013000Alva B. Adams Tunnel east portal, near Estes Park, Colo.2831970–present
6402231105291900Lake Estes near dam near Estes Park, Colo.161998–present
706734900Olympus Tunnel at Lake Estes, Colo.2471970–present
806742500Carter Lake near Berthoud, Colo.3091970–present
906737500Horsetooth Reservoir near Fort Collins, Colo. (Soldier Canyon Dam)5181969–present
10403147105083800Horsetooth Reservoir near Fort Collins, Colo. (Spring Canyon Dam)1001983–present
1109010500Colorado River below Baker Gulch near Grand Lake, Colo.--1953–present

Figure

Figure 1. Location of study area and sampling sites (modified from Mueller, 1990).


The CBT Project was authorized by Congress in 1937 and completed in 1956, resulting in the construction of several reservoirs, more than 120 miles of tunnels and canals, and numerous power-generation facilities. The USGS became involved in monitoring in the CBT Project area as part of a cooperative water-quality evaluation of the BOR Pick-Sloan Missouri Basin Program. When operation of the CBT Project was assumed by NCWCD, collection of baseline water-quality information continued. A report by Mueller (1990) summarized water-quality data available at the time, analyzed time-series trends, and evaluated the sampling network design for most of the sampling sites considered in this report and other sites on the eastern plains where the water is used for agricultural purposes.

Purpose and Scope

Part of the value of such a long-term monitoring record is the ability to test for temporal trends in water quality. Temporal trends in nutrients, major ions, trophic variables, and oxygen can indicate changes in water quality related to eutrophication. Ongoing changes in water use among agricultural and municipal users on the eastern slope of Colorado, changing land uses in reservoir watersheds, and other water-quality issues among NCWCD customers necessitated a reexamination of water-quality trends in CBT system reservoirs and related conveyances.

The overall objective of the project is to complete a retrospective analysis of reservoir profiles, nutrients, major ions, selected trace elements, chlorophyll-a, and hypolimnetic oxygen data from 1969 through 2000 in Lake Granby, Shadow Mountain Lake, and the Granby Pump Canal in Grand County, Colo., and Horsetooth Reservoir, Carter Lake, Lake Estes, east portal of the Alva B. Adams Tunnel (hereinafter "Adams Tunnel"), and Olympus Tunnel in Larimer County, Colo. Sampling sites in this report are in the headwater areas of the CBT Project where diversion and storage facilities are located. Areas downstream from Lake Granby on the western slope and water delivery areas downstream from Carter Lake and Horsetooth Reservoir are beyond the scope of this report. This report summarizes water-quality and field-measurement profile data collected by the USGS and stored in the USGS National Water Information System; assesses time-series trends of selected chemical constituents and physical properties, trends in oxygen deficits in the hypolimnion of the reservoirs in the late summer season by the seasonal Kendall trend test method, and nutrient limitation and trophic status indicators; and evaluates water-quality data in terms of Colorado water-quality standards.

Previous Studies and Acknowledgments

A report by Mueller (1990) discussed trends determined by using the seasonal Kendall method; general water-quality analysis and sampling network evaluation were the primary focus of that report. Trend analysis at the Adams Tunnel east portal that is included in this report also was computed for selected chemical constituents in Middelburg (1993, p. 204). Jassby and Goldman (1999) studied water-quality and biological conditions in Horsetooth Reservoir and Carter Lake in 1998–99. Greve (1999) designed a water-quality monitoring network for the Big Thompson watershed in cooperation with the Big Thompson Watershed Forum, a watershed group within the study area. Johnson and Goettl (1999) studied food web changes for 14 years following the introduction of rainbow smelt into Horsetooth Reservoir.

The author thanks Don Carlson at the NCWCD and Gene Price and Jeff Lucero at the BOR for their assistance in bringing this report to fruition, and Dave Dzurovshin at the Colorado State Engineers Office for assistance in retrieving discharge data at some sites.

Description of Study Area

The study area involves parts of two mountainous watersheds. On the western side of the Continental Divide, several major tributaries in the headwaters of the Colorado River (North Fork Colorado River, North Inlet, East Inlet, and Arapaho Creek) are connected to parts of the western slope CBT reservoir system (Lake Granby, Shadow Mountain Lake, and Grand Lake) and, thus, are the primary sources of water diverted by the CBT Project (fig. 2). Other water sources include the Willow Creek Canal, which conveys water stored in Willow Creek Reservoir, and water pumped from Windy Gap Reservoir (built in 1985 by the Municipal Subdistrict of the NCWCD to use the unused capacity in CBT facilities), located on the Colorado River downstream from the confluence with the Fraser River. Storage on the western slope is primarily in Lake Granby (539,800 acre-ft total capacity). Water is pumped through the Farr Pumping Plant and the Granby Pump Canal into Shadow Mountain Lake. Shadow Mountain Lake and Grand Lake are similar (hydraulically connected) and relatively nonfluctuating. Their function is to convey project water into the Adams Tunnel, the largest transcontinental diversion in Colorado (average 230,000 acre-ft diverted annually).


Figure

Figure 2. Schematic diagram of CBT Project facilities (from Mueller, 1990).


On the eastern side of the Continental Divide, project water is conveyed through tunnels and eventually to Lake Estes, where it mixes with water from the Big Thompson River, a watershed with headwaters located primarily within Rocky Mountain National Park. The Olympus Tunnel then conveys water from Lake Estes to a series of tunnels and small reservoirs to Carter Lake or the Hansen Feeder Canal. The Hansen Feeder Canal can convey the water (and accept water diverted at the mouth of Big Thompson Canyon through the Dille Tunnel) to Horsetooth Reservoir. Carter Lake (112,230 acre-ft total capacity) and Horsetooth Reservoir (156,735 acre-ft total capacity) are the primary storage facilities on the eastern slope and were constructed by damming valleys of intermittent drainages along the Front Range. Project water is then conveyed to water users by way of streams, canals, and pipelines within the NCWCD boundaries to Boulder Creek, Saint Vrain Creek, Big Thompson River, Cache La Poudre River, and the main-stem South Platte River. Drinking water is supplied to 30 Front Range cities and towns, and water for supplemental irrigation is provided to more than 600,000 acres of agricultural land. Project water was initially used primarily for agriculture, but municipal uses have increased in recent years (fig. 3).


Figure

Figure 3. Percentage of Colorado–Big Thompson (CBT) water supplied for municipal use.


Primary land uses within the parts of the watersheds where CBT project water is diverted and stored are undeveloped forest and rangelands under the management of the U.S. Department of Agriculture Forest Service and National Park Service (figs. 4 and 5). The facilities on the western slope and those near Estes Park are located in mountainous terrain (greater than 6,000-ft elevation) covered in coniferous forest, underlain by mostly crystalline bedrock. Annual precipitation is in the range of 16 to more than 40 inches per year (Colorado Climate Center, 1984). Horsetooth Reservoir and Carter Lake are located in more arid foothills on the edge of the mountain front, are surrounded by grass/shrublands, and are underlain by sedimentary rock (fig. 6). While one of the primary uses of water resources is recreation, there are two developed urban areas that could affect the water quality within the study area: Grand Lake and Estes Park. Grand Lake is a town of approximately 250 people, on the shores of Grand Lake. Similar, though less dense, residential development is present along the shores of Shadow Mountain and Granby Lakes (fig. 4). Before 1982, individual sewage disposal systems (ISDS) were used for sanitation. Due to increasing water-quality concerns, a municipal sanitation system that pumps waste downstream from Lake Granby for treatment was completed in 1982.


Figure

Figure 4. Land uses in watershed upstream from Lake Granby.



Figure

Figure 5. Land uses in watershed upstream from the mouth of Big Thompson Canyon.



Figure

Figure 6. Land uses in watershed draining directly to Horsetooth Reservoir.


The town of Estes Park is located around Lake Estes and has a population of approximately 3,000 (fig. 5). Less dense development is located along the Big Thompson Canyon from Estes Park to the mouth of the canyon west of Loveland. Water quality is potentially affected by treated sewage effluent from Estes Park discharged into the Big Thompson River upstream from Lake Estes. Smaller sewage-treatment facilities and ISDS's located along the Big Thompson River downstream from Lake Estes could affect the quality of water diverted to Horsetooth Reservoir through the Dille Tunnel (located just upstream from the mouth of Big Thompson Canyon) and Hansen Feeder Canal. Development around Carter Lake is minimal, but residential development around Horsetooth Reservoir could affect water quality (fig. 6).

Hydrology

The study area, because of its location in high mountains, derives annual streamflow primarily from snowmelt during May through July as shown in the streamflow statistics for the period of record at the Colorado River below Baker Gulch stream gage (fig. 7). The watershed area upstream from Lake Granby outlet is about 310 square miles. The watershed area upstream from the mouth of Big Thompson Canyon is about 306 square miles. The high-elevation areas of these catchments receive large amounts of snow from September to May. Maximum snowpack accumulation generally occurs about the beginning of May and is mostly melted by about mid-June when daily streamflow runoff peaks. Daily fluctuations in streamflow, which can substantially affect stream-water quality by influencing dilution and particle transport, may be largely due to diurnal variations in air temperatures, affecting snowmelt.


Figure

Figure 7. Maximum, mean, and minimum daily streamflows for the period of record (1953 to 2000) at Colorado River below Baker Gulch (09010500).


Operation of reservoirs and conveyances can affect seasonal water quality, streamflow, and limnological characteristics of storage reservoirs. Some streamflow and reservoir data were available from USGS sources, but most was obtained from either NCWCD or the Colorado State Engineers Office. Seasonal and annual operational characteristics of selected conveyances and reservoirs are shown in figure 8. Release and storage patterns are relatively similar over the period of record on a year-to-year basis at Shadow Mountain Lake, Horsetooth Reservoir, and Carter Lake (fig. 9). The contents of Shadow Mountain Lake, and consequently Grand Lake, vary little because the reservoir is controlled to minimize fluctuations in water-surface elevation. Multiyear patterns of storage can be seen in plots of contents for Lake Granby. Storage accumulated in wet years can persist for several years. Similarly, when depleted, Lake Granby can take several years to refill. The effects of drought years such as 1977 can be seen clearly in plots of reservoir contents for Lake Granby and Horsetooth Reservoir.


Figure

Figure 8. Time-series plots of operation of selected Colorado–Big Thompson conveyances.



Figure

Figure 9. Time-series plots of operation of selected Colorado–Big Thompson reservoirs.


Seasonal flow patterns among the conveyances are evident in the plots. Highest flow rates occur at the Willow Creek Canal and the Windy Gap pipeline during snowmelt season (fig. 10). Flow rates are highest in Granby Pump Canal, Adams Tunnel, and Carter Lake inflow during the winter months. Hansen Feeder Canal conveys flow mostly in winter and summer and generally maintains lower rates of flow in early spring and fall. On average, Lake Granby reaches maximum elevation in July of each year, falling steadily through summer and winter until April, when storage of the next snowmelt begins (fig. 11). Storage in Horsetooth Reservoir is somewhat different from Lake Granby in that average storage in Horsetooth increases from November through May or June and is then depleted during the summer season. In Carter Lake, storage increases from about October through April.


Figure

Figure 10. Monthly average discharge characteristics of selected Colorado–Big Thompson conveyances.



Figure

Figure 11. Monthly average reservoir contents of selected Colorado–Big Thompson reservoirs.


Water Quality

In this report, water quality will be described in two general categories: chemical characteristics and limnology. The discussion of chemical characteristics will present summary statistics, boxplots of chemical constituents, and descriptions of general water-quality conditions at the sampling sites. The number of samples may differ from boxplot to summary statistics because of the automated nature of the summary statistics/database and the use of estimated values in the boxplots. The discussion of limnology will present reservoir field-measurement profiles, hypolimnetic oxygen dynamics, trophic indicators, and nutrient limitation at reservoir sites.

Chemical Characteristics

The types of water-quality data collected at most sites that are part of the scope of this report include field measurements (water temperature, specific conductance, pH, and dissolved oxygen), major ions (calcium, magnesium, sodium, potassium, acid-neutralizing capacity or alkalinity, sulfate, chloride, fluoride), nutrients (ammonia plus organic nitrogen, ammonia, nitrite, nitrite plus nitrate, total phosphorus, dissolved phosphorus, orthophosphorus), and selected trace elements (iron, manganese, barium, strontium, and zinc). Summary statistics were compiled for the 10 sites in this study and are listed in tables 4 to 20 in the "Hydrologic Data" section (appendices at back of this report).

Surface and near-bottom sampling from each reservoir included field-measurement profiles. Some reservoir sites during the 1970's had only a surface sample collected or an additional intermediate depth sample collected in addition to the surface and near-bottom samples. Sampling frequency has not remained constant throughout the history of the program. The frequency history described here includes only the sites considered for this report. Other discontinued sampling sites on Carter Lake and Horsetooth Reservoir are not described in this report but are described in Mueller (1990).

Lake Granby (site 1, fig. 1) near the dam was sampled quarterly at the surface from 1973 to 1975. During 1979 to 1988, annual samples were collected from the surface and near bottom. From 1989 to 1990, samples were collected at surface and bottom, three times during the growing season (May to October). From 1991 to 1997, samples were collected approximately monthly from May to October. In 1998, sampling frequency decreased to four times per year during the growing season. Sampling at Lake Granby (site 2) in Rainbow Bay was started in 1989 and followed a schedule of one per month during May to September until 2000. Sampling at Granby Pump Canal (site 3) was approximately bimonthly on average from 1970 to 2000. The exact months sampled varied because samples were not collected in a particular month if there was no flow at the time of the visit. Sampling at Shadow Mountain Lake (site 4) began consistently in 1989 at a frequency of three per year during the growing season, increased to six per year from 1991 to 1997, and decreased to four per year from 1998 to 2000. Sampling at the Adams Tunnel East Portal (site 5) began in 1970 at a monthly frequency until 1988, when the bimonthly schedule that continued through 2000 was implemented. Sampling at Lake Estes (site 6) was begun in 1998 at a frequency of two per year, with samples collected at the surface and near the bottom in May and August. Monthly sampling at Olympus Tunnel (site 7) began in the fall of 1970 until 1986. By 1989, frequency had dropped to three samples per year, one each in spring, summer, and fall, and continued at the same frequency until 2000. Sampling at Carter Lake Reservoir (site 8) began in 1970 at a frequency of 9 or 10 per year, April to November until 1979. By 1983, sampling frequency was reduced to three per year during the spring through fall and continued to 2000. Sampling at Horsetooth Reservoir (Soldier Canyon Dam) (site 9) began in 1969, and samples were collected at surface, bottom, and an intermediate depth from April to October or November until 1978. By 1982, frequency was down to one surface and one near-bottom sample in spring, summer, and fall. Sample collection began at the Spring Canyon Dam (site 10) on Horsetooth in 1983 at a frequency of three per year, surface and near bottom.

