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WRIR 02-4030: Streamflow and Water-Quality Data for Selected Watersheds in the Lake Tahoe Basin, California and Nevada, through September 1998 |
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Streamflow summary statistics (period of record, average annual mean flows, highest and lowest annual mean flows, annual runoff, unit runoff, highest and lowest daily mean flow, instantaneous peaks, range and median of sampled flow, and number of samples) for the periods of record 1961-98 for the 21 streamflow (20 active and 1 discontinued) gaging stations at the primary and secondary stations are listed in table 4.
Periods of record for the 21 streamflow gaging stations ranged from 4 years (1995-98) at Edgewood Creek at Stateline to 38 years (1961-98) at Trout Creek at Tahoe Valley and Blackwood Creek near Tahoe City. Instantaneous streamflow during the study period for these stations ranged from 0 ft3/s at five stations during base-flow periods to 5,480 ft3/s at Upper Truckee River at South Lake Tahoe during a rain-on-snow flood event in January 1997.
Variations in streamflow in the Lake Tahoe Basin are largely due to differences in weather patterns and variations of precipitation amounts and intensity. Smaller variations are due to area and altitude distributions, air and soil temperature, amount of snow on the ground (if present), soil moisture conditions, types of soil and geology, slope and aspect (Jeton, 1999, figs. 6 and 7), type and amount of vegetation coverage (Jeton, 1999, fig. 8) and other natural conditions across the basin.
Streams in the same general area can differ widely in flow, for example, Third and Incline Creeks in the Incline Village area. During the January 1997 flood, the streamflow at Incline Creek near Crystal Bay was 179 ft3/s, establishing a new record peak, markedly greater than the previous peak of 87 ft3/s in 1970. The storm peak at Third Creek was only 108 ft3/s in 1997, well below the previous record peak of 150 ft3/s set in 1982 (Rowe and others, 1998). Previous studies in the Incline area (Rowe, 1993; Glancy, 1988) also noted that these two creeks are similar in size, in close proximity to each other, but exhibit markedly different runoff characteristics.
The highest mean annual daily mean streamflow was 106 ft3/s at Upper Truckee River at South Lake Tahoe and the lowest was 0.56 ft3/s at Logan House Creek near Glenbrook. The highest daily mean streamflow was 3,150 ft3/s on January 2, 1997, at Upper Truckee River at South Lake Tahoe. The lowest daily mean streamflow of 0 ft3/s occurred during summer months throughout the period of record at five stations.
Average annual runoff, and unit runoff and unit-runoff ranks for the 10 primary stations for the period of comparison (1988-98) are shown in figures 9 and 10, respectively. Upper Truckee River at South Lake Tahoe had the highest average annual runoff (65,100 acre-ft) and Logan House Creek near Glenbrook had the lowest (340 acre-ft). Blackwood Creek near Tahoe City had the highest unit runoff (2,280 acre-ft/mi2, rank of 1) and Logan House Creek near Glenbrook had the lowest unit runoff (163 acre-ft/mi2, rank of 10). Unit runoff on the California or western half of the Lake Tahoe Basin (627-2,280 acre-ft/mi2) was greater than on the Nevada or eastern half of the basin (163-929 acre-ft mi2).
Daily mean streamflow for the periods of record for two index stations, Incline Creek near Crystal Bay and Upper Truckee River at South Lake Tahoe, are shown in figures 11A and 12A. These stations were used to compare a large watershed and a medium-small watershed, a northern and southern watershed, a watershed with greater precipitation and one with less, and one watershed from Nevada and one from California. The annual seasonal pattern is typical of streams in the Lake Tahoe Basin, with most runoff occurring during the spring snowmelt period (April, May, and June). Other events include rainstorms in the fall period (October, November, and December), rain-on-snow storms in the winter period (January, February, and March), and convective thunderstorms in the summer period (July, August, and September). Median monthly runoff values are plotted in figures 11B and 12B. These plots show that more streamflow occurs in the spring period and less in the summer and fall periods.
