SURFACE- AND GROUND-WATER CONDITIONS

Streamflow and Seepage Estimates

The measured streamflows in the Upper Truckee River watershed ranged from zero to 11.6 ft³/s (table 3). Streamflow measured along the main stem of the Upper Truckee River increased from 2.6 ft³/s at site 45 to 11.6 ft³/s at site 3 in a downstream direction (fig. 3, pl. 1). At site 1, flow was not measured because the river was too deep to wade and velocities too slow for an accurate measurement due to backwater effects caused by the high level of Lake Tahoe on September 23, 1996. Three of the 13 remaining main-stem sites had no streamflow because these sites were on dry, divergent branches of the main stem. Of the 31 streamflow measurement sites that are tributary to the Upper Truckee River or are along the tributaries, 13 had no measurable streamflow. Major tributary inflows to the Upper Truckee River on September 23, 1996, included 2.0 ft³/s at sites 27 and 28 (sum of divergent flows in tributary), 0.9 ft³/s at site 37, and 0.8 ft³/s at site 12.

The measured streamflows in the Trout Creek watershed ranged from 0 to 23.0 ft³/s. Streamflow measured along the main stem of Trout Creek increased from 4.7 ft³/s at site 70 to 23.0 ft³/s for combined sites 46-48 (fig. 3, pl. 1). All the main stem sites had streamflow. Of the 14 streamflow measurement sites that are tributary to Trout Creek or are along the tributaries, only 1 (site 55) had no measurable streamflow. Major tributary inflows to Trout Creek on September 26, 1996, included 11.2 ft³/s at site 53, 2.4 ft³/s at site 61, 1.4 ft³/s at site 67, and 0.8 ft³/s at site 66.

The streamflows measured in September 1996, for both watersheds, were representative of base-flow conditions from August through December. The smallest daily streamflow for the 1996 water year at the most downstream gage on the Upper Truckee River (site 6) was 7.7 ft³/s in late October 1995 (Bostic and others, 1997, p. 268). The lowest monthly mean streamflow, 10.2 ft³/s, occurred during November 1995. The lowest daily streamflow for the 1996 water year at the most downstream gage on Trout Creek (site 52) was 14.0 ft³/s in late December 1995. The lowest monthly mean streamflow occurred during November 1995, and was 22.0 ft³/s (Bostic and others, 1997, p. 333). Hydrographs for these two streamgages are shown in figure 7, along with the daily precipitation record for a nearby NRCS SNOTEL site.

All streamflow data were entered into the USGS National Water Information System (NWIS) databases. Streamflow measurement data for September 1996 also appear in the annual data report for Nevada (Bostic and others, 1997).

Results of the streamflow measurements, seepage estimates, seepage rates per unit length, and unit runoffs are listed in table 4. In addition, the location of gaining, losing, and steady reaches are shown on plate 1 and figure 3.

Seepage estimates for reaches along the Upper Truckee River indicate that, of the 11.6 ft³/s streamflow near the mouth at site 3, 4.4 ft³/s (38 percent) was gained from ground-water seepage to the main stem, 4.5 ft³/s (39 percent) was gained from tributary inflows, and 2.6 ft³/s (23 percent) was the beginning streamflow at site 45. The average rate of gain per unit length along the main stem over the distance from site 45 to site 3 (13.3 mi) was 0.33 ft³/s per mile. Of the 10 reach segments along the main stem of the Upper Truckee River, 5 were gaining from ground-water seepage, 1 was losing due to ground-water seepage, 3 had no measurable influence due to ground water, and 1 was undetermined because a streamflow measurement was not possible (fig. 8).

Seepage estimates for reaches along Trout Creek indicate that, of the 23.0 ft³/s streamflow near the mouth at sites 46-48, 0.7 ft³/s (3 percent) was gained from ground-water seepage to the main stem, 17.4 ft³/s (76 percent) was gained from tributary inflows, and 4.7 ft³/s (21 percent) is the beginning streamflow at site 70. The average rate of gain per unit length along the main stem over the distance from site 70 (8.9 mi) to sites 46-48 was about 0.08 ft³/s per mile. Of the six reach segments along the main stem of Trout Creek, three were gaining from ground-water seepage, two were losing, and one had no measurable loss or gain (fig. 8).

The Upper Truckee River and Trout Creek have similar characteristics in the location of ground-water seepage contributions to their streamflows. Both streams are gaining in their upper reaches, both are steady or losing through their middle reaches, and both gain streamflow over a mile reach starting at about 2.5 mi upstream from Lake Tahoe (fig. 8).