Field Measurements and Major-Ion Concentrations

In snowmelt-dominated mountain streams, one of the primary processes controlling specific-conductance and major-ion concentrations is dilution; minimum values of specific conductance occur during peak snowmelt discharge and maximum values occur during winter low-flow conditions (Stevens, 2001; Clark and others, 2000). Daily fluctuations of specific conductance in streams not affected by reservoirs also reflect dilution by the daily diurnal fluctuation of flow during high snowmelt (Stevens, 2001). In reservoirs and conveyances that move water from reservoirs, these fluctuations are tempered by mixing that occurs during storage. Thus, concentrations are smoothed substantially in a reservoir like Lake Granby that can potentially store multiple years of runoff.

Water at CBT sampling sites was dilute and typical of high-elevation snowmelt runoff (fig. 12). Median concentrations of specific conductance at sites on the western slope (such as Lake Granby and Shadow Mountain Lake) and the high-elevation sites on the eastern slope (such as Adams Tunnel, Lake Estes, and Olympus Tunnel) were generally below 60 microsiemens per centimeter at 25 degrees Celsius (µS/cm). These sites are indicative of a hydrologic setting that is typical of snowmelt-source water—low rates of evapotranspiration (ET) and crystalline bedrock geology. Specific conductance at sites at Carter Lake and Horsetooth Reservoir were generally greater than 70 µS/cm. These sites possibly show the cumulative effects of development and ET from the rest of the storage and conveyance system. These sites are at lower elevations (higher ET) in areas of sedimentary rocks that can increase dissolved-solids content. Even so, all water sampled in this system should be considered to be high quality because of its low dissolved-solids content.


Figure

Figure

Figure

Figure 12. Boxplots of selected water-quality constituents.


Water temperature varied seasonally, with reservoir sites developing strong stratification patterns. The pH at CBT sites was generally in a neutral range of 7 to 8. Dissolved oxygen was generally in the range of 6 to 10 mg/L except in the hypolimnetic water of the reservoirs, where seasonal stratification sometimes produced dissolved-oxygen concentrations approaching 2 mg/L or less.

Major-ion composition of selected waters was plotted on a Piper diagram (fig. 13) to determine if the composition of water is changing as it moves from one part of the system to another. Adams Tunnel east portal was chosen because it represents the composition of water that is diverted from the western slope and reflects any urban influences present on the west side of the Continental Divide. Olympus Tunnel represents water with urban influences from Estes Park. Horsetooth Reservoir represents water with urban influences around the reservoir and sedimentary substrate, and any influences of wastewater treatment, septic tanks, and highway runoff in water diverted from the Dille Tunnel near the mouth of Big Thompson Canyon. Cation compositions in water from all three sites are somewhat mixed calcium-magnesium-sodium, but calcium is predominant (fig. 13). Anion compositions are primarily bicarbonate. Adams and Olympus Tunnel water had the most variability, with some samples containing relatively less bicarbonate and more sulfate. Horsetooth Reservoir composition was less variable but is mostly within the range of water sampled from the tunnels. Despite having larger concentration of dissolved solids, Horsetooth Reservoir water seemed to be a more concentrated version of the tunnel water.


Figure

Figure 13. Piper diagram showing major-ion composition of samples at selected sites.


Nutrients

Compounds of nitrogen and phosphorus are referred to as "nutrients" because they are the major building blocks of plant growth. In excess quantities, nutrients can promote nuisance algae growth in streams and reservoirs. Natural sources of nutrients include precipitation and biochemical processes in the watershed. Possible anthropogenic sources of nutrients in the CBT system include the following:

  1. Urban runoff.—Associated with developed areas and roads, storm runoff may have higher nutrient concentrations and, as road runoff, may bypass natural removal processes in soil by rapid delivery to a receiving stream or reservoir;
  2. Sewage effluent.—Septic-tank effluent can move with ground water into surface-water systems. Rural areas are often developed beyond the practical extent of municipal sewer lines, necessitating the use of septic leach fields. Although the town of Grand Lake has installed a sewer system (Three Lakes Sanitation District), not all residences within the watershed are connected to it. Estes Park effluent is treated (Estes Park Sanitation District and Upper Thompson Sanitation District) and returned to the Big Thompson River. Other scattered development in the Big Thompson watershed is not connected to municipal sewer systems; and
  3. Erosion of soil and plant materials exposed near roads or streams can cause loading of organic nitrogen and particulate phosphorus when transported to a stream.

Nutrient concentrations at CBT sites generally were low (fig. 12). Data censoring levels changed many times throughout the study, and data analyses at times were of insufficient precision to fully characterize concentration. Median concentrations of ammonia plus organic nitrogen were less than 0.3 mg/L at all sites. Median concentrations of dissolved nitrite plus nitrate concentrations were generally less than 0.2 mg/L, and median concentrations of total phosphorus were less than 0.05 mg/L at all sites. Concentrations of nitrite and orthophosphorus were rarely reported. The largest concentrations of nutrients tended to be at Carter Lake and Horsetooth Reservoir. The smallest concentrations were generally measured at Lake Granby and Shadow Mountain Lake. Bottom samples tended to have larger concentrations of most nutrients than surface samples at the same site.

Trace Elements

Trace elements, for the purpose of this report, are metal and transition metal elements commonly found in small (less than 1 milligram per liter) concentrations. Analytes commonly detected at most sites in this study included barium, iron, manganese, strontium, and zinc. Most samples also were analyzed for barium, beryllium, boron, cadmium, chromium, cobalt, copper, lead, lithium, molybdenum, nickel, silver, and vanadium. Trace elements are important indicators of water quality because, in large concentrations, they are toxic to aquatic life; a large proportion of Colorado's regulated constituents are trace elements (Colorado Department of Public Health and Environment, Water Quality Control Commission, 2001).

Concentrations of common dissolved trace elements (such as iron and manganese) were commonly above Minimum Reporting Limits (MRL's) but usually less than 1 milligram per liter (fig. 12). Others, such as beryllium, chromium, cobalt, lithium, molybdenum, nickel, silver, and vanadium, were almost never reported. Zinc, lead, cadmium, boron, and copper concentrations were reported infrequently, and when reported, ranged from a few micrograms per liter to a few tens of micrograms per liter (zinc). Sources of the more toxic trace elements (such as copper, cadmium, lead, and zinc) are generally associated with ore deposits or urban runoff. The relative lack of such ore deposits and the relatively low density of development in the drainage areas of the CBT project will tend to limit concentrations of these toxic trace elements to relatively low levels.

Colorado Water-Quality Standards

In Colorado, water-quality standards are determined by the Colorado Water Quality Control Commission and consist of a narrative or numeric restriction established to protect the beneficial uses of water. Water-quality standards for streams in Colorado are based on use classifications such as aquatic life, recreation, water supply, and agriculture. Some water-quality standards for streams are set to specific values or concentrations (such as dissolved oxygen, pH, nitrite, nitrate, chloride, sulfate, iron, and manganese). Others, such as un-ionized ammonia and some trace elements, are usually site specific and use hardness data and equations for each element to calculate standards (Colorado Department of Public Health and Environment, Water Quality Control Commission, 2001). In certain cases, fixed numeric standards have been established that reflect site-specific conditions and may specify trace-element phases and analytical techniques (such as dissolved or total recoverable methods).

Trace-element concentrations are assumed to be dissolved unless designated otherwise by the numeric standards (Colorado Department of Public Health and Environment, Water Quality Control Commission, 2001). Acute standards are values or concentrations not to be exceeded for 1 day. Chronic standards are not to be exceeded by the 85th percentile of sample concentrations in a representative period. Extraordinary ephemeral or seasonal conditions do not constitute violations. Stream-standard concentrations and values are not to be exceeded more than once every 3 years on average (Colorado Department of Public Health and Environment, Water Quality Control Commission, 2001). Thus, continued systematic monitoring is needed to determine compliance or noncompliance with a standard.

The State of Colorado specifies that hardness concentrations used to calculate the trace-element standards be computed from the lower 95-percent confidence limit of the mean hardness at the periodic low-flow criteria determined from regression analysis of site-specific or regional data. In this study, however, hardness concentrations were determined from analysis of water samples collected at the time of sampling. The comparison of standards in this report, therefore, is a general indicator.

The use classifications for CBT sampling sites for this report are Aquatic life cold 1, Recreation 1, Water supply, and Agriculture (Colorado Department of Public Health and Environment, Water Quality Control Commission, 2001). Estimates of the Colorado water-quality standards for samples collected at the 10 sampling sites were calculated on the basis of numeric fixed standards or equations (Table of Value Standards [TVS]) for each element (Colorado Department of Public Health and Environment, Water Quality Control Commission, 2001). Laboratory MRL's for trace elements such as cadmium, mercury, silver, and lead were commonly too high to determine concentrations at the level of the standard. These constituents might be present in the sample at concentrations lower than the laboratory MRL but higher than the standard. Samples were not analyzed for some constituents listed in the Colorado water-quality standards (for example, arsenic, chlorine, cyanide, selenium, and sulfide).

Based on all samples collected over the period of record, standards for chloride, sulfate, un-ionized ammonia, nitrate, dissolved chromium, and dissolved nickel were not exceeded at any of the sampling sites in the study area. Dissolved oxygen exceeded minimum concentrations, and pH was not below or above standards. Silver, lead, and cadmium concentrations occasionally exceeded computed standard values. The fixed standard for dissolved iron (300 µg/L) was exceeded at Lake Granby (two samples), Adams Tunnel (one sample), and Shadow Mountain Lake (one sample). The fixed standard for dissolved manganese (50 µg/L) was exceeded at Lake Granby (10 samples), Horsetooth Reservoir (17 samples), and Shadow Mountain Lake (17 samples). Many of the manganese exceedances were in the early 1990's, mainly in near-bottom samples. The fixed standard for manganese was not exceeded at Carter Lake. The presence of manganese is a common problem in the bottom water of lakes and reservoirs and can adversely affect drinking-water treatment, cause staining of laundry when used for washing, and when it precipitates in excessive quantities, can smother aquatic life (Niyogi and others, 1999).

Limnology

The following sections of the report will discuss issues of water quality that pertain to the reservoir sites. Conditions in the reservoirs affect not only that particular water body but the conveyances and streams into which that water is released, diverted, or pumped.

Reservoir Profile Characteristics

Reservoir profile data are routinely measured when water-quality samples are collected. These measurements are typically made with an instrument that contains sensors for water temperature, specific conductance, pH, and dissolved oxygen. The instrument is lowered beneath the surface from a boat that is held as stationary as possible over the sampling location. Successive readings are taken throughout the depth profile when the instrument stabilizes at a particular depth.

Plots of profiles measured in May and October at Lake Granby, Shadow Mountain Lake, Carter Lake, and Horsetooth Reservoir are shown in figures 14 to 17. The profiles are intended to show how these measurements vary with depth at the beginning of the ice-free season (May) and the end of the ice-free season (October). May and October also were the two months commonly measured at all sites a number of times during the study period.


Figure

Figure 14. May and October field-measurement profiles at Lake Granby (dam).



Figure

Figure 15. May and October field-measurement profiles at Shadow Mountain Lake.



Figure

Figure 16. May and October field-measurement profiles at Carter Lake.



Figure

Figure 17. May and October field-measurement profiles at Horsetooth Reservoir (Soldier Canyon Dam).


By May, water-temperature stratification had already begun at most of the reservoirs, particularly at the lower elevations of Carter Lake and Horsetooth Reservoir, where conditions may be ice-free a couple of months earlier. Specific conductance, pH, and dissolved oxygen have not yet become strongly stratified with depth in May, and dissolved-oxygen concentrations are relatively high throughout the profiles.

In the fall (October), conditions at the deeper reservoirs usually remained strongly stratified from summer heating except at Shadow Mountain Lake, where shallow depths (mean depth approximately 15 ft) interfere with the full development of a thermocline. In the other reservoirs, the top of the thermocline was at approximately 30 to 60 ft of depth most years in October (figs. 14–17). During stratification, specific conductance sometimes increased slightly with depth in Lake Granby and Horsetooth Reservoir. Dissolved oxygen was sometimes hypoxic in October profiles in the hypolimnion of the deeper reservoirs, and pH generally decreased with depth in May and October profiles at Lake Granby, Horsetooth Reservoir, and Carter Lake. The lowest hypolimnetic dissolved-oxygen concentrations in October were from Lake Granby and Horsetooth Reservoir, where near-bottom concentrations in some years were less than 2 mg/L.

Hypolimnetic Oxygen Dynamics

Dissolved-oxygen profiles show the distribution of oxygen concentrations with depth, but the shape of the profiles, their seasonal characteristics, reservoir water level, and hydraulic residence time (flushing rate) can be highly variable. This variation makes it difficult to compare from year to year or site to site. One approach to using the dissolved-oxygen profile data involves making computations that take into account reservoir bathymetry, reservoir contents, and the development of the oxygen profile during the growing season. In this way, there is the potential to compare dissolved-oxygen dynamics from year to year. Because low concentrations of dissolved oxygen can be cited as a consequence of eutrophic conditions, a comprehensive measure of those concentrations can contribute to the understanding of trends in eutrophication.

The zone in a reservoir where oxygen depletion is generally greatest is in the hypolimnion. The hypolimnion is the deep portion of the water-temperature profile, below the thermocline, that becomes isolated from the epilimnion and metalimnion during temperature stratification. After becoming isolated from the atmosphere and other sources of oxygen, the oxygen concentration begins to decrease steadily throughout the stratification season as a result of decay processes that steadily consume oxygen. The rate at which oxygen is consumed is indicative of the rate of biological activity, which is indicative of the level of eutrophication (Green, 1996). The hypolimnion does not remix and reoxygenate until fall overturn in lakes that are stratified in summer. These lakes restratify in winter and remix again in spring.

The technique of computing and using areal hypolimnetic oxygen deficits for analysis of eutrophication is described in detail in Green (1996) and will be described briefly in this report. Through the use of one of the PROFILE routines in the U.S. Army Corps of Engineers BATHTUB model (Walker, 1996), areal hypolimnetic oxygen deficit (AHOD) was computed for each growing season in each of four reservoirs (Lake Granby, Shadow Mountain Lake, Carter Lake, and Horsetooth Reservoir) when sufficient profile data were available. The model can be used with data inputs of bathymetry, profile data, estimates of the elevation of the hypolimnion for each profile, and time series of reservoir water levels to make computations of AHOD, which is defined as the change in oxygen per square meter of hypolimnetic surface per day through the stratification season (approximately May to September or October). To ensure that fall overturn reoxygenation has not begun, Walker (1996) recommends that the profile period of computation for AHOD begin with a spring profile in which stratification has already been established (usually May in this study) and end with a profile that is not beyond the peak of near-bottom oxygen depletion (usually July or August in this study). AHOD is computed as the rate of change (slope) of the areal hypolimnetic oxygen content (AHOC) computed by the model through the stratification season.