A flood-frequency summary for 20 streamflow gaging stations and 6 historical (discontinued) streamflow gaging stations in the monitored watersheds is listed in table 5. The 50-year and 100-year peak streamflows and maximum recorded peak streamflows through water year 1998 (Glen W. Hess, U.S. Geological Survey, written commun., 2000) also are listed in table 5.
Field measurements were made during sample collection. Summary statistics (minimums, maximums, medians, and number of samples) are listed in tables 6, 7, 8 and 9 for 10 primary, 10 secondary, and 14 miscellaneous water-quality stations.
Water temperatures (table 6) ranged from 0°C during winter periods at many locations to 23.0°C during summer periods at two lower elevation stations, Edgewood Creek at Lake Tahoe and Ward Creek at Highway 89 near Tahoe City. For all stations, the highest median temperature was 14.0°C at Tributary to Edgewood Creek and the lowest was 2.5°C at Ward Creek below confluence. For primary and secondary stations, the highest median temperature was 6.0°C at many locations and the lowest was 2.5°C at Ward Creek below confluence. Water temperatures typically increased slightly downstream.
Specific conductance (table 7) ranged from 8 microsiemens per centimeter (µS/cm) at General Creek near Meeks Bay during snowmelt in the spring to 900 µS/cm at Glenbrook Creek at Glenbrook during low summer streamflow. For all stations, the highest median specific conductance was 466 µS/cm at Glenbrook Creek at old highway and the lowest was 21 µS/cm at General Creek near Meeks Bay. For primary and secondary stations, the highest median specific conductance was 455 µS/cm at Glenbrook Creek at Glenbrook and the lowest was 21 µS/cm at General Creek near Meeks Bay. Specific conductance typically increases downstream, as noted for Incline Creek in Rowe (1999). In general, specific conductance is higher on the eastern side of the basin.
pH (table 8) ranged from 6.6 during spring snowmelt at Trout Creek at Pioneer Trail to 10.6 during summer low-flow periods at Edgewood Creek at Lake Tahoe. For all stations, the highest median pH was 8.8 at Edgewood Creek at Lake Tahoe and the lowest was 7.2 at General Creek near Meeks Bay. For primary and secondary stations, the highest median pH was 8.2 at Logan House Creek near Glenbrook and the lowest was 7.2 at General Creek near Meeks Bay. In general, pH appears to be higher on the eastern side of the basin and does not seem to vary in a downstream direction.
Dissolved oxygen (DO; table 9) ranged from 5.2 milligrams per liter (mg/L) at Edgewood Creek at Lake Tahoe to 12.6 mg/L at Logan House Creek near Glenbrook and Incline Creek near Crystal Bay. For all stations, the highest median DO was 10.2 mg/L at Ward Creek at Highway 89 near Tahoe City and the lowest was 8.3 mg/L at Edgewood Creek Tributary near Daggett Pass. For primary and secondary stations, the highest median DO was 10.2 mg/L at Ward Creek at Highway 89 and the lowest was 9.2 mg/L at Glenbrook Creek at Glenbrook.
Dissolved-oxygen concentration was near saturation (100 percent) for most stations. Dissolved-oxygen saturation ranged from 70 to 157 percent; both occurred at Edgewood Creek at Lake Tahoe. For all stations, the highest median saturation was 106 percent at Edgewood Creek at Lake Tahoe and the lowest was 92 percent at Edgewood Creek Tributary near Daggett Pass. For primary and secondary stations, the highest median saturation was 103 percent at Upper Truckee River at South Lake Tahoe and the lowest was 96 percent at Glenbrook Creek at Glenbrook.
Samples may have higher concentration values than from other studies in the basin, because LTIMP emphasized sampling during storm and snowmelt runoff. Summary statistics (minimums, maximums, medians, and number of samples) for nutrient and suspended-sediment concentrations from 10 primary, 10 secondary, and 14 miscellaneous sampling stations are listed in tables 10, 11, 12, 13, 14, 15, and 16.