The value obtained when discharge is divided by contributing drainage area, termed unit runoff, is often useful in comparing the magnitude of flow between two basins or the discharge at two or more locations in one basin. Unit runoff along the main stem of the Upper Truckee River ranged only slightly from 0.21 to 0.23 ft³/s/mi² while its tributaries had greater variation, from zero at many of the tributaries to 0.31 ft³/s/mi² at site 28 (table 3). Unit runoff along the main stem of Trout Creek ranged from 0.56 ft³/s/mi² in the lower reaches to 0.84 ft³/s/mi² in the upper reaches while its tributaries ranged from 0.07 ft³/s/mi² at site 50 to 1.00 ft³/s/mi² at site 67. The unit runoff in Trout Creek is larger than that of the Upper Truckee River. This is because most of the streamflow into Trout Creek is from the Cold Creek tributary whose unit runoff is 0.88 ft³/s/mi². The high unit runoff of the Cold Creek tributary is assumed to be from delayed snowpack melt because the drainage has a significant percentage of north-facing aspect (Peltz and others, 1994) or because the capacity of ground-water storage within the Cold Creek watershed is large.

Ground-Water Levels and Direction of Flow

The distribution of inventoried wells, their water use, and geology (consolidated rock or unconsolidated basin fill) of the study area are shown in figure 4. Because of the lack of drillers' reports for many wells, the distribution of wells completed in unconsolidated basin-fill deposits or consolidated rock is unknown. For wells with drillers' reports, most are completed in basin-fill deposits (unconsolidated) with a few wells completed in fractured granite (consolidated).

The median depth to water on the basis of measurements from 60 non-pumping wells was 12.7 ft below land surface and ranged from 1.33 ft below land surface at well 94 to 69.85 ft below land surface at well 137. Depths to water were generally shallow in observation wells in meadows and particularly along the meadow near the mouth of Angora Creek where it is tributary to the Upper Truckee River. Depths to water were the greatest in observation wells in the old landfill near Meyers.

Well locations, results of seepage estimates, water-level contours, generalized directions of ground-water flow, and consolidated and unconsolidated geology are shown on plate 1. Water-level contours derived from measured water levels and results of seepage estimates are represented on plate 1 by solid lines; contours determined by using a median depth to water of about 13 ft are represented on plate 1 by dashed lines. The interpretation of the water-level contours in areas with wells that have land-surface altitudes determined from topographic maps has an inherent uncertainty due to uncertainties associated with the water-level altitudes. In steeply sloping terrain, the horizontal uncertainty of the water-level contours is small. In the more gently sloping terrain, where the topographic-contour interval is 40 ft, this horizontal uncertainty can be greater. Where the topographic-contour interval is 20 ft or less, the horizontal uncertainty is less. Water-level contours exist only in the unconsolidated sediments of the study area and do not cross consolidated rock. The water-level contour interval on plate 1 is variable and in general increases to the south, from about 10 ft in South Lake Tahoe to 200 ft along Highway 89 near Luther Pass.

Ground-water altitudes (pl. 1) in the Upper Truckee River watershed range from about 6,220 ft at well 76 in the northern part of the study area near Lake Tahoe to 7,250 ft at well 130 in the southern part of the study area. Ground-water altitudes in the Trout Creek watershed range from 6,190 ft at well 137 in the northern part of the study area to 6,380 ft at well 148 in the old Meyers landfill. Ground-water altitudes in the study area generally mimic the topography, with higher altitudes in the upland areas and lower altitudes near Lake Tahoe.

Ground-water levels in two wells in the study area (wells 73 and 131) have been monitored by California Department of Water Resources since June 1962. These two wells have responded to climatic variations such as drought and wet years (fig. 9).

In general, ground water in the study area is flowing northward toward Lake Tahoe (pl. 1) and parallels surface-water flow. Ground water generally discharges to the Upper Truckee River and Trout Creek along the upper reaches, whereas in the middle reaches ground water is flowing parallel to both streams. In the middle reach, the Upper Truckee River is losing streamflow for about 1.9 mi and Trout Creek has a net loss over its middle reaches. Ground water discharges to both streams between river miles 1.5 and 2.8 as both streams have a net gain in streamflow that is not accounted for by tributary flows. Both streams show little gain in flow further downstream suggesting that little ground water discharges to the two streams close to Lake Tahoe (table 4, pl. 1).