Time-series plots of computed annual AHOD are shown in figure 18 for Shadow Mountain Lake, Lake Granby, Carter Lake, and Horsetooth Reservoir. There may be patterns present in the AHOD data, and the cycles are not necessarily the same for each reservoir. These patterns suggest that climatic, biological, or operational cycles may influence the rates of oxygen consumption in these reservoirs. The AHODs for each reservoir should not be directly compared to one another because they include the influences of water temperature and morphological characteristics, which may vary in each reservoir.


Figure

Figure 18. Time-series plots of annual areal hypolimnetic oxygen deficit in reservoirs.


Trophic Indicators and Nutrient Limitation

The relative fertility of a lake or reservoir can be evaluated by assessing the trophic status. Oligotrophic (nutrient-poor) lakes have characteristics such as high transparency, small organic-matter content, relatively large dissolved-oxygen concentrations, small nutrient concentrations, and small algal biomass. Eutrophic (nutrient-rich) lakes have the opposite characteristics (Woods, 1992). On the basis of data collected over the period of record at each site, CBT reservoirs were assessed using the method developed by Carlson (1977).

Trophic-state index (TSI) values were calculated from the following equations:

TSI(SD) = 60 – 14.41(ln SD)(1)

TSI(TP) = 14.42(ln TP) + 4.15(2)

TSI(CHLA) = 9.81(ln CHLA) + 30.6(3)

where

SDis Secchi-disk depth, in meters;
TPis total phosphorus concentration, in micrograms per liter;
CHLAis chlorophyll-a concentration (high-performance liquid chromatography was used), in micrograms per liter;
and
lnis the natural logarithm of the indicated variable.

Secchi-disk depth, total phosphorus (surface only), and chlorophyll-a values were averaged for each year to compute mean values. The annual mean values were then used to compute annual TSI values plotted in figure 19. Censored values of total phosphorus were common in the 1990's, and not all TSI's could be computed. The Carlson (1977) TSI index assumes phosphorus limitation. The boundary between oligotrophic (nutrient-poor) and mesotrophic (moderate nutrients) is a TSI value of 30. The boundary between mesotrophic and eutrophic (nutrient-rich) is a TSI value of 50. Most of the TSI values for the reservoirs in this study are in the mesotrophic range. A tendency toward decreasing trophic status in the 1990's at Lake Granby and Shadow Mountain Lake is based primarily on Secchi depth, and chlorophyll-a concentrations and can be seen in the plots.


Figure

Figure 19. Annual trophic status index (TSI) computations for Secchi depth, total phosphorus, and chlorophyll-a by the Carlson (1977) method.


The indices are meant to normalize the three indicators so that any of the three could describe the conditions in the water body. At Lake Granby and Shadow Mountain Lake, the three indices agree fairly well. At Carter Lake and Horsetooth Reservoir, the index for phosphorus does not compare well with the indices for Secchi depth and chlorophyll-a. Turbid conditions caused by suspended sediment could raise both the total phosphorus and Secchi-depth TSI's relatively but might not affect or might even decrease the chlorophyll-a index due to reduced light penetration. Other cyclic factors such as grazing of phytoplankton by zooplankton can make interpretation of chlorophyll-a indices problematic. The use of TSI can give a qualitative indication of relative status of water bodies but should not be used exclusively to evaluate whether lakes and reservoirs are meeting the criteria of their water-quality classifications.

Nitrogen (N) to phosphorus (P) mass ratios have been used to characterize the limiting nutrient for algal growth present in water (Britton and Gaggiani, 1987; Woods, 1992). If a nutrient is limiting, the hypothesis is that the addition of that nutrient would then cause an increase in algal production (in the absence of other limiting factors) (Britton and Gaggiani, 1987; Woods, 1992). Due to the large number of censored nitrogen and phosphorus concentrations, the N/P ratios were not analyzed. An analysis of N/P ratios would indicate only certain conditions when nutrient concentrations were high (greater than minimum reporting limits) and not the full range of ratios for different concentration conditions in the reservoirs.

Trend Analysis

Time-series trend analysis is a common approach to evaluating the temporal changes in long-term data sets. The method chosen for the CBT data is the seasonal Kendall nonparametric test. The test is robust in that it does not require normality, can be used with censored data, and seasonal effects that can obscure the detection of a trend can be removed. The test is computed on the ranks of the data and will indicate the significance of monotonic trends (generally increasing or decreasing with time) and provide an estimate of the slope of the change in the median of the data. Further details on the method and applications can be found in publications such as Helsel and Hirsch (1995), Middelburg (1993), Clark and others (2000), and Mueller (1990).

Time-Series Data

Time-series data were compiled from the USGS database for the period of record at each site. To help with visual assimilation of these large data sets, scatterplots of each constituent of interest were made and a trace of a LOWESS smooth regression was drawn unless periods of data were missing or a series of values were less than MRL (see figs. 23–37 in "Hydrologic and Water-Quality Data" section at back of report). The LOWESS smooth curve was estimated using the MRL in cases where data were censored. Total ammonia and total nitrate were considered similar to dissolved ammonia and dissolved nitrite plus nitrate for plotting and trend-testing purposes. Mueller (1990) made a comparison of the total and dissolved data for these constituents and found no statistical difference. If a field specific conductance or pH value was missing or erroneous, the laboratory value was substituted.

Certain characteristics of the data are visible in the plots that are not evident in the application of the statistical trend test. Variability, nonmonotonic trends, and nonseasonal cycles are important information that are not conveyed by a p-value or slope.

A few general observations about the data from the time-series plots (see figs. 23–37 in "Hydrologic and Water-Quality Data" section at back of report) are listed here:

  1. Many time-series data are not exclusively monotonic, a condition that could obscure small-scale trends in the trend testing. For example, a normal distribution curve would represent strong trends if it were a time series, but the symmetrical qualities would cancel out the monotonic properties and no significant trends would be computed. This problem is particularly evident in the nutrient data at some of the sites that have long periods of record. Sites such as Granby Pump Canal, Olympus Tunnel, Adams Tunnel east portal, Horsetooth Reservoir at Soldier Canyon Dam, and Lake Granby Dam show clearly some peaking of nutrient concentrations in the early 1980's (despite the problematic changes in minimum reporting limits) that might not be assessed adequately by monotonic trend testing of the full period of record (fig. 20). Other constituents are well characterized as monotonic distributions (fig. 21).

Figure

Figure 20. Nonmonotonic distribution and seasonal Kendall test results for pre-1983 and post-1983 of ammonia plus organic nitrogen (total) at Adams Tunnel east portal.



Figure

Figure 21. Monotonic distribution and seasonal Kendall test results for all samples of dissolved iron at Adams Tunnel east portal.


  1. In general, specific conductance, dissolved solids, and most of the major ions at most sites have similar LOWESS curve shapes, indicating that some of the data for constituents and properties may be providing redundant trend information.
  2. Because the data at most sites are collected in a very limited number of seasons, some of the variability and representativeness of the annual water-quality changes at the sites are potentially lost. Trend results apply to seasons represented in the data set.
  3. Apparent decreases in nutrient concentrations throughout the system are accompanied by decreases in iron (Fe) and manganese (Mn) concentrations. This pattern could indicate a relation between fertility and bottom-water hypoxia (which can cause reducing conditions that release Fe and Mn).
  4. The degree of variability is high in the chlorophyll-a data, indicating that sampling frequency may not be sufficient to adequately describe algal population dynamics that occur in short time scales.

Some qualitative temporal trend information based on visual inspection of the time-series plots is summarized here:

  1. Field measurements of dissolved oxygen and pH are relatively stable through time at most sites. Small fluctuations are probably related to factors such as changes in water temperature, water depth, collection date, and reservoir contents. Slight decreases in dissolved oxygen are shown by the LOWESS curves in the near-bottom samples at Lake Granby (Rainbow Bay) (fig. 26), Carter Lake (fig. 33), and Horsetooth Reservoir (fig. 35).
  2. Specific conductance, dissolved solids, and most major ions show decreases in concentrations at most sites over time. Since 1998, however, downward trends have flattened at sites on the western slope (Lake Granby, Shadow Mountain Lake, Granby Pump Canal), and since 1998, upward trends are evident at Horsetooth Reservoir. Some decreases might be related to high runoff years when large volumes of water containing low dissolved solids are stored, influencing concentrations for many years afterward, as that dilute water is pumped throughout the system from the Lake Granby storage. The dilutional effects of high runoff in 1995 can be seen in many plots (figs. 23–37).
  3. At sites with 20 years or more years of record, nutrient concentrations have decreased (sometimes substantially) from early 1980's levels. Multiple MRL's and large amounts of data below the MRL in recent years have made it difficult to characterize the most recent trends. It is clear, however, that nutrient concentrations seem to have peaked sometime in the 1970's and early 1980's and have stayed at relatively low levels during the 1990's. Good examples of this pattern can be seen in some plots of ammonia plus organic nitrogen, ammonia, nitrite plus nitrate, and total phosphorus for Adams Tunnel (fig. 30), Olympus Tunnel (fig. 31), Granby Pump Canal (fig. 27), Horsetooth Reservoir (Soldier Canyon Dam) (figs. 34 and 35), and Carter Lake (figs. 32 and 33). This pattern is much less definitive in Lake Granby (figs. 23 and 24).
  4. Trace elements such as iron (Fe) and manganese (Mn) have a high degree of variability that is probably caused by seasonal reducing conditions. Reducing conditions can cause the release of Fe and Mn from bottom sediments and are proportional to the degree of hypolimnetic oxygen depletion in a given growing season and the proximity to which the near-bottom sample can be collected close to the reservoir bottom sediment (a variable and uncertain exercise). Similar to nutrients, concentrations of Fe and Mn in the long records at Adams Tunnel (fig. 30), Olympus Tunnel (fig. 31), and Horsetooth Reservoir (figs. 33 and 35, iron only) peak in the early 1980's and generally decline through 2000.
  5. Miscellaneous measurements such as transparency, chlorophyll-a, and bacteria also show patterns that can be observed on time-series plots. Records of transparency generally show increasing clarity in CBT system reservoirs from 1970 to 2000, although seasonal variability is fairly high (often 50 to 100 inches or more over a few months). The record for chlorophyll-a is relatively short (about 11 years). Two peaks of concentration can be seen at Lake Granby (spillway) (fig. 23), Shadow Mountain Reservoir (fig. 28), and Horsetooth Reservoir (figs. 34 and 36) corresponding to approximately 1990–91 and 1995–96. Concentrations of bacteria are generally very low (less than 10 colonies per 100 milliliters) for all sites. Some large spikes in concentrations of fecal and total coliform bacteria (more than 100 colonies per 100 milliliters) fecal coliform and more than 1,000 colonies per 100 milliliters total coliform were reported in the early 1980's at Olympus Tunnel (fig. 31).

Statistical Testing

Seasonal Kendall test (Helsel and Hirsch, 1995) was used to determine the significance of monotonic temporal trends over the period of record for selected constituents. Constituents chosen for analysis included field measurements, major ions, nutrients, chlorophyll-a and -b, fecal and total coliform bacteria, and trace elements that were routinely reported above censoring levels.

Trend for a particular constituent was only computed when less than 50 percent of the data was below the MRL and more than 5 years of data was available. Lake Estes had less than 5 years of data and was therefore excluded. If more than 5 percent of the data was censored, the slope of the median was not reported. Small breaks in the time series were acceptable if they occurred in the middle one-third of the data. Many of the nutrient species at particular sites were mostly censored data and were not tested. Top- and bottom-sample constituents were tested separately. No flow adjustments were made because all sites were reservoirs or conveyances receiving water from reservoirs. Because multiple MRL's were common, all censored data were recensored to the highest MRL. Or, in a few cases, the highest MRL data in a series were ignored if the data could be eliminated by minor shortening of the beginning or ending of the period of record for the analysis. Estimated data were treated as legitimate. Most outliers were allowed to remain, as the data sets commonly were very large, and the seasonal Kendall method is relatively unaffected by outliers.

The choice of available seasons was limited as sampling frequency at most sites was down to three samples per year near the end of the period of record. Seasons were chosen to maximize the length of record for testing. The seasons and periods of record used for testing each site are listed in table 2. The period of record tested may differ slightly depending on the particular constituent or property and the sampling history. When multiple samples were available for a particular season, the most complete set of sample constituents was first criterion, and the sample closest to the beginning of the target month was the next criterion for choosing among multiple samples.

Table 2. Seasonal periods and period of record tested by seasonal Kendall test.

[Seasons are shown as ranges in months of the year; the hyphens connect months that represent ranges of the seasonal periods]

Site number (fig. 1)Site nameSeasonsPeriod of record
1Lake Granby near Granby, Colo. (near spillway)May–June, July–Aug, Sept, Oct1973–present
2Lake Granby near Granby, Colo. (near dam in Rainbow Bay)May–June, July–Aug, Sept, Oct1989–present
3Granby Pump Canal near Grand Lake, Colo.No seasonal partition1970–present
4Shadow Mountain Lake near Grand Lake, Colo.May, July–Aug, Sept1989–present
5Alva B. Adams Tunnel east portal, near Estes Park, Colo.Bimonthly1970–present
6Lake Estes near dam near Estes Park, Colo.Insufficient record (less than 5 years)1998–present
7Olympus Tunnel at Lake Estes, Colo.Mar–Apr, July, Nov1970–present
8Carter Lake near Berthoud, Colo.Apr–May, July–Aug, Sept–Oct1970–present
9Horsetooth Reservoir near Fort Collins, Colo. (Soldier Canyon Dam)May–June, July–Aug, Sept–Oct1970–present
10Horsetooth Reservoir near Fort Collins, Colo. (Spring Canyon Dam)May–June, July–Aug, Sept–Oct1983–present

Summary results of trend testing are summarized in table 3. Detailed summaries by site that include p-values and any trend slopes that were computed are listed in Hydrologic and Water-Quality Data tables 21 to 35 at back of report. Three alpha-significance levels were used in the summaries: greater than 90 percent was considered significant; greater than 95 percent was moderately significant; and greater than 99 percent was highly significant. The slope is the median of all slopes between data pairs in the same season and is reported in the tables in units of change per year for the median. Positive slopes indicate upward trends (increasing concentrations or values with time); negative slopes indicate downward trends (decreasing concentrations or values) with time.

Table 3. Summary of significant upward and downward temporal trends at Colorado-Big Thompson sampling sites.