Concentrations of dissolved nitrite plus nitrate nitrogen (table 10) ranged from less than 0.002 mg/L at three stations to 2.25 mg/L at Highway 50 Culvert to Edgewood Creek during storm runoff. For all stations, the highest median concentration was 0.134 mg/L at Highway 50 Culvert to Edgewood Creek and the lowest was 0.004 mg/L at Second Creek at Lakeshore Drive. For primary and secondary stations, the highest median concentration was 0.030 mg/L at Incline Creek near Crystal Bay and the lowest was 0.005 mg/L at Trout Creek at U.S. Forest Service Road 12N01 and General Creek near Meeks Bay. Concentrations of dissolved nitrite plus nitrate nitrogen made-up about 10 percent of the total nitrogen component for all stations. Dissolved nitrite plus nitrate nitrogen concentrations were observed to increase slightly in a downstream direction, based on median values for the five multiple-station watersheds.
Concentrations of dissolved ammonia nitrogen (table 11) were low, ranging from less than 0.003 mg/L at many stations to 1.39 mg/L at Edgewood Creek Tributary above Clubhouse during storm runoff. For all stations, the highest median concentration was 0.166 mg/L at Highway 50 Culvert to Edgewood Creek and the lowest was less than 0.003 mg/L at many stations. For primary and secondary stations, the highest median concentration was 0.003 mg/L at the three Edgewood Creek stations and the lowest was less than 0.003 mg/L at many stations. All stations had medians less than 0.003 mg/L except the eight stations in the Edgewood Creek watershed. In the Edgewood Creek watershed, ammonia did not increase in a downstream direction. Dissolved ammonia nitrogen made-up less than 1 percent of the total nitrogen component for all stations.
Total ammonia and organic nitrogen concentrations (table 12) ranged from less than 0.04 to 24.0 mg/L; both values occurred at Third Creek near Crystal Bay, with the higher reading observed during summer thunderstorm runoff. For all stations, the highest median concentration was 4.15 mg/L at Highway 50 Culvert to Edgewood Creek and the lowest was 0.07 mg/L at Ward Creek below confluence. For primary and secondary stations, the highest median was 0.21 mg/L at Incline Creek near Crystal Bay and the lowest was 0.07 mg/L at Ward Creek below confluence. Total ammonia and organic nitrogen concentrations were observed to increase only slightly in a downstream direction.
Concentrations of soluble reactive phosphorus (table 13) ranged from less than 0.001 mg/L at Logan House Creek near Glenbrook to 1.55 mg/L at Edgewood Creek Tributary above Clubhouse during storm runoff. For all stations, the highest median concentration was 0.090 mg/L at Highway 50 Culvert to Edgewood Creek and the lowest was 0.002 mg/L at Logan House Creek near Glenbrook. For primary and secondary stations, the highest median concentration was 0.013 mg/L at Glenbrook Creek at Glenbrook and the lowest was 0.002 mg/L at Logan House Creek near Glenbrook. Soluble reactive phosphorus concentrations made-up about 24 percent of the total phosphorus component. Soluble reactive phosphorus concentrations were observed to increase only slightly or not at all in a downstream direction.
Total phosphorus concentrations (table 14) ranged from less than 0.002 mg/L at Logan House Creek near Glenbrook to 11.1 mg/L at Highway 50 Culvert to Edgewood Creek during storm runoff. For all stations, the highest median concentration was 4.30 mg/L at Highway 50 Culvert to Edgewood Creek and the lowest was 0.0021 mg/L at three stations. For primary and secondary stations, the highest median concentration was 0.051 mg/L at Incline Creek near Crystal Bay and the lowest was 0.021 mg/L at several stations. Total phosphorus concentrations were observed to increase slightly in a downstream direction in the Incline, Trout, and Ward Creeks and Upper Truckee River watersheds, and decrease slightly in a downstream direction in the Edgewood Creek watershed.
Concentrations of biologically reactive iron (table 15) ranged from 8 micrograms per liter (µg/L) at Ward Creek at Highway 89 and below confluence stations to 131,000 µg/L at Highway 50 Culvert to Edgewood Creek during storm runoff. For all stations, the highest median concentration was 44,200 µg/L at Highway 50 Culvert to Edgewood Creek and the lowest was 63 µg/L at Ward Creek below confluence. For primary and secondary stations, the highest median concentration was 1,120 µg/L at Third Creek near Crystal Bay and the lowest was 63 µg/L at Ward Creek below confluence. Iron concentrations were observed to increase in a downstream direction in the Incline, Trout, and Ward Creeks and Upper Truckee River watersheds, and decrease in a downstream direction in the Edgewood Creek watershed.
