Water Resources of Colorado

Hydrology and Water-Quality Characteristics
of Muddy Creek and Wolford Mountain
Reservoir near Kremmling, Colorado,
1990 through 2001

By Michael R. Stevens, and Lori A. Sprague

Available from the U.S. Geological Survey, Branch of Information Services, Box 25286, Denver Federal Center, Denver, CO 80225, USGS Water-Resources Investigations Report 03-4073, 82 p., 35 figs.

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A water-quality monitoring program was begun in March 1985 on Muddy Creek in anticipation of the construction of a reservoir water-storage project. Wolford Mountain Reservoir was constructed by the Colorado River Water Conservation District during 199294. The reservoir began to be filled in 1995.

Water quality generally was good in Muddy Creek and Wolford Mountain Reservoir throughout the period of record (collectively, 1990 through 2001), with low concentrations of nutrients (median total nitrogen less than 0.6 and median total phosphorus less than 0.05 milligrams per liter) and trace elements (median dissolved copper less than 2, median dissolved lead less than 1, and median dissolved zinc less than 20 micrograms per liter). Specific conductance ranged from 99 to 1,720 microsiemens per centimeter. Cation compositions at Muddy Creek sites were mixed calcium-magnesium-sodium. Anion compositions were primarily bicarbonate and sulfate. Suspended-sediment concentrations ranged from less than 50 milligrams per liter during low-flow periods to hundreds of milligrams per liter during snowmelt. Turbidity in prereservoir Muddy Creek generally was measured at less than 10 nephelometric turbidity units during low-flow periods and ranged to more than 360 nephelometric turbidity units during snowmelt. Compared to prereservoir conditions, turbidity in Muddy Creek downstream from the reservoir was substantially reduced because the reservoir acted as a sediment trap.

During most years, peak flows were slightly reduced by the reservoir or similar to peaks upstream from the reservoir. The upper first to fifteenth percentiles of flows were decreased by operation of the reservoir compared to prereservoir flows. Generally, the fifteenth to one-hundredth percentiles of flow were increased by operation of the reservoir outflow compared to prereservoir flows.

Nutrient transport in the inflow is proportional to the amount of inflow-water discharge in a given year. Some nitrogen was stored in the water column and gain/loss patterns for total nitrogen were somewhat related to reservoir storage. Nitrogen tended to move through the reservoir, whereas phosphorus was mostly trapped within the reservoir in bottom sediments. The reservoir gained phosphorus every year (1996 2001) and, as a percentage, more phosphorus was retained than nitrogen in years when both were retained in the reservoir due to stronger phosphorus tendencies for adsorption, coprecipitation, and settling. Only small amounts of phosphorus were available in the water column at the outflow, and reservoir water-column storage did not influence phosphorus outflowloading patterns as much as settling further upstream in the reservoir.

From 1990 to 2001, upstream from the reservoir, concentrations and values of dissolved solids, turbidity, some major ions, and dissolved iron increased (p-value less than 0.10), and acid-neutralizing capacity decreased. From 1990 to 2001, there were no significant (p-value less than 0.10) trends in nutrient concentrations upstream from the reservoir. From 1990 to 2001, downstream from the reservoir, trends in concentrations and values of dissolved solids, turbidity, major ions, total ammonia plus organic nitrogen, dissolved and total-recoverable iron, and total-recoverable manganese were downward.

Upstream and downstream water-quality constituents for the prereservoir (1990 to 1995) period were compared. Concentrations and values of dissolved solids, major ions, turbidity, and manganese were greater (p-value less than 0.10) at the downstream site.

From 1995 to 2001 (postconstruction), upstream and downstream water-quality constituents also were compared. Concentrations of specific conductance and major ions increased at the downstream site when compared to the upstream site (p-value less than 0.10), except for acid-neutralizing capacity and silica, which decreased. Turbidity, concentrations of total-recoverable and dissolved manganese, and total-recoverable iron also were smaller downstream from the reservoir.

Results indicate that concentrations of dissolved solids increased downstream in Muddy Creek before the reservoir was constructed. This trend continued after construction, but the difference between upstream and downstream median concentrations of dissolved solids and major ion concentrations was less than in the prereservoir period.

Spring runoff temperatures and fall temperatures in Muddy Creek were lower than in the reservoir. Thus, inflows to the reservoir tended to settle near the thermocline. In summer, inflow-water temperature was similar to the surface layer, and flows were routed through the reservoir near the surface.

In winter, Muddy Creek stream temperatures were near 0C, and surface water at the ice cover interface was 0C, but the temperature in the reservoir subsurface probably increased with depth to 4C in the bottom waters (water is most dense at about 4C). Although no winter stratification measurements were made under the ice, the reservoir was assumed to be similar to other dimictic, montane reservoirs. Thus, inflow will tend to be routed just under the ice. Flow patterns within the reservoir could be important because residence time varies from season to season, and in the event of a chemical spill upstream, knowledge of timing and probable vertical location of the plume would be important for outflow gate configurations to manage the spill.

Near-bottom samples of ammonia and nitrite plus nitrate generally had larger concentrations than concurrent surface samples. Surface-sample concentrations of nitrite plus nitrate were substantially depleted throughout the growing season. Temporally, surface and bottom concentrations tended to decrease and stabilize for ammonia plus organic nitrogen and ammonia throughout the period of record. Nitrite plus nitrate seemed to increase in bottom samples with time. Spatial variation in nutrient concentrations from inflow to dam tended to decrease for total ammonia plus organic nitrogen and total phosphorus in surface samples, whereas bottom-sample concentrations of nitrite plus nitrate tended to increase from inflow to dam.

Spatially, total recoverable and dissolved iron and manganese and total recoverable aluminum show decreasing median concentrations from inflow to dam in surface samples. Bottomsample median concentrations of total recoverable iron and aluminum decreased from inflow to dam. The spatial increases in manganese along the reservoir axis probably are related to redox reactions in the hypolimnion, which release manganese during periods of hypoxia.

Bacteria counts were low to not detected in the reservoir samples. Bacteria in Muddy Creek may be associated with suspended material that rapidly settles out in the reservoir.

Trophic conditions generally were upper oligotrophic to mesotrophic from 1995 to 2001 in Wolford Mountain Reservoir based on available Secchi depth, total phosphorus concentrations, and chlorophyll-a concentrations. Dissolved-oxygen concentrations generally were in the range of 3 to 7 milligrams per liter except in late summer when oxygen concentrations approached 1 milligram per liter or less in the hypolimnion.

Constituent concentrations generally were acceptable and met Colorado water-quality standards. Selenium concentrations exceeded chronic aquatic standards twice in Muddy Creek at the Kremmling site (prereservoir, 198295). Six selenium concentrations in near-bottom samples collected from the reservoir near the dam during 199597 exceeded chronic aquatic standards. Aquatic standards for iron and manganese were occasionally exceeded in Muddy Creek and in near-bottom samples from Wolford Mountain Reservoir.




Purpose and Scope

Previous Studies

Description and Background Information of Study Area


Methods of Investigation

Field Methods

Data-Analysis Methods

Data-Set Construction


Water Quality

Streamwater Quality

Field Properties

Major Ions


Trace Elements

Biological Indicators

Suspended Sediment

Reservoir Water Quality

Field Properties

Major Ions


Trace Elements

Biological Indicators

Trophic Status

Water-Quality Standards

Summary and Conclusions

References Cited

Hydrologic and Water-Quality Data


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Water Resources of Colorado

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