[--, insufficient data; (+), upward trend; (–), downward trend; 0, trend not significant; significance: *, greater than 90 percent; **, greater than 95 percent; ***, greater than 99 percent; A, no trend computation because censored values exceeded 50 percent of data; site 6 was not tested because period of record was less than 5 years; S, surface sample; B, near-bottom sample; site names listed in table 1; mg/L, milligrams per liter; µS/cm, microsiemens per centimeter at 25 degrees Celsius; °, degree Celsius; mL, milliliter; µg/L, micrograms per liter]

Property or constituent Significant trends by study site
Site 1(S)Site 1(B)Site 2(S)Site 2(B)Site 3Site 4(S)Site 4(B)Site 5Site 7Site 8(S)Site 8(B)Site 9(S)Site 9(B)Site 10(S)Site 10(B)
Oxygen, dissolved (mg/L)(–)**(–)***0(–)*00(+)***0(+)**0(–)***0(–)***0(–)*
pH, field (standard units)000(–)**(+)***00(+)***(+)*(+)**0(+)***00(–)***0
Specific conductance (µS/cm)(–)***(–)***(–)***(–)***(–)***0(–)***(–)***(–)***(–)***(–)***(–)***(–)***(–)***(–)***
Water temperature (°C)0(–)***0(–)***(+)*----(+)***00(+)**0(+)*(–)*(+)**
Hardness, total (mg/L)(–)***(–)***(–)***(–)***(–)***(–)*(–)***0(–)**(–)***(–)***(–)***(–)***(–)**(–)***
Calcium (mg/L)(–)***(–)***(–)***(–)***(–)***(–)**(–)***0(–)**(–)***(–)***(–)***(–)***(–)**(–)***
Magnesium (mg/L)(–)***(–)***(–)***(–)***(–)***00(–)***(–)***(–)***(–)***(–)*(–)***(–)***
Potassium (mg/L)(–)***(–)***(–)***(–)***(–)***00(–)***0000000
Sodium adsorption ratio000000000000000
Sodium, dissolved (mg/L)(–)***(–)***(–)***(–)***(–)***(–)***(–)***(–)***0(–)***(–)***(–)**(–)**(–)**(–)*
Sodium (percent)(–)***(–)***(–)***(–)***(–)***(–)***(–)***0000000(+)**
Acid-neuralizing capacity (mg/L as CaCO3)(–)***(–)***(–)***(–)***(–)***(–)*(–)***00(–)***(–)***(–)***(–)***(–)*(–)***
Chloride, dissolved (mg/L)0000(–)**0000(+)***(+)***(+)***(+)***(+)***(+)***
Fluoride, dissolved (mg/L)000000000000000
Silica, dissolved (mg/L)00000(+)**0(+)***(+)***(+)***(+)***(+)**(+)*(+)*(+)***
Sulfate, dissolved (mg/L)(–)***(–)***(–)***(–)***(–)***(–)***(–)***(–)***(–)***(–)******--0(–)*(–)***
Nitrogen, ammonia plus organic, total (mg/L as N)0A00(–)***(–)***0(–)***00A0A0A
Nitrogen, ammonia, dissolved (mg/L as N)AAAAAA0A00000A0
Nitrogen, nitrite plus nitrate, dissolved (mg/L as N)AAAA(–)***AA00AAA0A(+)*
Nitrogen, nitrite, dissolved (mg/L as N)AAAAAAAAAAAAAAA
Phosphorus, dissolved (mg/L as P)AAAAAAA0(–)***AAAAAA
Phosphorus, ortho, dissolved (mg/L as P)AAAAAAA0AAAAAAA
Phosphorus, total (mg/L as P)A000(–)***00A(–)***(–)***(–)***(–)***0A0
Residue, dissolved at 180°C (mg/L)(–)**00(–)*(–)*0(–)*00(–)***(–)***(–)***(–)***00
Dissolved solids, sum of constituents (–)***(–)***(–)***(–)***(–)***0(–)**0(–)**(–)***(–)***(–)**(–)***0(–)*
Transparency, Secchi disk (inches)(–)***--(+)***----0------0--(+)***--0--
Coliform, fecal (colonies per 100 mL)A--A----A------A--A--0--
Chlorophyll-a (µg/L)(–)***--(–)***----0------0--0--0--
Chlorophyll-b (µg/L)A--A----A----------A--A--
Barium, dissolved (µg/L)000000(0)**00(–)*(–)*0(+)*00
Iron, dissolved (µg/L)(–)***(–)***(–)***(–)***(–)***0(–)***(–)***(–)***(–)**0000
Manganese, dissolved (µg/L)A(–)***(–)***(–)***(–)***(–)***(–)***--(–)**A00A00
Strontium, dissolved (µg/L)(–)***(–)***(–)***(–)***(–)***(–)**(–)***(–)***(–)**(–)***(–)***(–)***(–)***(–)***(–)***
Zinc, dissolved (µg/L)A0AA0A00(–)**0000AA

Among field measurements, a downward (decreasing) trend in specific conductance was common at all sites except surface samples in Shadow Mountain Lake (site 4S). An upward (increasing) temporal trend for pH was noted at all the conveyances and the surface samples of Carter Lake (site 8S) and Horsetooth Reservoirs (sites 9S only). A negative trend in pH was computed for the near-bottom samples from Lake Granby Rainbow Bay (site 2B). Trends for dissolved-oxygen concentrations decreased in the near-bottom samples at Lake Granby Rainbow Bay (site 2B), Carter Lake (site 8B), and Horsetooth Reservoir (sites 9B and 10B), and increased in the near-bottom samples of Lake Granby spillway (site 1B) and Shadow Mountain Lake (site 4B).

In general, major ions and hardness at all sites showed significant downward trends. The exceptions were silica, which indicated upward trends at Shadow Mountain (site 4), Adams Tunnel (site 5), and all eastern slope sites (sites 6–10), and chloride, which indicated upward trends at Carter Lake (site 8) and Horsetooth Reservoir (sites 9 and 10). Trends for fluoride and sodium adsorption ratio were not significant. When significant, dissolved-solids trends were downward.

Among nutrients, a small number of significant trends were mostly stable (no trend) or downward. Downward trends were computed for total phosphorus at the Granby Pump Canal (site 3), Olympus Tunnel (site 7), Carter Lake (site 8), and the surface samples at Horsetooth Reservoir (site 9S). Downward trends also are noted for ammonia plus organic nitrogen (total) at Granby Pump Canal (site 3), Shadow Mountain Lake (site 4S), and Adams Tunnel east portal (site 5). Nitrite plus nitrate had a significant downward trend at the Granby Pump Canal (site 3) and an upward trend at Horsetooth Reservoir (site 10B) in the near-bottom sample.

Transparency and chlorophyll-a variables, which are directly related to perceptions of eutrophication, showed some improvement with time in Lake Granby (sites 1S and 2S). Transparency showed an upward trend and chlorophyll-a a downward trend at both of the surface sites in Lake Granby. The only other significant trend in transparency was an upward trend at Horsetooth Reservoir (site 9S). No significant trends in fecal coliform bacteria were detected.

Among trace elements, only barium, iron, manganese, strontium, and zinc had sufficient uncensored data for trend testing. Downward trends in iron and manganese were notable at Lake Granby (sites 1 and 2, Fe-Mn), Granby Pump Canal (site 3, Fe-Mn), and Shadow Mountain Lake (site 4S and 4B, Mn only). On the eastern slope, iron showed downward trends at Adams Tunnel (site 5), Olympus Tunnel (site 7), and Carter Lake (site 8S and 8B). The trend in manganese was downward at Olympus Tunnel (site 7). Strontium, a minor ion with similar chemistry to the major ions, showed a downward trend at all sites. Zinc was a downward trend at Olympus Tunnel (site 7), the only significant trend for zinc. The trend in barium was downward at Carter Lake (site 8S and 8B) and upward in the near-bottom samples at Horsetooth Reservoir (site 9B).

The AHOD data computed for Lake Granby, Shadow Mountain Lake, Carter Lake, and Horsetooth Reservoir at the dam locations (related to hypolimnetic oxygen dynamics described earlier) also were tested for temporal trends. First, the annual AHOD data were correlated with year (temporal variable) by using a parametric Pearson correlation. R-squared values ranged from less than 0.1 (Carter Lake Reservoir) to 0.14 (Granby Reservoir), indicating no linear correlation. Green (1996) established a correlation between AHOD and reciprocal hydraulic residence time (RHRT) that explained additional variance in the temporal correlation. Plots of time-series RHRT (May to August each year) at four CBT reservoirs were computed using annual inflow or outflow data and average reservoir contents for each year (fig. 22). With CBT reservoir data, a correlation of log(AHOD) with log(RHRT) yielded r-squared values ranging from 0.11 to 0.65. Encouraged by the potential of the residence time to improve the temporal correlation, a regression model for log(AHOD) that included log(RHRT) and the year was computed and yielded r-squared values ranging from 0.12 to 0.65. Residuals from a logAHOD/logRHRT regression (effects of RHRT removed) were then regressed with year. The largest r-squared obtained was 0.13, indicating little evidence of AHOD changes with time (temporal trend). The AHOD data and RHRT data were then tested for trend using Kendall's tau and testing correlation with time. None of the AHOD, RHRT, or residuals from the log(AHOD) and log(RHRT) relation data had significant temporal trends.


Figure

Figure 22. Time-series plots of reservoir reciprocal hydraulic residence time in terms of the proportion of water entering or leaving the water body.


Summary and Conclusions

The U.S. Geological Survey (USGS), in an ongoing cooperative monitoring program with the Northern Colorado Water Conservancy District (NCWCD), Bureau of Reclamation (BOR), and City of Fort Collins, has collected water-quality data in reservoirs and conveyances such as canals and tunnels related to the Colorado–Big Thompson (CBT) Project in north-central Colorado since 1969.

This report assessed and summarized (1) water-quality and field-measurement profile data collected by USGS and stored in the USGS National Water Information System, (2) time-series trends of chemical constituents and physical properties by using the seasonal Kendall trend test method, (3) oxygen deficits in the hypolimnion (reservoirs only), (4) trophic status indicators, and (5) water-quality data in terms of Colorado water-quality standards.

Water at CBT sites is dilute and typical of high-elevation snowmelt runoff. Median concentrations of specific conductance at sites on the western slope (Lake Granby and Shadow Mountain Lake) and the high-elevation sites on the eastern slope (Adams Tunnel east portal, Lake Estes, and Olympus Tunnel) were generally below 60 microsiemens per centimeter at 25 degrees Celsius. These sites are indicative of a hydrologic setting that is typical of snowmelt source water—low rates of evapotranspiration (ET) and crystalline bedrock geology. Sites with large specific conductances (generally greater than 70 microsiemens per centimeter at 25 degrees Celsius) and concentrations of major ions are Carter Lake and Horsetooth Reservoir. These sites show the cumulative effects of urban development and ET from the rest of the storage and conveyance system. These sites are in lower elevations (higher ET) in areas of sedimentary rocks that can increase dissolved-solids content. Even so, all water sampled in this system should be considered to be of high quality in terms of low dissolved-solids content.

Water temperature varied seasonally, with reservoir sites developing strong stratification patterns. The pH at CBT sites was generally in a neutral range of 7 to 8. Dissolved oxygen was generally in the range of 6 to 10 milligrams per liter, except in the hypolimnetic water of the reservoirs, where seasonal stratification sometimes produced dissolved-oxygen concentrations approaching 2 milligrams per liter or less.

Cation compositions at Adams Tunnel, Olympus Tunnel, and Horsetooth Reservoir are somewhat mixed calcium-magnesium-sodium, but calcium is predominant. Anion compositions are primarily bicarbonate. Despite having larger concentrations of dissolved solids, Horsetooth Reservoir water seems to be simply a more concentrated version of the tunnel waters.

Nutrient concentrations at CBT sites generally were low. Data censoring levels changed many times throughout the study and at times had insufficiently high reporting levels to fully characterize concentration. Largest nutrient concentrations tended to occur at Carter Lake and Horsetooth Reservoirs. The smallest nutrient concentrations were generally measured at Lake Granby and Shadow Mountain Lake. Bottom samples tended to have larger concentrations of most nutrients than did surface samples at the same site.

Concentrations of common dissolved trace elements such as iron and manganese commonly exceeded MRL's, but concentrations were usually less than 1 milligram per liter. Beryllium, chromium, cobalt, lithium, molybdenum, nickel, silver, and vanadium were almost never detected. Zinc, lead, cadmium, boron, and copper concentrations were reported infrequently, and concentrations ranged from a few micrograms per liter to a few tens of micrograms per liter (zinc).

Based on all samples collected over the period of record, standards for dissolved oxygen, pH, chloride, sulfate, un-ionized ammonia, nitrate, dissolved chromium, and dissolved nickel were not exceeded at any of the sampling sites in the study area. Silver, lead, copper, cadmium, and zinc concentrations occasionally exceeded computed standard values. The fixed standard for dissolved iron (300 micrograms per liter) was exceeded in two samples at Lake Granby, one sample at Adams Tunnel east portal, and one sample at Shadow Mountain Lake. The fixed standard for dissolved manganese (50 micrograms per liter) was exceeded at Lake Granby (10 samples), Horsetooth Reservoir (17 samples), and Shadow Mountain Lake (17 samples). Many of the exceedances of manganese were in the early 1990's in mainly near-bottom samples. The fixed standard for manganese was not exceeded at Carter Lake.

Time-series of computed annual areal hypolimnetic oxygen deficit (AHOD) were plotted for Shadow Mountain Lake, Lake Granby, Carter Lake, and Horsetooth Reservoir. Multiyear patterns are evident in the AHOD data, and the cycles are not necessarily the same for each reservoir. This pattern suggests that climatic, biological, or operational cycles may influence the rates of oxygen consumption in these reservoirs.

On the basis of data collected over the period of record at each site, CBT reservoirs were assessed for trophic status using the Trophic-State Index (TSI) method developed by Carlson (1977). Most of the index values for the reservoirs are in the mesotrophic range most years. A tendency toward decreasing trophic status in the 1990's at all reservoirs based mostly on Secchi depth and chlorophyll-a can be seen in time-series plots of index values. The use of TSI can give a qualitative indication of relative status of water bodies but should not be used exclusively to evaluate whether lakes and reservoirs are meeting the criteria of their water-quality classifications.

Time-series data were compiled from the USGS database for the period of record at each site. To help with visual assimilation of these large data sets, scatterplots of each constituent of interest were made and a trace of a LOWESS smooth regression was drawn when sufficient data over the minimum reporting limit were present.

A few general observations and interpretations from the time-series plots were made:

  1. Many time-series data are not exclusively monotonic, a condition that could obscure small-scale trends in the trend testing. Other constituents are well characterized as monotonic distributions.
  2. Specific conductance, dissolved solids, and most of the major ions have similar LOWESS curve shapes, indicating that some of the constituent concentrations may be providing redundant trend information.
  3. Because the data at most sites are collected in a very limited number of seasons, some of the variability and representativeness of the annual water-quality changes at the sites are potentially lost. Trend results should be applied cautiously to seasons not represented in the sampling.
  4. Apparent decreases in nutrient concentrations throughout the system are accompanied by decreases in iron (Fe) and manganese (Mn) concentrations. This pattern could indicate a relation between fertility and bottom-water hypoxia (which can cause reducing conditions that release Fe and Mn).
  5. The degree of variability in the chlorophyll-a data is high, indicating that sampling frequency is not dense enough to adequately describe algal population dynamics that occur on short time scales.