Suspended-sediment concentrations (table 16) ranged from less than 1 mg/L at five stations to 12,500 mg/L at Highway 50 Culvert to Edgewood Creek during storm runoff. For all stations, the highest median concentration was 3,900 mg/L at Highway 50 Culvert to Edgewood Creek and the lowest was 3 mg/L at three stations. For primary and secondary stations, the highest median concentration was 59.5 mg/L at Third Creek near Crystal Bay and the lowest was 3 mg/L at Logan House Creek near Glenbrook and Upper Truckee River at Highway 50 above Meyers. Suspended sediment increased in a downstream direction in the Incline, Trout, and Ward Creeks and Upper Truckee River watersheds, and decreased in a downstream direction in the Edgewood Creek watershed.
Nutrient and suspended-sediment concentrations for the two index stations, Incline Creek near Crystal Bay and Upper Truckee River at South Lake Tahoe, are shown as time-series plots in figures 13 and 14. The data in these figures exhibit wide scatter, especially for the Incline Creek stations, and trends are difficult to discern. Nutrient and suspended-sediment concentrations versus streamflow for the two index stations are plotted in figures 15 and 16. The data in these plots exhibit wide scatter also; relations between concentration and streamflow are complex.
A non-parametric Wilcoxon signed-rank test (Helsel and Hirsch, 1992) was performed on the ESTIMATOR and FLUX estimates for each constituent across the 10 primary stations for the study period. Comparisons between ESTIMATOR and FLUX load estimation programs for the 10 primary stations are listed in table 17. The results of the two load estimation programs were found to be not significantly different for only three of the seven constituents tested. The range of difference was from -96 to 710 percent.
Examination of table 17 shows ESTIMATOR-FLUX monthly load differences of dissolved nitrite plus nitrate nitrogen ranged from -39 to 17 percent and the Wilcoxon test found the loads not to be statistically different. Dissolved ammonia nitrogen load differences ranged from -33 to 119 percent and the Wilcoxon test found the loads to be statistically different. Total nitrogen load differences ranged from -13 to -2 percent and the Wilcoxon test found the loads to be statistically different. Soluble reactive phosphorus load differences ranged from -78 to 4 percent and the Wilcoxon test found the loads to be statistically different. Total phosphorus load differences ranged from -65 to -3 percent and the Wilcoxon test found the results to be statistically different. Biologically reactive iron load differences ranged from -96 to 90 percent and the Wilcoxon test found the loads not to be statistically different. Suspended-sediment loads differences ranged from -33 to 710 percent and the Wilcoxon test found the loads not to be statistically different.
Summary statistics (median, maximum, and minimum) for estimated monthly loads of nutrients (from ESTIMATOR) and suspended sediment (from FLUX) for the 10 primary and 10 secondary stations for various periods of record are listed in tables 18, 19, 20, 21, 22, and 23 and are discussed below.
Estimated monthly loads of dissolved nitrite plus nitrate nitrogen for the study period ranged from 0.01 kilogram per month (kg/mo) at Logan House Creek near Glenbrook and Ward Creek below confluence to 848 kg/mo at Upper Truckee River at South Lake Tahoe. The highest median monthly load was 64.0 kg/mo at Upper Truckee River at South Lake Tahoe and the lowest median was 0.20 kg/mo at Logan House Creek. Dissolved nitrite plus nitrate nitrogen accounted for about 25 percent of the total nitrogen load for all stations.
Estimated monthly loads of dissolved ammonia nitrogen for the study period ranged from 0.01 kg/mo at Logan House Creek, Glenbrook Creek at Glenbrook, Ward Creek at Highway 89 and Ward Creek below confluence to 120 kg/mo at Upper Truckee River at South Lake Tahoe. The highest median monthly load was 10.5 kg/mo at Upper Truckee River at South Lake Tahoe and the lowest median was 0.07 kg/mo at Logan House Creek near Glenbrook. Dissolved ammonia nitrogen accounted for about 7 percent of the total nitrogen load for all stations.