From July to November 1996, altitude of ground water in wells in the area between Lake Tahoe and Highway 50 (about river mile 1.5 on both stems) was nearly the same as the lake-surface altitude (table 5, pl. 1). This suggests that the ground-water flow beneath the Upper Truckee River and Trout Creek drainages between Highway 50 and Lake Tahoe was minimal during the study. Much of the ground-water discharge in these drainages was to the channels of the Upper Truckee River and Trout Creek upstream from Highway 50 (pl. 1).

Hydraulic gradients in the study area upstream from Highway 50 ranged from 10 to 1,400 ft/mi. Hydraulic gradients in the Upper Truckee River watershed are greatest in the upland areas. For example, the gradient near Luther Pass is 700 to 1,400 ft/mi. Hydraulic gradients tend to decrease rapidly in the lower areas, such as Christmas Valley, where gradients ranged from 30 to 60 ft/mi. In the Tahoe Paradise area, the hydraulic gradients ranged from about 20 to 40 ft/mi. In the northern part of the study area, the hydraulic gradients ranged from 10 ft/mi along the Upper Truckee River near the airport to as much as 50 ft/mi in the South Lake Tahoe area near the intersection of Highway 50 and Highway 89 in the Upper Truckee River watershed. The hydraulic gradients in the Trout Creek watershed ranged from about 420 ft/mi for areas along Saxon Creek to about 20 ft/mi along the lower reaches of Trout Creek upstream from the confluence of Heavenly Valley Creek. In the South Lake Tahoe area of the Trout Creek watershed, just south of Highway 50, the hydraulic gradient is about 30 ft/mi except in the area of well 137, where a cone of depression is caused by municipal pumping (Woodling, 1987, p. 21) and the hydraulic gradient is as high as 300 ft/mi. Hydraulic gradients vary on either side of the large lateral glacial moraine that divides the Trout Creek watershed from the Upper Truckee River watershed. Hydraulic gradients ranged from 170 to 1,300 ft/mi on the west side of the moraine and from 60 to 730 ft/mi on the east side.

Water Quality

Specific conductance of surface-water samples from sites in the Upper Truckee River watershed on September 23, 1996, ranged from 31 µS/cm at site 16 to 148 µS/cm at site 19 (table 3). Specific conductance in the main channel of the Upper Truckee River increased in a downstream direction from 50 µS/cm at site 45 to 99 µS/cm at site 15 and then remained relatively constant from site 15 to site 1 with a range of only 96 to 99 µS/cm (fig. 10). The relatively large increase in specific conductance with downstream direction for the upper half of the Upper Truckee River was probably caused by the relatively large component of higher conductance ground water contributing to the rivers streamflow for this segment (fig. 3, tables 3 and 4). The lower half of the Upper Truckee River has relatively constant specific conductance probably because streamflow has almost no gain from ground-water seepage for this segment (fig. 3, tables 3 and 4). The specific conductance values found along the main stem of the Upper Truckee River during base-flow conditions are similar to the highest values found during the 1996 water year. For the 1996 water year, specific conductance ranged from 22 to 96 µS/cm at site 6 near the mouth to 14 to 51 µS/cm at site 43 (Bostic and others, 1997, p. 263-269). Specific conductances are usually greatest during the low streamflow of late summer through fall and immediately following some storms prior to snowmelt. Specific conductances are lowest during snowmelt runoff, which generally peaks in late spring through early summer.

Specific conductance of surface-water samples from sites in the Trout Creek watershed on September 26, 1996, ranged from 43 µS/cm at site 53 to 92 µS/cm at site 58 (table 3). The specific conductance measured in the main channel of Trout Creek ranged from 49 to 54 µS/cm (fig. 10). The lack of increase in specific conductance with downstream direction in Trout Creek as compared with the Upper Truckee River might be due to the minimal contribution of ground-water seepage to streamflow. The specific conductance values found along the main stem of Trout Creek during base-flow conditions are similar to the highest values found during the 1996 water year. For the 1996 water year, specific conductance ranged from 25 to 54 µS/cm at site 49 near the mouth and from 19 to 53 µS/cm at site 68 (Bostic and others, 1997, p. 329 and 334). Specific conductances also are the greatest during the low-flow periods of late summer through fall and the smallest during snow melt runoff in late spring to early summer.

Specific conductance of surface-water samples for the three Upper Truckee River water-quality sites from early July through mid-December 1996, ranged from 17 µS/cm at site 43 to 101 µS/cm at site 17 (table 6). Specific conductances for the three Trout Creek water-quality network sites for the same period ranged from 31 µS/cm at site 68 to 55 µS/cm at site 57.