Seasonal Kendall test was used to determine the significance of monotonic temporal trends over the period of record for field measurements, major ions, nutrients, chlorophyll-a and -b, fecal and total coliform bacteria, and trace elements that were routinely reported above censoring levels.

Among field measurements, a downward trend in specific conductance was common to all sites except surface samples in Shadow Mountain Lake. In general, major ions and hardness at all sites showed significant downward trends. The exceptions were (1) silica, which had an upward trend at Shadow Mountain Lake, Adams Tunnel east portal, and all eastern slope sites, and (2) chloride, which indicated upward trends at Carter Lake and Horsetooth Reservoir. Fluoride and sodium adsorption ratio had no significant trends. When significant, dissolved-solids trends were downward.

Among nutrients that met testing criteria, most either had no evidence of trend or a downward trend. Transparency and chlorophyll-a, variables that are directly related to perceptions of eutrophication, showed some improvement with time in Lake Granby. The trend for transparency was upward and chlorophyll-a was downward at both of the surface sites in Lake Granby. The only other significant trend in these variables was an upward trend in transparency at the Horsetooth Reservoir (Soldier Canyon Dam). No significant trends in fecal coliform bacteria were detected. Among trace elements, only barium, iron, manganese, strontium, and zinc had sufficient uncensored data for trend testing. Downward trends in iron and manganese were notable at Lake Granby (Fe-Mn), Granby Pump Canal (Fe-Mn), and Shadow Mountain Lake (Mn only). On the eastern slope, the trend in iron was upward at Adams Tunnel, Olympus Tunnel, and Carter Lake. Manganese trend was downward at Adams and Olympus Tunnels. The zinc trend was downward at Olympus Tunnel, the only significant trend for zinc.

The AHOD data (computed for Lake Granby, Shadow Mountain Lake, Carter Lake, and Horsetooth Reservoir, at the dam locations) related to hypolimnetic oxygen dynamics also were tested for temporal trend. A regression model that included log(AHOD), log(RHRT), and the year, was computed. The model yielded r-squared values that ranged from 0.12 to 0.65. Residuals from a logAHOD/logRHRT regres-sion (effects of RHRT removed) were then regressed with year. The largest r-squared obtained was 0.13, indicating little evidence of AHOD changes with time (temporal trend). The AHOD data and RHRT data were then tested for trend by using Kendall's tau and testing correlation with time. None of the AHOD, RHRT, or residuals from the log(AHOD) and log(RHRT) relation data had significant temporal trends.

Major conclusions of the study were as follows:

  1. Water quality was generally good throughout the CBT system over the period of record, which ranged from 1969 to 2000. Dissolved solids and nutrient concentrations were low and typical of a forested/mountainous/crystalline bedrock hydrologic setting. Most of the more toxic trace elements were rarely detected or were found in low concentrations, which was due at least in part to a relative lack of ore-mineral deposits within the drainage areas of the CBT project.
  2. Constituent concentrations consistently met water-quality standard thresholds set by the State of Colorado.
  3. Trophic-state index values indicated mesotrophic conditions generally prevail at reservoirs based on available Secchi depth, total phosphorus concentrations, and chlorophyll-a concentrations.
  4. Based on plots of time-series values and concentrations and seasonal Kendall nonparametric trend testing, concentrations of dissolved solids and most major ions were decreasing at most sites. Many of the nutrient time-series data did not meet the minimum criteria for testing. But for those that did, nutrient concentrations had insufficient evidence of trends or downward temporal trends (ammonia plus organic nitrogen and total phosphorus). Iron and manganese data either had insufficient evidence of trend or indicated downward trends. Chlorophyll-a data were collected for only 11 years but generally indicated insufficient evidence of trend or downward temporal trends.

References Cited

Britton, L.J., and Gaggiani, N.G., 1987, Water-quality assessment of Arvada Reservoir, Denver Metropolitan area, Colorado: U.S. Geological Survey Water-Resources Investigations Report 87–4107, 66 p.

Carlson, R.E., 1977, A trophic state index for lakes: Limnology and Oceanography, v. 22, no. 2, p. 361–369.

Clark, M.L., Eddy-Miller, C.A., and Mast, M.A., 2000, Environmental characteristics and water quality of Hydrologic Benchmark Network stations in West-Central United States, 1963–95: U.S. Geological Survey Circular 1173–C, 115 p.

Colorado Climate Center, 1984, Colorado average annual precipitation map 1951–1980: Fort Collins, Colorado Climate Center map, scale 1:500,000.

Colorado Department of Public Health and Environment, Water Quality Control Commission, 2001, Classifications and numeric standards for South Platte River Basin; Laramie River Basin; Republican River Basin; Smoky Hill River Basin: Denver, Colorado Department of Public Health and Environment, Water Quality Control Commission Report, 3.8.0 (5 CCR 1002–8), variously paginated.

Green, W.R., 1996, Eutrophication trends inferred from hypolimnetic dissolved-oxygen dynamics within selected White River reservoirs, northern Arkansas-southern Missouri, 1974–94: U.S. Geological Survey Water-Resources Investigations Report 96–4096, 22 p.

Greve, A.I., 2002, Data-quality measures of stakeholder-implemented watershed-monitoring programs: U.S. Geological Survey Open-File Report 02–141, 19 p.

Helsel, D.R., and Cohn, T.A., 1988, Estimation of descriptive statistics for multiply censored water-quality data: Water Resources Research, v. 24, p. 1997–2004.

Helsel, D.R., and Hirsch, R.M., 1995, Statistical methods in water resources, studies in environmental science 49, 3d ed.: New York, Elsevier, 529 p.

Jassby, A.D., and Goldman, C.R., 1999, Horsetooth and Carter Lake Reservoirs, water quality comparisons: Davis, Calif., Report of Ecological Research Associates, 49 p.

Johnson, B.M., and Goettl, J.P., 1999, Food web changes over fourteen years following introduction of rainbow smelt into a Colorado reservoir: North American Journal of Fisheries Management, v. 19, p. 629–642.

Middelburg, R.F., 1993, Colorado stream water quality, in Paulson, R.W., and others, National water summary, Hydrologic events and stream water quality: U.S. Geological Survey Water-Supply Paper 2400, p. 204.

Mueller, D.K., 1990, Analysis of water-quality data and sampling programs at selected sites in north-central Colorado: U.S. Geological Survey Water-Resources Investigations Report 90–4005, 79 p.

Niyogi, D.K., McKnight, D.M., and Lewis, W.M., 1999, Influences of water and substrate quality for periphyton in a montane stream affected by acid mine drainage: Limnology and Oceanography v. 44, no. 3, p. 804–814.

Stevens, M.R., 2001, Assessment of water quality, road runoff, and bulk atmospheric deposition, Guanella Pass area, Clear Creek and Park counties, Colorado, water years 1995–97: U.S. Geological Survey Water-Resources Investigations Report 00–4186, 183 p.

Walker, W.W., 1996, Simplified procedures for eutrophication assessment and prediction, user manual: U.S. Army Corps of Engineers, Waterways Experiment Station, Instruction Report W–96–2, 227 p.

Woods, P.F., 1992, Limnology of Big Lake, south-central Alaska, 1983–84: U.S. Geological Survey Water-Supply Paper 2382, 108 p.


Hydrologic and Water-Quality Data

Abbreviations

CaCO3Calcium carbonate

CaCalcium

MgMagnesium

KPotassium

NaSodium

ClChloride

FFluoride

SiO2Silicon dioxide

SO4Sulfate

NNitrogen

PPhosphorus

AsArsenic

BaBarium

BeBeryllium

BBoron

CdCadmium

CrChromium

CoCobalt

CuCopper

FeIron

PbLead

LiLithium

MnManganese

MoMolybdenum

NiNickel

AgSilver

SrStrontium

VVanadium

ZnZinc


Figure

Figure

Figure

Figure

Figure

Figure

Figure 23. Temporal variations in periodic measurements and concentrations at Lake Granby near spillway, sampled near the surface.



Figure

Figure

Figure

Figure

Figure

Figure 24. Temporal variations in periodic measurements and concentrations at Lake Granby near spillway, sampled near the bottom.



Figure

Figure

Figure

Figure

Figure

Figure

Figure 25. Temporal variations in periodic measurements and concentrations at Lake Granby near Rainbow Bay, sampled near the surface.



Figure

Figure

Figure

Figure

Figure

Figure 26. Temporal variations in periodic measurements and concentrations at Lake Granby near Rainbow Bay, sampled near the bottom.



Figure

Figure

Figure

Figure

Figure

Figure 27. Temporal variations in periodic measurements and concentrations at Granby Pump Canal.



Figure

Figure

Figure

Figure

Figure

Figure

Figure 28. Temporal variations in periodic measurements and concentrations at Shadow Mountain Lake, sampled at the surface.



Figure

Figure

Figure

Figure

Figure

Figure 29. Temporal variations in periodic measurements and concentrations at Shadow Mountain Lake, sampled near the bottom.



Figure

Figure

Figure

Figure

Figure

Figure 30. Temporal variations in periodic measurements and concentrations at Adams Tunnel, east portal, near Estes Park.



Figure

Figure

Figure

Figure

Figure

Figure 31. Temporal variations in periodic measurements and concentrations at Olympus Tunnel.



Figure

Figure

Figure

Figure

Figure

Figure

Figure 32. Temporal variations in periodic measurements and concentrations at Carter Lake, sampled near the surface.



Figure

Figure

Figure

Figure

Figure

Figure 33. Temporal variations in periodic measurements and concentrations at Carter Lake, sampled near the bottom.



Figure

Figure

Figure

Figure

Figure

Figure

Figure 34. Temporal variations in periodic measurements and concentrations at Horsetooth Reservoir, near Soldier Canyon Dam, sampled near the surface.



Figure

Figure

Figure

Figure

Figure

Figure 35. Temporal variations in periodic measurements and concentrations at Horsetooth Reservoir, near Soldier Canyon Dam, sampled near the bottom.



Figure

Figure

Figure

Figure

Figure

Figure

Figure 36. Temporal variations in periodic measurements and concentrations at Horsetooth Reservoir, near Spring Canyon Dam, sampled near the surface.



Figure

Figure

Figure

Figure

Figure

Figure 37. Temporal variations in periodic measurements and concentrations at Horsetooth Reservoir, near Spring Canyon Dam, sampled near the bottom.


Table 4. Summary statistics for site 1, Lake Granby, near spillway, near surface.

Figure

Table 5. Summary statistics for site 1, Lake Granby, near spillway, near bottom.

Figure

Table 6. Summary statistics for site 2, Lake Granby (Rainbow Bay), sampled near surface.

Figure

Table 7. Summary statistics for site 2, Lake Granby (Rainbow Bay), sampled near bottom.

Figure

Table 8. Summary statistics for site 3, Granby Pump Canal.

Figure

Table 9. Summary statistics for site 4, Shadow Mountain Lake, sampled at the surface.

Figure

Table 10. Summary statistics for site 4, Shadow Mountain Lake, sampled near bottom.

Figure

Table 11. Summary statistics for site 5, Alva B. Adams Tunnel east portal.

Figure

Table 12. Summary statistics for site 6, Lake Estes, sampled at surface.

Figure

Table 13. Summary statistics for site 6, Lake Estes, sampled near bottom.

Figure

Table 14. Summary statistics for site 7, Olympus Tunnel.

Figure

Table 15. Summary statistics for site 8, Carter Lake, sampled near surface.

Figure

Table 16. Summary statistics for site 8, Carter Lake, sampled near bottom.

Figure

Table 17. Summary statistics for site 9, Horsetooth Reservoir (Soldier Canyon Dam), sampled near surface.

Figure

Table 18. Summary statistics for site 9, Horsetooth Reservoir (Soldier Canyon Dam), sampled near bottom.

Figure

Table 19. Summary statistics for site 10, Horsetooth Reservoir (Spring Canyon Dam), sampled at surface.

Figure

Table 20. Summary statistics for site 10, Horsetooth Reservoir (Spring Canyon Dam), sampled near bottom.

Figure

Table 21. Trend analysis results for site 1, Granby Reservoir, near dam, surface samples.

[0, no trend; (+), upward trend; (–), downward trend; significance: *, greater than 90 percent; **, greater than 95 percent; ***, greater than 99 percent; change is percent change in median since the first year of record; A, no trend computation because nondetects greater than 50 percent; B, no slope computation because nondetects greater than 20 percent; mg/L, milligrams per liter; µS/cm, microsiemens per centimeter at 25 degrees Celsius; °C, degree Celsius; in., inch; mL, milliliter; hplc, high-performance liquid chromatography; µg/L, micrograms per liter]

Seasonal Kendall season-edited dataNumberCensoredPercentage censoredPeriod record (years)p-valueSlopeMedianSignificanceChange
Oxygen, dissolved (mg/L)4600110.0383–0.050017.5(–)**–7.1
pH, field (standard units)4600110.1719–0.025197.90–3.4
Specific conductance (µS/cm at 25°C)4600110.0000–1.76854(–)***–30.1
Water temperature (°C)4600110.7760–0.0201415.20–1.4
Hardness, total (mg/L as CaCO3)4500110.0000–0.554322(–)***–24.1
Calcium, dissolved (mg/L)4500110.0000–0.16626.85(–)***–23.3
Magnesium, dissolved (mg/L)4500110.0000–0.031411.2(–)***–24.9
Potassium, dissolved (mg/L)4500110.0000–0.01250.7(–)***–17.7
Sodium adsorption (ratio)4500111.000000.200.0
Sodium, dissolved (mg/L)4500110.0000–0.087762.2(–)***–35.4
Sodium (percent)4500110.0000–0.200416(–)***–12.8
Acid-neutralizing capacity (mg/L)4500110.0000–0.499724(–)***–20.4
Chloride, dissolved (mg/L)4500110.055400.400.0
Fluoride, dissolved (mg/L)45920110.037400.10B
Silica, dissolved (mg/L)4500111.72470.042814.85010.3
Sulfate, dissolved (mg/L)4500110.0000–0.11892.95(–)***–35.7
Nitrogen, ammonia plus organic (mg/L as N)451329110.002400.30B
Nitrogen, ammonia (mg/L as N)45327111AAAAA
Nitrite plus nitrate (mg/L as N)454510011AAAAA
Nitrogen, nitrite (mg/L as N)45439611AAAAA
Phosphorus, dissolved (mg/L as P)39348711AAAAA
Phophorus, ortho, dissolved (mg/L as P)45419111AAAAA
Phosphorus, total (mg/L as P)41256111AAAAA
Residue, dissolved at 180°C (mg/L)4400110.0296–0.670737(–)**–18.0
Dissolved solids, sum of constituents (mg/L)4500110.00000.664335(+)***23.6
Transparency, Secchi disk (in.)4400110.00028.037122(+)***119.8
Coliform, fecal (colonies per 100 mL)42307111AAAAA
Chlorophyll-a, hplc (µg/L)4600110.0056–0.22682.05(–)***–73.1
Chlorophyll-b, hplc (µg/L)45337311AAAAA
Barium, dissolved (µg/L)4500110.22630800.0
Iron, dissolved (µg/L)452147110.0000B26(–)***B
Manganese, dissolved (µg/L)39246211AAAAA
Strontium, dissolved (µg/L)4500110.0000–1.21942(–)***–27.2
Zinc, dissolved (µg/L)34205911AAAAA

Table 22. Trend analysis results for site 1, Granby Reservoir, near dam, bottom samples.