Estimated monthly loads of total nitrogen for the study period ranged from 0.07 kg/mo at Logan House Creek near Glenbrook and Ward Creek below confluence to 13,100 kg/mo at Upper Truckee River at South Lake Tahoe. The highest median monthly load was 413 kg/mo at Upper Truckee River at South Lake Tahoe and the lowest median was 2.65 kg/mo at Logan House Creek near Glenbrook.
Estimated monthly loads of soluble reactive phosphorus for the study period ranged from 0.01 kg/mo at Logan House Creek near Glenbrook and Ward Creek below confluence to 392 kg/mo at Upper Truckee River at South Lake Tahoe. The highest median monthly load was 16.9 kg/mo at Trout Creek at South Lake Tahoe and the lowest median was 0.05 kg/mo at Logan House Creek near Glenbrook. Soluble reactive phosphorus load accounted for about 18 percent of the total phosphorus load for all stations.
Estimated monthly loads of total phosphorus for the study period ranged from 0.02 kg/mo at Logan House Creek near Glenbrook to 6,180 kg/mo at Blackwood Creek near Tahoe City. The highest median monthly load was 101 kg/mo at Upper Truckee River at South Lake Tahoe and the lowest median was 0.25 kg/mo at Logan House Creek near Glenbrook.
Estimated monthly loads of biologically reactive iron for the study period ranged from 0.04 kg/mo at Logan House Creek near Glenbrook to 183,000 kg/mo at Blackwood Creek near Tahoe City. The highest median monthly load was 1,160 kg/mo at Upper Truckee River at South Lake Tahoe and the lowest median was 1.30 kg/mo at Logan House Creek near Glenbrook.
Estimated monthly loads of suspended sediment for the study period ranged from 0.1 kg/mo at Logan House Creek near Glenbrook to 6,500,000 kg/mo at Blackwood Creek near Tahoe City. The highest median monthly load was 40,900 kg/mo at Upper Truckee River at South Lake Tahoe and the lowest median was 36.8 kg/mo at Logan House Creek near Glenbrook.
Plots of estimated monthly loads for nutrients and suspended sediment for the two index primary stations, Incline Creek near Crystal Bay and Upper Truckee River at South Lake Tahoe, for various periods of record are shown in figures 17, 18, 19, 20, 21, and 22. Monthly loads for Incline Creek near Crystal Bay are grouped by nitrogen (fig. 17), phosphorus (fig. 18), and biologically reactive iron and suspended sediment (fig. 19). Upper Truckee River at South Lake Tahoe monthly loads are grouped by nitrogen (fig. 20), phosphorus (fig. 21), and biologically reactive iron and suspended sediment (fig. 22). Median monthly loads for the period of record for these two stations were summarized by months and seasons and also included in figures 17-22.
The monthly load plots for Incline Creek near Crystal Bay (figs. 17, 18, and 19) show a pattern typical for the smaller streams (drainage area less than 8 mi2) in the basin. Loads during water years with below-normal precipitation (1988-92 and 1994) generally were low, whereas loads for water years of above-normal precipitation (1993 and 1995-98) were higher. The highest monthly load at this medium-smaller watershed station occurred in spring of 1995 for all nutrients and sediment except dissolved nitrite plus nitrate and ammonia nitrogen. Dissolved nitrite plus nitrate nitrogen had the highest monthly load in the spring (June) of 1996. Dissolved ammonia nitrogen had the highest monthly load in spring (June) of 1998. The highest monthly load for the non-spring period all occurred during January 1997 for all nutrients and suspended sediment.
The seasonal load plots for Incline Creek near Crystal Bay (figs. 17, 18, and 19) show the highest median seasonal loads for this station occur in the spring during periods of snowmelt. The highest median monthly loads occurred in April for dissolved nitrite plus nitrate nitrogen, soluble reactive phosphorus, and total phosphorus; May for dissolved ammonia nitrogen, total nitrogen, and suspended-sediment; and June for iron. The lowest median seasonal loads occurred during the summer period (July, August and September) for nitrite plus nitrate, soluble reactive phosphorus and total phosphorus; and the fall period (October, November, and December) for ammonia, total nitrogen, iron, and suspended sediment. The lowest median monthly loads occurred in various months but usually in October.
