WRIR
00-4001
Surface- and Ground Water Characteristics in the Upper Truckee River and Trout
Creek Watersheds
Lake Tahoe is an outstanding natural resource and known for its deep, clear waters. Increased nutrient concentrations within Lake Tahoe are considered the leading cause of algal growth and loss of clarity in the lake. Surface- and ground-water discharge throughout the Lake Tahoe Basin are assumed to be significant mechanisms for nutrient transport to Lake Tahoe. The Tahoe Regional Planning Agency has primary responsibility for the environmental protection of Lake Tahoe with an emphasis on reducing the loss of lake clarity in Lake Tahoe.
The Upper Truckee River and Trout Creek watersheds are the two largest watersheds and have the greatest areas of urban land use within the Lake Tahoe Basin. In 1996, the USGS, in cooperation with TRPA, began a study to improve the understanding of the surface-water and ground-water systems and their interactions within the Upper Truckee River and Trout Creek watersheds.
The contribution of ground water to surface-water streamflow, the unit runoff, the general direction of ground-water flow, and the comparisons of water quality from the surface-water system to the ground-water system during a period of minimal snowmelt runoff for the Upper Truckee River and Trout Creek watersheds were evaluated. Streamflow and water-quality data were collected at existing and supplemental surface-water streamflow and water-quality sites and water-level and water-quality data were collected at existing and supplemental ground-water sites.
Seepage estimates were determined for the Upper Truckee River and Trout Creek by measuring streamflow at designated sites used to define reach segments. Seepage gains and losses were determined for the selected reaches by subtracting the sum of the flow at the upstream end of the reach plus any tributary inflows from the flow at the downstream end of the reach. Unit-runoff values were determined by normalizing streamflow to contributing drainage-area size. Specific conductance and water temperature were determined at the time of streamflow measurements to provide synoptic field water-quality conditions for both watersheds.
Water levels were determined for wells within the study area and were used to produce a water-level altitude map, to determine directions of ground-water flow, and to determine hydraulic gradients.
Samples from six surface-water-quality and eight ground-water-quality sites were collected for nutrient species and iron as well as the basic field measurements of specific conductance, pH, and water temperature. Summary statistics for the chemical and field data were computed for surface- and ground-water-quality sites.
Streamflows measured during the seepage run were during a base flow period for both the Upper Truckee River and Trout Creek. All but 3 of the 13 streamflow measurement sites on the main stem of the Upper Truckee River had measurable streamflow. The three dry sites were divergent branches of the main stem. Forty-eight percent of the streamflow measurement sites that are tributary to the Upper Truckee River or along the tributaries were dry. All the streamflow measurement sites on the main stem of Trout Creek had measurable flow. Only one of the streamflow sites measured in the Trout Creek watershed was dry. This indicates that streamflows in the Trout Creek watershed are more perennial than those in the Upper Truckee River.
The largest tributary inflow into the Upper Truckee River was from Grass Lake Creek, which accounted for 17 percent of the total flow near the mouth of the Upper Truckee River. The largest tributary inflow into Trout Creek was Cold Creek, which accounted for 49 percent of the total flow near the mouth of Trout Creek.
The Upper Truckee River has greater ground-water seepage contributing to its overall streamflow than Trout Creek, while Trout Creek has greater tributary inflows contributing to its overall streamflow. Both streams had a similar proportion of streamflow at their uppermost main stem sites (when computed as a percentage of the most downstream sites). The total streamflow of the Upper Truckee River near its mouth was 38 percent ground-water seepage to the main stem, 39 percent tributary inflows, and 23 percent was the streamflow at the uppermost main stem site. The total streamflow of Trout Creek near its mouth was 4 percent ground-water seepage to the main stem, 76 percent tributary inflows, and 20 percent streamflow from the upper most main stem site.
Both the Upper Truckee River and Trout Creek had streamflow that was gaining from ground-water seepage in their upper reaches, both were steady or losing to ground-water seepage in their middle reaches, and both were again gaining flow from ground-water seepage in their lower reaches.
Unit runoff values for the Upper Truckee River watershed were less than those of the Trout Creek watershed. This was mainly due to the large contribution of flow from the Cold Creek tributary to Trout Creek. The large unit runoff of the Cold Creek tributary is assumed to be due to protracted snowmelt resulting from the high percentage of north-facing aspect or due to the delayed release of ground water from storage.