[0, no trend; (+), upward trend; (–), downward trend; significance: *, greater than 90 percent; **, greater than 95 percent; ***, greater than 99 percent; change is percent change in median since the first year of record; A, no trend computation because nondetects greater than 50 percent; B, no slope computation because nondetects greater than 20 percent; mg/L, milligrams per liter; µS/cm, microsiemens per centimeter at 25 degrees Celsius; °C, degree Celsius; µg/L, micrograms per liter]

Seasonal Kendall season-edited dataNumberCensoredPercentage censoredPeriod record (years)p-valueSlopeMedianSignificanceChange
Oxygen, dissolved (mg/L)4400110.00060.21824.8(+)***68.8
pH, field (standard units)4500110.822407.100.0
Specific conductance (µS/cm at 25°C)4500110.0000–2.1555.5(–)***–34.6
Water temperature (°C)4500110.0016–0.12456.5(–)***–18.9
Hardness, total (mg/L as CaCO3)4500110.0000–0.551324(–)***–22.2
Calcium, dissolved (mg/L)4500110.0000–0.17897.3(–)***–23.5
Magnesium, dissolved (mg/L)4500110.0000–0.02961.3(–)***–22.0
Potassium, dissolved (mg/L)4500110.0001–0.016580.7(–)***–22.8
Sodium adsorption (ratio)4500110.478900.200.0
Sodium, dissolved (mg/L)4500110.0000–0.097872.3(–)***–37.3
Sodium (percent)4500110.0000–0.200817(–)***–12.1
Acid-neutralizing capacity (mg/L)4500110.0000–0.663626(–)***–24.3
Chloride, dissolved (mg/L)4500110.517000.400.0
Fluoride, dissolved (mg/L)451329110.0730B0.10B
Silica, dissolved (mg/L)4500110.1085–0.080685.90–13.9
Sulfate, dissolved (mg/L)4500110.0000–0.088593(–)***–27.6
Nitrogen, ammonia plus organic (mg/L as N)46245211AAAAA
Nitrogen, ammonia (mg/L as N)45296411AAAAA
Nitrite plus nitrate (mg/L as N)45255611AAAAA
Nitrogen, nitrite (mg/L as N)45429311AAAAA
Phosphorus, dissolved (mg/L as P)39287211AAAAA
Phophorus, ortho, dissolved (mg/L as P)45378211AAAAA
Phosphorus, total (mg/L as P)421536110.4151B0.020B
Residue, dissolved at 180°C (mg/L)4400110.3325–0.377400–9.8
Dissolved solids, sum of constituents (mg/L)4500110.0000–0.984438(–)***–24.7
Barium, dissolved (µg/L)4500110.10230800.0
Iron, dissolved (µg/L)45818110.0002B19.5(–)***B
Manganese, dissolved (µg/L)391231110.0001B10(–)***B
Strontium, dissolved (µg/L)4500110.0000–1.21846.5(–)***–24.9
Zinc, dissolved (µg/L)34175080.3221B80B

Table 23. Trend analysis results for site 2, Granby Reservoir (Rainbow Bay), surface samples.

[0, no trend; (+), upward trend; (–), downward trend; significance: *, greater than 90 percent; **, greater than 95 percent; ***, greater than 99 percent; change is percent change in median since the first year of record; A, no trend computation because nondetects greater than 50 percent; B, no slope computation because nondetects greater than 20 percent; mg/L, milligrams per liter; µS/cm, microsiemens per centimeter at 25 degrees Celsius; °C, degree Celsius; in., inch; mL, milliliter; hplc, high-performance liquid chromatography; µg/L, micrograms per liter]

Seasonal Kendall season-edited dataNumberCensoredPercentage censoredPeriod record (years)p-valueSlopeMedianSignificanceChange
Oxygen, dissolved (mg/L)4500110.2757–0.025097.40–3.7
pH, field (standard units)4600110.4714–0.0099727.80–1.4
Specific conductance (µS/cm at 25°C)4600110.0000–1.83854.75(–)***–30.7
Water temperature (°C)4600110.74860.0363313.4503.0
Hardness, total (mg/L as CaCO3)4500110.0000–0.637723(–)***–26.1
Calcium, dissolved (mg/L)4500110.0000–0.19057.1(–)***–25.4
Magnesium, dissolved (mg/L)4500110.0000–0.027911.3(–)***–20.9
Potassium, dissolved (mg/L)4500110.0000–0.014190.7(–)***–19.9
Sodium adsorption (ratio)4500110.058600.200.0
Sodium, dissolved (mg/L)4500110.0000–0.097532.3(–)***–37.2
Sodium (percent)4500110.0000–0.201316(–)***–12.9
Acid-neutralizing capacity (mg/L)4500110.0000–0.626226(–)***–23.1
Chloride, dissolved (mg/L)4400110.114800.400.0
Fluoride, dissolved (mg/L)44920110.4901B0.10B
Silica, dissolved (mg/L)4500110.79900.01451503.2
Sulfate, dissolved (mg/L)4400110.0000–0.13712.9(–)***–40.5
Nitrogen, ammonia plus organic (mg/L as N)461635110.004400.20B
Nitrogen, ammonia (mg/L as N)45306711AAAAA
Nitrite plus nitrate (mg/L as N)3932829AAAAA
Nitrogen, nitrite (mg/L as N)45429311AAAAA
Phosphorus, dissolved (mg/L as P)3528809AAAAA
Phophorus, ortho, dissolved (mg/L as P)45378211AAAAA
Phosphorus, total (mg/L as P)411844110.1100B0.020B
Residue, dissolved at 180°C (mg/L)4400110.1052–0.627138.50–16.3
Dissolved solids, sum of constituents (mg/L)4400110.0000–0.852636(–)***–22.8
Transparency, Secchi disk (in.)4300110.00097.043113(+)***109.5
Coliform, fecal (colonies per 100 mL)42256011AAAAA
Chlorophyll-a, hplc (µg/L)4500110.0059–0.24871.8(–)***–83.1
Chlorophyll-b, hplc (µg/L)44347711AAAAA
Barium, dissolved (µg/L)4500110.00360800.0
Iron, dissolved (µg/L)352690.0000B20(–)***B
Manganese, dissolved (µg/L)34154480.0010B4(–)***B
Strontium, dissolved (µg/L)4500110.0000–1.1944(–)***–25.6
Zinc, dissolved (µg/L)3418538AAAAA

Table 24. Trend analysis results for site 2, Granby Reservoir (Rainbow Bay), bottom samples.

[0, no trend; (+), upward trend; (–), downward trend; significance: *, greater than 90 percent; **, greater than 95 percent; ***, greater than 99 percent; change is percent change in median since the first year of record; A, no trend computation because nondetects greater than 50 percent; B, no slope computation because nondetects greater than 20 percent; mg/L, milligrams per liter; µS/cm, microsiemens per centimeter at 25 degrees Celsius; °C, degree Celsius; µg/L, micrograms per liter]

Seasonal Kendall season-edited dataNumberCensoredPercentage censoredPeriod record (years)p-valueSlopeMedianSignificanceChange
Oxygen, dissolved (mg/L)4500110.0878–0.12855.1(–)*–24.1
pH, field (standard units)4600110.0433–0.033467.4(–)**–4.8
Specific conductance (µS/cm at 25°C)4600110.0000–2.0254.5(–)***–33.4
Water temperature (°C)4600110.0000–0.61368.35(–)***–56.1
Hardness, total (mg/L as CaCO3)4500110.0000–0.570323(–)***–23.7
Calcium, dissolved (mg/L)4500110.0000–0.19567(–)***–26.3
Magnesium, dissolved (mg/L)4500110.0000–0.032581.3(–)***–24.0
Potassium, dissolved (mg/L)4500110.0009–0.011090.7(–)***–15.9
Sodium adsorption (ratio)4500110.116400.200.0
Sodium, dissolved (mg/L)4500110.0000–0.084572.3(–)***–33.1
Sodium (percent)4500110.0001–0.167716(–)***–10.8
Acid-neutralizing capacity (mg/L)4500110.0000–0.532525(–)***–20.8
Chloride, dissolved (mg/L)4400110.162100.400.0
Fluoride, dissolved (mg/L)441227110.088400.1000B
Silica, dissolved (mg/L)4500110.7429–0.016085.60–3.1
Sulfate, dissolved (mg/L)4400110.0000–0.099722.900(–)***–31.4
Nitrogen, ammonia plus organic (mg/L as N)451840110.006700.20B
Nitrogen, ammonia (mg/L as N)45265811AAAAA
Nitrite plus nitrate (mg/L as N)40287011AAAAA
Nitrogen, nitrite (mg/L as N)45439611AAAAA
Phosphorus, dissolved (mg/L as P)3527779AAAAA
Phophorus, ortho, dissolved (mg/L as P)45388411AAAAA
Phosphorus, total (mg/L as P)401333110.6043B0.020B
Residue, dissolved at 80°C (mg/L)4400110.0897–0.747939(–)*–18.9
Dissolved solids, sum of constituents (mg/L)4400110.0000–0.753736(–)***–20.5
Barium, dissolved (µg/L)4500110.00070800.0
Iron, dissolved (µg/L)4537110.0002B15(–)***B
Manganese, dissolved (µg/L)451227110.0076B7(–)***B
Strontium, dissolved (µg/L)4512110.0000–1.15144(–)***–24.9
Zinc, dissolved (µg/L)46265711AAAAA

Table 25. Trend analysis results for site 3, Granby Pump Canal, samples.

[0, no trend; (+), upward trend; (–), downward trend; significance: *, greater than 90 percent; **, greater than 95 percent; ***, greater than 99 percent; change is percent change in median since the first year of record; A, no trend computation because nondetects greater than 50 percent; B, no slope computation because nondetects greater than 20 percent; mg/L, milligrams per liter; µS/cm, microsiemens per centimeter at 25 degrees Celsius; °C, degree Celsius; µg/L, micrograms per liter]

Seasonal Kendall season-edited dataNumberCensoredPercentage censoredPeriod record (years)p-valueSlopeMedianSignificanceChange
Oxygen, dissolved (mg/L)17000300.36100.01337.5505.4
pH, field (standard units)17300300.00040.017367.3(+)***7.4
Specific conductance (µS/cm at 25°C)17900300.0001–0.248260(–)***–11.7
Water temperature (°C)18000300.05770.019984.1(+)*15.8
Hardness, total (mg/L as CaCO3)5600110.0000–0.528224(–)***–21.4
Calcium, dissolved (mg/L)5600110.0000–0.1767.4(–)***–22.9
Magnesium, dissolved (mg/L)5600110.0000–0.024491.3(–)***–18.6
Potassium, dissolved (mg/L)5600110.0000–0.016490.7(–)***–22.7
Sodium adsorption (ratio)5600111.000000.200.0
Sodium, dissolved (mg/L)5600110.0000–0.086412.3(–)***–33.7
Sodium (percent)5600110.0000–0.182217(–)***–11.1
Acid-neutralizing capacity (mg/L)5600110.0000–0.618726(–)***–22.9
Chloride, dissolved (mg/L)5612110.0160–0.018370.4(–)**–39.6
Fluoride, dissolved (mg/L)561018110.1912B0.10B
Silica, dissolved (mg/L)5600110.14790.066615.4014.7
Sulfate, dissolved (mg/L)5600110.0000–0.098683(–)***–30.2
Nitrogen, ammonia plus organic (mg/L as N)1392719220.0000–0.016520.38(–)***–63.8
Nitrogen, ammonia (mg/L as N)53377011AAAAA
Nitrite plus nitrate (mg/L as N)1285946220.0010B0.1(–)***B
Nitrogen, nitrite (mg/L as N)55498911AAAAA
Phosphorus, dissolved (mg/L as P)54407411AAAAA
Phophorus, ortho, dissolved (mg/L as P)54407411AAAAA
Phosphorus, total (mg/L as P)1343123220.0000B0.02(–)***B
Residue, dissolved at 180°C (mg/L)5500110.0502–0.475840(–)*–12.2
Dissolved solids, sum of constituents (mg/L)5500110.0000–0.74538(–)***–19.3
Barium, dissolved (µg/L)5612110.47830700.0
Iron, dissolved (µg/L)4836110.0032B23(–)***B
Manganese, dissolved (µg/L)46920110.0000B4(–)***B
Strontium, dissolved (µg/L)5600110.0000–1.13745(–)***–24.1
Zinc, dissolved (µg/L)471940110.7031B5.50B

Table 26. Trend analysis results for site 4, Shadow Mountain Reservoir, surface samples.

[0, no trend; (+), upward trend; (–), downward trend; significance: *, greater than 90 percent; **, greater than 95 percent; ***, greater than 99 percent; change is percent change in median since the first year of record; A, no trend computation because nondetects greater than 50 percent; B, no slope computation because nondetects greater than 20 percent; mg/L, milligrams per liter; µS/cm, microsiemens per centimeter at 25 degrees Celsius; °C, degree Celsius; in., inch; mL, milliliter; hplc, high-performance liquid chromatography; µg/L, micrograms per liter]

Seasonal Kendall season-edited dataNumberNondetects% nondetectPeriod record (years)p-valueSlopeMedianSignificanceChange
Oxygen, dissolved (mg/L)3500110.86878655340.014297.402.2
pH, field (standard units)3500110.6171454802–0.0099137.60–1.4
Specific conductance (µS/cm at 25°C)3600110.119470507–0.527449.60–11.0
Water temperature (°C)------------------
Hardness, total (mg/L as CaCO3)3600110.0898858202–0.200322(–)*–9.5
Calcium, dissolved (mg/L)3600110.0469222566–0.097986.5(–)**–15.2
Magnesium, dissolved (mg/L)3600110.644583414801.300.0
Potassium, dissolved (mg/L)3600110.145392698300.700.0
Sodium adsorption (ratio)3600110.853307492100.200.0
Sodium, dissolved (mg/L)3600110.0005205513–0.058381.9(–)***–28.5
Sodium (percent)3600110.0000174818–0.250116(–)***–15.7
Acid-neutralizing capacity (mg/L)3600110.0690640193–0.2523.5(–)*–11.0
Chloride, dissolved (mg/L)3526110.9999999996B0.40B
Fluoride, dissolved (mg/L)3513110.64322104400.100.0
Silica, dissolved (mg/L)3600110.03155510140.09915.55(+)**22.0
Sulfate, dissolved (mg/L)3500110.0018395843–0.057312.8(–)***–20.1
Nitrogen, ammonia plus organic (mg/L as N)36514110.0016781137B0.3(–)***B
Nitrogen, ammonia (mg/L as N)36236411AAAAA
Nitrite plus nitrate (mg/L as N)30217011AAAAA
Nitrogen, nitrite (mg/L as N)36339211AAAAA
Phosphorus, dissolved (mg/L as P)27238511AAAAA
Phophorus, ortho, dissolved (mg/L as P)36359711AAAAA
Phosphorus, total (mg/L as P)291138110.04626130700.020B
Residue, dissolved at 180°C (mg/L)3500110.967008774903800.0
Dissolved solids, sum of constituents (mg/L)3200110.1267610508–0.25340–7.7
Transparency, Secchi disk (in.)3613110.38266299091.41583020.9
Coliform, fecal (colonies per 100 mL)33206111AAAAA
Chlorophyll-a, hplc (µg/L)3413110.1967038082–0.11292.250–42.4
Chlorophyll-b, hplc (µg/L)35277711AAAAA
Barium, dissolved (µg/L)3613110.8990986180700.0
Iron, dissolved (µg/L)3613110.2332613662–2.406560–37.6
Manganese, dissolved (µg/L)36617110.0091607158B4(–)***B
Strontium, dissolved (µg/L)3613110.038358664–0.635538.685(–)**–16.5
Zinc, dissolved (µg/L)271141110.0453922501060B

Table 27. Trend analysis results for site 4, Shadow Mountain Reservoir, bottom samples.