The monthly load plots for Upper Truckee River at South Lake Tahoe (figs. 20, 21, and 22) show a pattern typical for larger streams (greater than drainage area of 10 mi2) in the basin. Loads for water years with below-normal precipitation years (1985, 1987-92, and 1994) generally were low, whereas loads for water years of above-normal precipitation (1984, 1986, 1993, and 1995-98) were much higher. The largest monthly load for this larger watershed station occurred in January of 1997 for all nutrients and sediment except ammonia, which occurred in April 1989.
The seasonal load plots for Upper Truckee River at South Lake Tahoe (figs. 20, 21, and 22) show the highest median seasonal loads for all nutrient and suspended-sediment constituents occurred in the spring (April, May, and June) during periods of snowmelt. March also had high loads when snowmelt begins. For all constituents, the largest median monthly loads occurred in May and the lowest median seasonal loads occurred during the summer period (July, August and September). The lowest median monthly loads occurred in various months but usually in August, September, or October.
The period of comparison is 1988-98, except Edgewood Creek at Stateline, which was sampled only from 1992 to 1998 after this station was relocated in 1992. The original station, Edgewood Creek at Lake Tahoe, was located about 0.5 mi downstream and was in operation from 1989 to 1992. Because streamflow at Edgewood Creek at Lake Tahoe was affected by various diversions upstream and impoundments downstream, the stage-streamflow relation was determined to be of poor quality. Although samples were collected from this discontinued gaging station, estimated loads were not calculated due to the poor streamflow record. Median monthly loads for nutrients and suspended sediment for the 10 primary stations for the comparison period are shown in figures 23, 24, 25, 26, 27, 28, and 29.
The Upper Truckee River at South Lake Tahoe had the largest median monthly loads for five of the six nutrients with 53.0 kg/mo nitrite plus nitrate; 10.5 kg/mo ammonia; 413 kg/mo total nitrogen; 76.7 kg/mo total phosphorus; 1,200 kg/mo biologically reactive iron; and 40,900 kg/mo suspended sediment. Trout Creek at South Lake Tahoe had the largest median of monthly loads for soluble reactive phosphorus at 13.8 kg/mo. Logan House Creek near Glenbrook had the smallest median monthly loads for all nutrients and suspended sediment with 0.20 kg/mo dissolved nitrite plus nitrate, 0.07 kg/mo dissolved ammonia, 2.63 kg/mo total nitrogen, 0.05 kg/mo soluble reactive phosphorus, 0.25 kg/mo total phosphorus, 1.30 kg/mo biologically reactive iron, and 36.8 kg/mo suspended sediment.
The within-watershed comparison of summary statistics (sum, median, maximum and minimum), percent input of estimated monthly loads of nutrients and suspended sediment and percent differences in sums between 10 secondary stations the period of comparison for stations, by area, are listed in tables 19, 20, 21, 22, 23, and 24. The five watersheds that are compared are Incline Creek, Trout Creek, and Upper Truckee River, 1990-98; and Edgewood Creek and Ward Creek, 1992-98.
For each watershed, summary statistics were calculated for runoff and constituent loads using methods described in Helsel and Hirsch (1992). Median values were chosen as preferable summary statistics because they are not strongly influenced by extreme values. For the within-watershed comparison for the five multiple-station watersheds (Incline, Edgewood, Trout and Ward Creeks and Upper Truckee River), monthly loads were compared among stations. Each multiple-station watershed was measured at no less than three points: an upstream station in the headwaters, a middle station between the headwaters and the mouth, and a downstream station at or near the mouth of the stream. For each watershed, the monthly load for the upstream station was considered an "input" amount. The differences in monthly loads between the upstream/middle and middle/downstream stations were then calculated. These values were divided the downstream station load to calculate the percent monthly load contributed by the reach in question. Where more than one station was measured in the headwaters part of the watershed, all headwater stations were totaled as the upstream input.