The median depth to water in the study area during this period was 12.7 ft below land surface with a range of 1.33 to 69.85 ft below land surface. Depths to water were generally least in meadows and greatest in the old Meyers landfill. Ground-water altitudes in the study area ranged from 6,190 to 7,250 ft and generally mimicked the land-surface topography.
Ground-water in the study area generally flows parallel to surface water. In the upper reaches of both watersheds, ground water flows towards the stream channels and in the middle reaches it flows parallel to the main channels. In the lower reaches near Lake Tahoe, ground-water levels and the water level at Lake Tahoe are nearly equal resulting in a very small hydraulic gradient. This suggests that ground-water discharge directly to Lake Tahoe is minimal.
Hydraulic gradients in the study area varied greatly, ranging from nearly zero to 1,400 ft/mi upstream from Highway 50. Hydraulic gradients were the greatest in upland areas and least near Lake Tahoe and along the middle reaches of the main stems of both streams.
The specific conductance of surface water measured during the seepage study had a greater range in the Upper Truckee River watershed than in the Trout Creek watershed and was generally greater in value also. In the Upper Truckee River watershed, specific conductance ranged from 31 to 148 µS/cm and in the Trout Creek watershed it ranged from 43 to 92 µS/cm. The specific conductance of water in the upper half of the main stem of the Upper Truckee River increased in the downstream direction and was consistent for the lower half. The specific conductance for Trout Creek was consistent throughout the length of its main stem. This is likely attributed to the larger ground-water seepage component of total streamflow in the upper half of the Upper Truckee River than in Trout Creek.
Specific conductance for surface water was much less than that of ground water and had a much smaller range. Specific conductance for the six surface-water-quality sites for the period of study ranged from 17 to 101 µS/cm. Specific conductance for the ground-water-quality sites for the same period ranged from 94 to 305 µS/cm for wells that were considered representative of general ground-water conditions.
Temperature of surface water measured during the seepage study was generally lowest at upstream sites and highest at downstream sites in both the Upper Truckee River and Trout Creek. The overall range was 4.5 to 13.5°C. Air temperatures ranged from 3.5 to 30.0°C during the seepage study.
Median values of water temperature for both surface water and ground water were similar. Surface-water temperatures (0.5 to 16.0°C) had a significantly greater range than those measured in ground water (8.0 to 14.5°C).
Median values of pH for surface and ground water were similar; however, pH ranges for ground water (5.5 to 9.0) were significantly greater than those measured for surface water (6.6 to 7.8).
Concentrations of nitrite plus nitrate, ammonia, and orthophosphorus were greater for the ground-water samples than for the surface-water samples collected. Concentrations of bioreactive iron were generally greater for ground-water samples than for surface-water samples. Both surface- and ground-water samples had similar concentrations of phosphorous and kjeldahl (ammonia plus organic nitrogen). Ground water typically had greater variation in nitrite plus nitrate, ammonia, kjeldahl, and bioreactive iron concentrations than surface water. Surface- and ground-water samples had similar variations in phosphorous and orthophosphorus.
The most important results of this study are that, even though the Upper Truckee River and Trout Creek share many similarities in geology, vegetation, land use, and location, they had significantly different characteristics with respect to their interactions with the ground-water system. In particular, 38 percent of the streamflow of the Upper Truckee River near its mouth originated from ground-water seepage to its main channel while that of Trout Creek was only 4 percent. Ground-water contribution to streamflow also can be seen in the field measurement of specific conductance because ground water generally has greater conductivity. At the upper sites of both streams, specific conductance values are similar. However, the specific conductance increased in the downstream direction along the upper half of the Upper Truckee River but remained relatively constant along the lower half of the main stem. Specific conductance remained fairly constant for the entire length of Trout Creek.
Another important result is that during July to November 1996, the altitude of ground-water between Lake Tahoe and Highway 50 was nearly the same as the lake-surface altitude. This suggests ground-water discharge beneath the Upper Truckee River and Trout Creek drainages directly to Lake Tahoe was minimal and that much of the ground-water discharge was to the channels of the Upper Truckee River and Trout Creek upstream from Highway 50.
Table
of Contents||Next Section: References Cited
USGS Water home page || USGS
home page
The pathname for this page is <summary.htm>
These pages are maintained by S.C. DeMeo <email: scdemeo@usgs.gov>
Last modified: Wednesday, April 12, 2000