[0, no trend; (+), upward trend; (–), downward trend; significance: *, greater than 90 percent; **, greater than 95 percent; ***, greater than 99 percent; change is percent change in median since the first year of record; A, no trend computation because nondetects greater than 50 percent; B, no slope computation because nondetects greater than 20 percent; mg/L, milligrams per liter; µS/cm, microsiemens per centimeter at 25 degrees Celsius; °C, degree Celsius; µg/L, micrograms per liter]

Seasonal Kendall season-edited dataNumberCensoredPercentage censoredPeriod record (years)p-valueSlopeMedianSignificanceChange
Oxygen, dissolved (mg/L)3500110.00441346410.26935.6(+)***74.4
pH, field (standard units)3500110.2542864030.012327.301.9
Specific conductance (µS/cm at 25°C)3600110.0003232226–0.977552(–)***–18.6
Water temperature (°C)----------------
Hardness, total (mg/L as CaCO3)3600110.0023115152–0.327822(–)***–15.0
Calcium, dissolved (mg/L)3600110.0009600972–0.126.8(–)***–17.6
Magnesium, dissolved (mg/L)3600110.291425377201.300.0
Potassium, dissolved (mg/L)3600110.181549397200.700.0
Sodium adsorption (ratio)3600110.862965852700.200.0
Sodium, dissolved (mg/L)3600110.0000623096–0.060041.9(–)***–29.2
Sodium (percent)3600110.0003709347–0.200316(–)***–12.8
Acid-neutralizing capacity (mg/L)3600110.004847576–0.298824(–)***–12.7
Chloride, dissolved (mg/L)3513110.1113847049–0.009990.40–23.9
Fluoride, dissolved (mg/L)3526110.9520599211B0.10B
Silica, dissolved (mg/L)3600110.6332197237–0.018216.20–3.2
Sulfate, dissolved (mg/L)3500110.000936414–0.071282.9(–)***–23.6
Nitrogen, ammonia plus organic (mg/L as N)36617110.030082576300.250B
Nitrogen, ammonia (mg/L as N)361747110.1713121915B0.020B
Nitrite plus nitrate (mg/L as N)30206711AAAAA
Nitrogen, nitrite (mg/L as N)36349411AAAAA
Phosphorus, dissolved (mg/L as P)27228111AAAAA
Phophorus, ortho, dissolved (mg/L as P)36339211AAAAA
Phosphorus, total (mg/L as P)36925110.1247838628B0.020B
Residue, dissolved at 180°C (mg/L)3500110.0736147125–0.499739(–)*–13.1
Dissolved solids, sum of constituents (mg/L)3200110.028596739–0.493834(–)**–14.7
Barium, dissolved (µg/L)3613110.04659079530700.0
Iron, dissolved (µg/L)3613110.1891830763–1.917770–23.8
Manganese, dissolved (µg/L)3638110.0000252162B15(–)***B
Strontium, dissolved (µg/L)3613110.0002823951–0.99339(–)***–24.3
Zinc, dissolved (µg/L)27155611AAAAA

Table 28. Trend analysis results for site 5, Adams Tunnel east portal.

[0, no trend; (+), upward trend; (–), downward trend; significance: *, greater than 90 percent; **, greater than 95 percent; ***, greater than 99 percent; change is percent change in median since the first year of record; A, no trend computation because nondetects greater than 50 percent; B, no slope computation because nondetects greater than 5 percent; ft3/s, cubic feet per second; mg/L, milligrams per liter; µS/cm, microsiemens per centimeter at 25 degrees Celsius; °C, degree Celsius; µg/L, micrograms per liter]

Seasonal Kendall season-edited dataNumberCensoredPercentage censoredPeriod record (years)p-valueSlopeMedianSignificanceChange
Instantaneous discharge ( ft3/s)17500300.2191–0.9974550–6.4
Oxygen, dissolved (mg/L)17600300.384108.300.0
pH, field (standard units)17600300.00000.027267.5(+)***11.6
Specific conductance (µS/cm at 25°C)16500300.0025–0.26750(–)***–14.8
Water temperature (°C)17900300.00220.022735(+)***14.7
Hardness, total (mg/L as CaCO3)14100240.1679–0.05411190–6.6
Calcium, dissolved (mg/L)14100240.2766–0.016595.90–6.5
Magnesium, dissolved (mg/L)14100240.0078–0.0053931.1(–)***–11.1
Potassium, dissolved (mg/L)13900240.0015–0.0055480.7(–)***–17.3
Sodium adsorption (ratio)14100240.130000.200.0
Sodium, dissolved (mg/L)14100240.0006–0.015451.9(–)***–17.7
Sodium (percent)13900240.006601700.0
Acid-neutralizing capacity (mg/L)14100240.23480.049842105.9
Chloride, dissolved (mg/L)13911240.017300.400.0
Fluoride, dissolved (mg/L)1401410240.1741B0.10B
Silica, dissolved (mg/L)13900240.00000.050044.4(+)***31.8
Sulfate, dissolved (mg/L)13800240.0000–0.14853.3(–)***–69.1
Nitrogen, ammonia plus organic (mg/L as N)1333224220.0000B0.3(–)***B
Nitrogen, ammonia (mg/L as N)67426311AAAAA
Nitrite plus nitrate (mg/L as N)55244490.003000.10B
Nitrogen, nitrite (mg/L as N)67609011AAAAA
Phosphorus, dissolved (mg/L as P)1165850200.002000.020B
Phophorus, ortho, dissolved (mg/L as P)67517611AAAAA
Phosphorus, total (mg/L as P)1203529200.005700.020B
Residue, dissolved at 180°C (mg/L)6600110.776003400.0
Dissolved solids, sum of constituents (mg/L)13100240.3101–0.04997310–3.8
Barium, dissolved (µg/L)6700110.91590600.0
Iron, dissolved (µg/L)14100240.0000–1.79130(–)***–82.1
Strontium, dissolved (µg/L)6700110.0000–1.39235(–)***–35.3
Zinc, dissolved (µg/L)722636190.7935B80B

Table 29. Trend analysis results for site 7, Olympus Tunnel, samples.

[0, no trend; (+), upward trend; (–), downward trend; significance: *, greater than 90 percent; **, greater than 95 percent; ***, greater than 99 percent; change is percent change in median since the first year of record; A, no trend computation because nondetects greater than 50 percent; B, no slope computation because nondetects greater than 20 percent; mg/L, milligrams per liter; µS/cm, microsiemens per centimeter at 25 degrees Celsius; °C, degree Celsius; µg/L, micrograms per liter]

Seasonal Kendall season-edited dataNumberCensoredPercentage censoredPeriod record (years)p-valueSlopeMedianSignificanceChange
Oxygen, dissolved (mg/L)8800300.04020.020069.1(+)**6.8
pH, field (standard units)8800300.05300.010517.6(+)*4.2
Specific conductance (µS/cm at 25°C)8100300.0008–0.375448(–)***–20.9
Water temperature (°C)8900300.311104.500.0
Hardness, total (mg/L as CaCO3)7200250.0215–0.131317.5(–)**–17.1
Calcium, dissolved (mg/L)7200250.0271–0.038475.15(–)**–17.0
Magnesium, dissolved (mg/L)7200250.0100–0.0072121.1(–)***–15.1
Potassium, dissolved (mg/L)7100250.015400.600.0
Sodium adsorption (ratio)7100250.470400.200.0
Sodium, dissolved (mg/L)7100250.3087–0.0049920–6.0
Sodium (percent)7100250.139901900.0
Acid-neutralizing capacity (mg/L)7200250.908401800.0
Chloride, dissolved (mg/L)7200250.493000.600.0
Fluoride, dissolved (mg/L)7057250.4827B0.10B
Silica, dissolved (mg/L)7200250.00280.033564.6(+)***20.2
Sulfate, dissolved (mg/L)7100250.0000–0.12873.4(–)***–63.4
Nitrogen, ammonia plus organic (mg/L as N)341029110.3054B0.30B
Nitrogen, ammonia (mg/L as N)691522250.9754B0.040B
Nitrite plus nitrate (mg/L as N)742736250.4603B0.0890B
Nitrogen, nitrite (mg/L as N)71699725AAAAA
Phosphorus, dissolved (mg/L as P)602948220.0000B0.01(–)***B
Phophorus, ortho, dissolved (mg/L as P)33278211AAAAA
Phosphorus, total (mg/L as P)681015250.0067B0.02(–)***B
Residue, dissolved at 180°C (mg/L)3100110.69430.17783306.1
Dissolved solids, sum of constituents (mg/L)7000250.0137–0.167130(–)**–13.0
Barium, dissolved (µg/L)3100110.95770600.0
Iron, dissolved (µg/L)7200250.0050–1.49762.5(–)***–45.7
Manganese, dissolved (µg/L)5359180.0413B4(–)**B
Strontium, dissolved (µg/L)3213110.0478–0.734728.5(–)**–24.6
Zinc, dissolved (µg/L)251040110.0257B7(–)**B

Table 30. Trend analysis results for site 8, Carter Reservoir, surface samples.

[0, no trend; (+), upward trend; (–), downward trend; significance: *, greater than 90 percent; **, greater than 95 percent; ***, greater than 99 percent; change is percent change in median since the first year of record; A, no trend computation because nondetects greater than 50 percent; B, no slope computation because nondetects greater than 20 percent; mg/L, milligrams per liter; µS/cm, microsiemens per centimeter at 25 degrees Celsius; °C, degree Celsius; in., inch; mL, milliliter; hplc, high-performance liquid chromatography; µg/L, micrograms per liter]

Seasonal Kendall season-edited dataNumberCensoredPercentage censoredPeriod record (years)p-valueSlopeMedianSignificanceChange
Oxygen, dissolved (mg/L)8500300.1114–0.0081127.60–3.2
pH, field (standard units)8800300.01210.014148(+)**5.5
Specific conductance (µS/cm at 25°C)8800300.0000–0.995780(–)***–31.3
Water temperature (°C)8800300.907101700.0
Hardness, total (mg/L as CaCO3)3600110.0002–0.572330(–)***–18.8
Calcium, dissolved (mg/L)3600110.0002–0.19799.8(–)***–19.8
Magnesium, dissolved (mg/L)3600110.0001–0.014171.3(–)***–11.3
Potassium, dissolved (mg/L)3600110.037000.700.0
Sodium adsorption (ratio)3600111.000000.200.0
Sodium, dissolved (mg/L)3600110.0000–0.042272.2(–)***–19.0
Sodium (percent)3600110.484801300.0
Acid-neutralizing capacity (mg/L)3600110.0009–0.49932(–)***–15.7
Chloride, dissolved (mg/L)3400110.00630.033130.7(+)***72.7
Fluoride, dissolved (mg/L)34515110.3807B0.10B
Silica, dissolved (mg/L)3600110.00050.15522.5(+)***108.8
Sulfate, dissolved (mg/L)3413110.0004–0.073952.9(–)***–24.3
Nitrogen, ammonia plus organic (mg/L as N)351440110.046600.20B
Nitrogen, ammonia (mg/L as N)743142300.021500.030B
Nitrite plus nitrate (mg/L as N)77516629AAAAA
Phosphorus, dissolved (mg/L as P)29258611AAAAA
Phophorus, ortho, dissolved (mg/L as P)35318911AAAAA
Phosphorus, total (mg/L as P)722839290.0000B0.03(–)***B
Residue, dissolved at 180°C (mg/L)7900300.0000–0.434447(–)***–24.3
Dissolved solids, sum of constituents (mg/L)3300110.0025–0.489139.5(–)***–12.7
Transparency, Secchi disk (in.)7200300.34520.408196013.7
Coliform, fecal (colonies per 100 mL)56386824AAAAA
Chlorophyll-a, hplc (µg/L)3500110.14130.049641.2060.5
Barium, dissolved (µg/L)3600110.0943–0.249823(–)*–11.2
Iron, dissolved (µg/L)27830110.0030B7(–)***B
Manganese, dissolved (µg/L)27197011AAAAA
Strontium, dissolved (µg/L)3600110.0000–1.11641(–)***–25.7
Zinc, dissolved (µg/L)271141110.6150B7.50B

Table 31. Trend analysis results for site 8, Carter Reservoir, bottom samples.