Monthly nutrient and suspended load plots and the hydrograph for the comparison period (1991-98) for the two index watersheds (Incline Creek and Upper Truckee River) are shown in figures 30 and 31. The three Incline Creek watershed stations (near mouth = near Crystal Bay, middle = at Highway 28, and upstream = above Tyrol Village) are shown in figure 29 and the three Upper Truckee River watershed stations (near mouth = at South Lake Tahoe, middle = above Meyers, and upstream = at South Upper Truckee Road) are shown in figure 31.
The upper Ward Creek watershed station (below confluence) had largest inputs, in terms of percentage, from the upstream watershed area for runoff (60 percent of the total watershed runoff comes from above this station), nitrite plus nitrate (75 percent), ammonia (70 percent) and soluble reactive phosphorus (43 percent). Edgewood Creek at Palisade Drive had largest inputs, in terms of percentage, for total nitrogen (49 percent), total phosphorus (47 percent), iron (56 percent) and suspended sediment (91 percent). The station with the largest average input percentage for all eight constituents was Ward Creek below confluence at 46 percent. The station with the lowest average input percentage for all eight constituents was Eagle Rock Creek (the other Edgewood Creek watershed input station) at 11 percent. The upstream Edgewood Creek watershed stations are not in direct sequence and both are considered "input" stations, so were included in an upstream/ downstream station comparison. Edgewood Creek at Stateline station did not show increases from the combined two upstream "input" stations, except with suspended sediment. A portion of the suspended-sediment load in the Edgewood Creek watershed may be stored in a pond upstream of Edgewood Creek at Stateline station.
A summary of average percent changes of sums for the five multiple-station watersheds is listed in table 24. Four of the multiple-station watersheds showed increases in average percent gains going downstream, between the upstream "input" and middle station and the middle and downstream "near mouth" stations. Ward Creek watershed had decreasing percent gains going downstream, with the largest average percent gain coming from above the upstream station. The largest increase between the upstream (input) and middle stations occurred in Trout Creek watershed at 36 percent. The largest increase between the middle and downstream (near mouth) stations occurred in Upper Truckee River watershed at 42 percent. Two watersheds had losses (minus percent) between stations for two constituents. Edgewood Creek watershed had a -34 percent change in suspended sediment between the downstream and two combined upstream stations (table 20). Ward Creek watershed had a -3 percent change in dissolved ammonia between middle and downstream stations (table 23); this may be due to the presence of beaver dams between the stations.
Tables of monthly and daily load estimation values, along with statistical estimates (SE, SE PRED, and 95-percent confidence intervals), for periods of record for the 20 primary and secondary stations are presented in appendix 2.
Median monthly yields, in kilograms per square kilometers per month (kg/km2/mo), were calculated by dividing median monthly loads by drainage area, in square kilometers. The resulting yields were compared and ranked for each constituent for each of the 10 sampled watersheds. Median monthly yields for each constituent were assigned a rank 1 for the highest yield to a rank 10 for the lowest yield. Overall ranks were determined by averaging the ranks of all seven constituents for each watershed station and ranking these average ranks among the stations.
Median monthly yields for nutrients and suspended sediment for the 10 primary stations are listed with load summary data in table 18 and shown in figures 32, 33, 34, 35, 36, 37, and 38. Unit values for average annual runoff for the period of record are listed in table 4 and shown in figure 10.
Comparing primary stations for the study period (1988-98), Incline Creek near Crystal Bay had the largest median monthly yields for dissolved nitrite plus nitrate, 0.56 kg/km2/mo, and soluble reactive phosphorus, 0.30 kg/km2/mo. Third Creek had the largest median monthly yields for total nitrogen, 7.50 kg/km2/mo, total phosphorus, 1.55 kg/km2/mo, biologically reactive iron, 45.9 kg/km2/mo, and suspended sediment, 1,360 kg/km2/mo. Edgewood Creek at Stateline had the largest yield for ammonia, 0.12 kg/km2/mo. Logan House Creek had the smallest yields for all nutrients and sediment with nitrite plus nitrate, 0.04 kg/km2/mo, ammonia, 0.01 kg/km2/mo, total nitrogen, 0.49 kg/km2/mo, soluble reactive phosphorus, 0.01 kg/km2, total phosphorus, 0.05 kg/km2/mo, biologically reactive iron, 0.24 kg/km2/mo, and suspended sediment, 17.3 kg/km2/mo.