[0, no trend; (+), upward trend; (–), downward trend; significance: *, greater than 90 percent; **, greater than 95 percent; ***, greater than 99 percent; change is percent change in median since the first year of record; A, no trend computation because nondetects greater than 50 percent; B, no slope computation because nondetects greater than 20 percent; mg/L, milligrams per liter; µS/cm, microsiemens per centimeter at 25 degrees Celsius; °C, degree Celsius; µg/L, micrograms per liter]

Seasonal Kendall season-edited dataNumberCensoredPercentage censoredPeriod record (years)p-valueSlopeMedianSignificanceChange
Oxygen, dissolved (mg/L)790030<.00001–0.083666.9(–)***–30.6
pH, field (standard units)7400300.432407.400.0
Specific conductance (µS/cm at 25°C)750030<.00001–0.948871(–)***–33.2
Water temperature (°C)8700300.03850.027986.5(+)**13.8
Hardness, total (mg/L as CaCO3)360011<.00001–0.589728(–)***–20.6
Calcium, dissolved (mg/L)360011<.00001–0.20468.8(–)***–22.4
Magnesium, dissolved (mg/L)360011<.00001–0.02231.3(–)***–17.1
Potassium, dissolved (mg/L)3600110.001600.600.0
Sodium adsorption (ratio)3600111.000000.200.0
Sodium, dissolved (mg/L)3600110.0001–0.046812.2(–)***–20.8
Sodium (percent)3600110.789801400.0
Acid-neutralizing capacity (mg/L)360011<.00001–0.623130(–)***–20.3
Chloride, dissolved (mg/L)3400110.00040.043910.6(+)***143.5
Fluoride, dissolved (mg/L)34412110.8823B0.10B
Silica, dissolved (mg/L)3600110.00030.15733.8(+)***60.6
Sulfate, dissolved (mg/L)330011<.00001–0.068112.9***–22.6
Nitrogen, ammonia plus organic (mg/L as N)35185111AAAAA
Nitrogen, ammonia (mg/L as N)641625300.6375B0.030B
Nitrite plus nitrate (mg/L as N)34175017AAAAA
Phosphorus, dissolved (mg/L as P)29269011AAAAA
Phophorus, ortho, dissolved (mg/L as P)35329111AAAAA
Phosphorus, total (mg/L as P)61223629<.00001B0.02(–)***B
Residue, dissolved at 180°C (mg/L)640030<.00001–0.457743(–)***–27.4
Dissolved solids, sum of constituents (mg/L)3300110.0003–0.658138(–)***–17.3
Barium, dissolved (µg/L)3600110.0612–0.30499(–)*–31.0
Iron, dissolved (µg/L)2741580.0296B4(–)**B
Manganese, dissolved (µg/L)2772680.5328B9.950B
Strontium, dissolved (µg/L)360011<.00001–1.16139.5(–)***–27.5
Zinc, dissolved (µg/L)27103780.3872B70B

Table 32. Trend analysis results for site 9, Horsetooth Reservoir (Soldier Canyon Dam), surface samples.

[0, no trend; (+), upward trend; (–), downward trend; significance: *, greater than 90 percent; **, greater than 95 percent; ***, greater than 99 percent; change is percent change in median since the first year of record; A, no trend computation because nondetects greater than 50 percent; B, no slope computation because nondetects greater than 20 percent; mg/L, milligrams per liter; µS/cm, microsiemens per centimeter at 25 degrees Celsius; °C, degree Celsius; in., inch; mL, milliliter; hplc, high-performance liquid chromatography; µg/L, micrograms per liter]

Seasonal Kendall season-edited dataNumberCensoredPercentage censoredPeriod record (years)p-valueSlopeMedianSignificanceChange
Oxygen, dissolved (mg/L)8600300.3364–0.003847.80–1.5
pH, field (standard units)8600300.00020.021047.6(+)***8.7
Specific conductance (µS/cm at 25°C)8800310.0000–0.660775(–)***–23.9
Water temperature (°C)8800310.8649015.4500.0
Hardness, total (mg/L as CaCO3)3600110.0100–0.396928.5(–)***–14.1
Calcium, dissolved (mg/L)3600110.0097–0.13839.05(–)***–15.4
Magnesium, dissolved (mg/L)3600110.0515–0.011021.5(–)*–7.7
Potassium, dissolved (mg/L)3600110.043200.700.0
Sodium adsorption (ratio)3600111.000000.200.0
Sodium, dissolved (mg/L)3600110.0396–0.019732.4(–)**–8.6
Sodium (percent)3600110.223201500.0
Acid-neutralizing capacity (mg/L)3600110.0025–0.492629(–)***–17.0
Chloride, dissolved (mg/L)3400110.00010.06420.7(+)***224.3
Fluoride, dissolved (mg/L)34515110.5380B0.10B
Silica, dissolved (mg/L)3600110.04170.10412.75(+)**53.9
Sulfate, dissolved (mg/L)3.4--
Nitrogen, ammonia plus organic (mg/L as N)361439110.033800.30B
Nitrogen, ammonia (mg/L as N)823948310.034800.040B
Nitrite plus nitrate (mg/L as N)81577030AAAAA
Nitrogen, nitrite (mg/L as N)77577431AAAAA
Phosphorus, dissolved (mg/L as P)30289311AAAAA
Phophorus, ortho, dissolved (mg/L as P)36328911AAAAA
Phosphorus, total (mg/L as P)672943290.0000B0.04(–)***B
Residue, dissolved at 180°C (mg/L)7600310.0000–0.562550(–)***–29.6
Dissolved solids, sum of constituents (mg/L)3400110.0350–0.493739(–)**–12.9
Transparency, Secchi disk (in.)6800300.00651.06675(+)***54.7
Coliform, fecal (colonies per 100 mL)52316023AAAAA
Chlorophyll-a, hplc (µg/L)3500110.1716–0.07841.90–36.4
Chlorophyll-b, hplc (µg/L)35288011AAAAA
Barium, dissolved (µg/L)3600110.7456018.500.0
Iron, dissolved (µg/L)4312190.4434–0.2706130–32.7
Manganese, dissolved (µg/L)421536190.4685B30B
Strontium, dissolved (µg/L)3600110.0001–1.22937.5(–)***–30.1
Zinc, dissolved (µg/L)27114180.5916B5.50B

Table 33. Trend analysis results for site 9, Horsetooth Reservoir (Soldier Canyon Dam), bottom samples.

[0, no trend; (+), upward trend; (–), downward trend; significance: *, greater than 90 percent; **, greater than 95 percent; ***, greater than 99 percent; change is percent change in median since the first year of record; A, no trend computation because nondetects greater than 50 percent; B, no slope computation because nondetects greater than 20 percent; mg/L, milligrams per liter; µS/cm, microsiemens per centimeter at 25 degrees Celsius; °C, degree Celsius; µg/L, micrograms per liter]

Seasonal Kendall season-edited dataNumberCensoredPercentage censoredPeriod record (years)p-valueSlopeMedianSignificanceChange
Oxygen, dissolved (mg/L)7800300.0010–0.070197.5(–)***–24.5
pH, field (standard units)4900170.6109 07.200.0
Specific conductance (µS/cm at 25°C)7300300.0000–0.499773(–)***–18.6
Water temperature (°C)8400300.08060.02127.1(+)*9.4
Hardness, total (mg/L as CaCO3)3600110.0001–0.504828.5(–)***–17.6
Calcium, dissolved (mg/L)3600110.0001–0.17428.95(–)***–19.2
Magnesium, dissolved (mg/L)3600110.0033–0.019571.5(–)***–13.3
Potassium, dissolved (mg/L)3600110.0054–0.0095590.7(–)***–13.9
Sodium adsorption (ratio)3600111.000000.200.0
Sodium, dissolved (mg/L)3600110.0151–0.024982.3(–)**–11.2
Sodium (percent)3600110.042301500.0
Acid-neutralizing capacity (mg/L)3600110.0001–0.669229(–)***–22.3
Chloride, dissolved (mg/L)3400110.00000.076290.7(+)***346.4
Fluoride, dissolved (mg/L)34515110.3881B0.10B
Silica, dissolved (mg/L)3600110.08760.11253.65(+)*41.6
Sulfate, dissolved (mg/L)3400110.0019–0.098843.20–28.7
Nitrogen, ammonia plus organic (mg/L as N)36195311AAAAA
Nitrogen, ammonia (mg/L as N)452044170.3865B0.040B
Nitrite plus nitrate (mg/L as N)391026170.4192B0.120B
Nitrogen, nitrite (mg/L as N)43419517AAAAA
Phosphorus, dissolved (mg/L as P)36256917AAAAA
Phophorus, ortho, dissolved (mg/L as P)36256911AAAAA
Phosphorus, total (mg/L as P)371232170.2979B0.030B
Residue, dissolved at 180°C (mg/L)6400310.0017–0.288148(–)***–17.0
Dissolved solids, sum of constituents (mg/L)3400110.0006–0.433338.5(–)***–11.6
Barium, dissolved (µg/L)3600110.08750.109817(+)*7.4
Iron, dissolved (µg/L)3326110.4980B150B
Manganese, dissolved (µg/L)36195311AAAAA
Strontium, dissolved (µg/L)3600110.0000–1.33740(–)***–30.6
Zinc, dissolved (µg/L)271141110.8970B60B

Table 34. Trend analysis results for site 10, Horsetooth Reservoir (Spring Canyon Dam), surface samples.

[0, no trend; (+), upward trend; (–), downward trend; significance: *, greater than 90 percent; **, greater than 95 percent; ***, greater than 99 percent; change is percent change in median since the first year of record; A, no trend computation because nondetects greater than 50 percent; B, no slope computation because nondetects greater than 20 percent; mg/L, milligrams per liter; µS/cm, microsiemens per centimeter at 25 degrees Celsius; °C, degree Celsius; in., inch; mL, milliliter; hplc, high-performance liquid chromatography; µg/L, micrograms per liter]

Seasonal Kendall season-edited dataNumberCensoredPercentage censoredPeriod record (years)p-valueSlopeMedianSignificanceChange
Oxygen, dissolved (mg/L)4800111.062007.600.0
pH, field (standard units)5000111.88700.012517.6501.8
Specific conductance (µS/cm at 25°C)5000110.0043–0.792866(–)***–12.3
Water temperature (°C)4800110.0597–0.15816.65(–)*–9.9
Hardness, total (mg/L as CaCO3)3600110.0161–0.335328(–)**–12.3
Calcium, dissolved (mg/L)3600110.0234–0.11268.95(–)**–12.9
Magnesium, dissolved (mg/L)3600110.384601.400.0
Potassium, dissolved (mg/L)3600110.114300.600.0
Sodium adsorption (ratio)3600111.000000.200.0
Sodium, dissolved (mg/L)3600110.0243–0.032512.4(–)**–13.8
Sodium (percent)3600111.100701500.0
Acid-neutralizing capacity (mg/L)3600110.0546–0.352929(–)*–12.5
Chloride, dissolved (mg/L)3400111.99960.066870.8(+)***184.5
Fluoride, dissolved (mg/L)34618110.6458B0.10B
Silica, dissolved (mg/L)3600111.94830.13762.75(+)*78.7
Sulfate, dissolved (mg/L)3400110.0688–0.079483.2(–)*–23.8
Nitrogen, ammonia plus organic (mg/L as N)361644110.056700.30B
Nitrogen, ammonia (mg/L as N)36215811AA0.03AA
Nitrite plus nitrate (mg/L as N)30268711AA0.07AA
Nitrogen, nitrite (mg/L as N)36359711AA0.03AA
Phosphorus, dissolved (mg/L as P)30299711AA0.01AA
Phophorus, ortho, dissolved (mg/L as P)36349411AA0.02AA
Phosphorus, total (mg/L as P)32206311AA0.01AA
Residue, dissolved at 180°C (mg/L)3300111.37930.24864007.1
Dissolved solids, sum of constituents (mg/L)3400110.1645–0.2356380–6.6
Transparency, Secchi disk (in.)4600111.9991079.500.0
Coliform, fecal (colonies per 100 mL)321238110.5629B20B
Chlorophyll-a, hplc (µg/L)3500110.2313–0.074862.10–32.3
Chlorophyll-b, hplc (µg/L)35246911AA0.1AA
Barium, dissolved (µg/L)3600111.5075018.500.0
Iron, dissolved (µg/L)361644111.2370B21.50B
Manganese, dissolved (µg/L)321134111.8638B30B
Strontium, dissolved (µg/L)3600110.0000–1.25838(–)***–30.4
Zinc, dissolved (µg/L)2720748AA14AA

Table 35. Trend analysis results for site 10, Horsetooth Reservoir (Spring Canyon Dam), bottom samples.

[0, no trend; (+), upward trend; (–), downward trend; significance: *, greater than 90 percent; **, greater than 95 percent; ***, greater than 99 percent; change is percent change in median since the first year of record; A, no trend computation because nondetects greater than 50 percent; B, no slope computation because nondetects greater than 20 percent; mg/L, milligrams per liter; µS/cm, microsiemens per centimeter at 25 degrees Celsius; °C, degree Celsius; µg/L, micrograms per liter]

Seasonal Kendall season-edited dataNumberCensoredPercentage censoredPeriod record (years)p-valueSlopeMedianSignificanceChange
Oxygen, dissolved (mg/L)4800110.0709–0.098755.2(–)*–18.8
pH, field (standard units)5000110.1716–0.008267.30–1.2
Specific conductance (µS/cm at 25°C)5000110.0069–0.899470(–)***–13.1
Water temperature (°C)4800110.01340.066567.45(+)**10.4
Hardness, total (mg/L as CaCO3)3600110.0002–0.498229(–)***–17.1
Calcium, dissolved (mg/L)3600110.0001–0.16349.1(–)***–17.8
Magnesium, dissolved (mg/L)3600110.0070–0.017361.5(–)***–11.9
Potassium, dissolved (mg/L)3600110.023900.700.0
Sodium adsorption (ratio)3600111.000000.200.0
Sodium, dissolved (mg/L)3600110.0759–0.019572.3(–)*–8.9
Sodium (percent)3600111.98920.13315(+)**10.3
Acid-neutralizing capacity (mg/L)3600110.0020–0.524930(–)***–17.4
Chloride, dissolved (mg/L)3400110.00000.096650.7(+)***885.2
Fluoride, dissolved (mg/L)34412110.9048B0.10B
Silica, dissolved (mg/L)3600110.00990.19713.85(+)***81.3
Sulfate, dissolved (mg/L)3400110.0029–0.099413.2(–)***–28.8
Nitrogen, ammonia plus organic (mg/L as N)36205611AAAAA
Nitrogen, ammonia (mg/L as N)361439110.1157B0.040B
Nitrite plus nitrate (mg/L as N)311135110.0518B0.155(+)*B
Nitrogen, nitrite (mg/L as N)36349411AAAAA
Phosphorus, dissolved (mg/L as P)32216611AAAAA
Phophorus, ortho, dissolved (mg/L as P)36195311AAAAA
Phosphorus, total (mg/L as P)341132111.0000B0.02950B
Residue, dissolved at 180°C (mg/L)3300110.27880.426342011.9
Dissolved solids, sum of constituents (mg/L)3300110.0568–0.265940(–)*–7.0
Barium, dissolved (µg/L)3613110.2815017.500.0
Iron, dissolved (µg/L)3539110.7407B210B
Manganese, dissolved (µg/L)3539110.1998B130B
Strontium, dissolved (µg/L)3500110.0001–1.05640.5(–)***–24.8
Zinc, dissolved (µg/L)25135211AAAAA

Document Accessibility: Adobe Systems Incorporated has information about PDFs and the visually impaired. This information provides tools to help make PDF files accessible. These tools convert Adobe PDF documents into HTML or ASCII text, which then can be read by a number of common screen-reading programs that synthesize text as audible speech. In addition, an accessible version of Acrobat Reader 6.0, which contains support for screen readers, is available. These tools and the accessible reader may be obtained free from Adobe at Adobe Access.


U.S. Department of the Interior, U.S. Geological Survey
Persistent URL: http://pubs.water.usgs.gov/wri034044
Page Contact Information: GS Pubs Web Contact
Last modified: Thursday, September 01 2005, 05:08:22 PM
FirstGov button  Take Pride in America button