Median monthly suspended-sediment yields were highest for Third Creek, followed in order by Incline Creek, Blackwood Creek, and the Upper Truckee River. These yields are similar to another suspended-sediment study from 1981 to 1985 on nine Lake Tahoe Basin watersheds by Hill and Nolan (1988). This study found that the highest annual suspended-sediment yields were from, in descending order, Blackwood Creek, Ward Creek, Upper Truckee River, and Third Creek. Note that eight of their stations are existing LTIMP stations.
Ranks of median monthly yields for each primary station for each constituent are shown along with yields in figures 31, 32, 33, 34, 35, 36, and 37. The overall average ranks are shown in figure 39. Incline Creek had the highest average rank of median monthly yield and Logan House Creek near Glenbrook had the lowest. Edgewood Creek actually came in third, but was downgraded to fourth as the comparison period for that watershed was only from 1992 to 1998.
Median monthly nutrient and suspended-sediment yields and average annual unit runoff data for the five multiple stations watersheds are presented with the load summary data in tables 19, 20, 21, 22, and 23. All five multiple-station watersheds decreased in unit runoff from the upstream station to the downstream station, except Edgewood Creek. For example, Incline Creek decreased from 1,180 acre-ft/mi2 at the upstream site to 1,040 acre-ft/mi2 at the middle station to 930 acre-ft/mi2 at the downstream (near mouth) station, which is typical for the smaller streams. Yields for dissolved nitrite plus nitrate nitrogen increased slightly to moderately in downstream order in four of five watersheds. Ward Creek stations decreased from 0.50 to 0.22 kg/km2/mo between the upstream and downstream stations. Dissolved ammonia nitrogen yields increased slightly in four watersheds between the upstream and downstream stations, with Ward Creek showing a decrease. Total nitrogen and total phosphorus yields increased between the upstream and downstream stations in four watersheds but decreased in Edgewood Creek. Soluble reactive phosphorus yields increased in Incline, Edgewood and Ward Creeks and decreased in Trout Creeks and Upper Truckee River. Iron increased between the upstream and downstream stations in all five watersheds while suspended sediment increased in Incline, Trout and Upper Truckee River watersheds and decreased in the Ward and Edgewood Creeks.
Trends in constituent concentrations over time from the Seasonal-Kendall test for all 10 primary and 10 secondary stations are summarized in table 25 and for the 10 primary stations by constituent in figures 40, 41, 42, 43, 44, 45, and 46. Over time 69 percent of constituent concentrations decreased, 26 percent had no significant trend, and 5 percent increased.
Comparison of the primary stations (fig. 40) showed five stations with decreasing trends in dissolved nitrite plus nitrate nitrogen concentrations and five stations with no significant trends. Seven stations had decreasing trends in dissolved ammonia nitrogen concentrations and three stations had no significant trends. Six stations had decreasing trends in total nitrogen concentrations and four had no significant trends. Four stations had decreasing trends in soluble reactive phosphorus concentrations and six had no significant trends. Nine of the 10 stations had decreasing trends in total phosphorus concentrations and one had no significant trend. Three stations in General, Blackwood, and Ward Creeks had increasing trends in biologically reactive iron, four had decreasing trends, and three had no significant trends. The same three stations had increasing trends in suspended-sediment concentrations, four stations had decreasing trends, and three stations had no significant trends.
Decreasing trends in undeveloped watersheds, such as Logan House Creek, may be due to a variety of causes, although none are clearly indicated. Potential causes include changes in weather patterns during the study period, such as the lack of intense summer thunderstorms. Cooler spring snowmelt periods during the latter years of the study led to longer, more moderate runoff patterns, in general, and may have resulted in less channel erosion. In addition, decreasing trends for developed watersheds also may be due to the increased restoration projects and installation of best management practices, resulting in overall greater efficiency in these watersheds. Increasing concentrations in biologically reactive iron and suspended sediment at the three stations noted previously seem to be based primarily on samples collected during and after the January 1997 flood.
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