Specific conductance of ground-water samples for wells in the Upper Truckee River and Trout Creek watersheds from mid-July through mid-December 1996, ranged from 94 µS/cm at well 137 to 542 µS/cm at well 143 (table 7). As stated earlier, the water-quality results from well 143 may not represent the overall ground-water conditions due to the proximity of the old Meyers landfill. The next highest value of specific conductance is 305 µS/cm at well 135. Specific conductances varied in only two wells between summer and fall samples (fig. 11). Specific conductance did not appear to have any trend with respect to distance from Lake Tahoe (fig. 11).

Water temperatures measured at streamflow sites in the Upper Truckee River watershed on September 23, 1996, ranged from 4.5°C at site 14 to 13.5°C in the lower reaches of the main channel at site 6 (table 3). The main channel water temperatures generally increased in a downstream direction. Water temperatures ranged from 6.0 to 9.5°C at the upper sites and ranged from 11.5 to 13.5°C at the lower sites. Water temperatures can be affected by air temperatures, which ranged from 3.5°C in the morning to 25.0°C in the afternoon. Water temperature measured at streamflow sites in the Trout Creek watershed on September 26, 1996, ranged from 5.0°C at site 68 to 11.5°C near the mouth at sites 46 and 48. Water temperatures also increased in a downstream direction with a range of 5.0 to 6.5°C in the upper reaches to 7.0 to 11.5°C in the lower reaches. The air temperatures ranged from 9.5°C in the morning to 30.0°C in the afternoon. Weather was clear and warm on both days of the seepage run.

Water temperatures for the six surface-water-quality sites in the Upper Truckee River and Trout Creek watersheds ranged from 0.5°C at site 49 in early December to 16.0°C at site 6 in mid-July. Water temperatures of ground water for the seven wells in both watersheds ranged from 8.0°C at wells 71 and 77 in late November and mid-December to 14.5°C at well 97 in mid-July. Ground-water temperatures varied seasonally by more than a half degree Celsius at only three wells (fig. 10).

Values of pH in surface water for the six sites for the Upper Truckee River and Trout Creek had a narrow range from 6.6 at site 57 to 7.8 at sites 6, 17, and 43 (table 6). Values of pH in ground water for the seven wells in both watersheds had a greater range from 5.5 at well 71 to 9.0 at well 80 (table 7). About 53 percent of ground-water quality sites had pH values from 6 to 8. Determination of the cause of this variability is beyond the scope of this study. The variation of values between summer and fall samples were small except for well 71, which varied by 1 pH unit (fig. 11). Values of pH did not appear to have any trends with respect to distance from Lake Tahoe (fig. 11).

Nutrient data collected from the six surface-water-quality sites for July through December 1996 are listed in table 6. Nitrite plus nitrate (NO₂+NO₃) concentrations ranged from 0.002 to 0.036 mg/L. The NO₂+NO₃ concentrations are well below the USEPA drinking water standard of 10 mg/L (U.S. Environmental Protection Agency, 1996). Ammonia nitrogen (NH₄) concentrations ranged from less than the detection limit of 0.001 to 0.013 mg/L. Kjeldahl (NH₄ plus organic nitrogen) concentrations ranged from 0.04 to 0.51 mg/L. Phosphorous (P) concentrations ranged from 0.014 to 0.241 mg/L. Orthophosphorus concentrations ranged from 0.003 to 0.032 mg/L. Bioreactive iron (Fe) concentrations ranged from 45 to 2,650 µg/L. Some of these extreme values were from samples collected during storms and are not representative of normal flow conditions. Samples collected during storms were not used in the summary statistic comparisons between surface- and ground-water quality in figure 6 because they were not randomly collected.

Nutrient data collected from nine ground-water-quality sites in July through December 1996 are listed in table 7. NO₂+NO₃ concentrations ranged from 0.002 to 3.24 mg/L. Three samples, all from the Trout Creek watershed, were greater than 1.8 mg/L, whereas, 75 percent of concentrations were below 0.76 mg/L. These NO₂+NO₃ concentrations are below the USEPA drinking water standard of 10 mg/L (U.S. Environmental Protection Agency, 1996). Ammonia (NH₄) concentrations ranged from 0.001 to 0.523 mg/L, with 75 percent below 0.2 mg/L. Kjeldahl (ammonia plus organic nitrogen) concentrations ranged from less than 0.01 to 1.7 mg/L, with two samples greater than 1.2 mg/L and 75 percent below 0.18 mg/L. Phosphorus (P) concentrations ranged from 0.018 to 0.101 mg/L, with 75 percent below 0.06 mg/L. Phosphorus concentrations were lower in samples collected in the fall than in the summer at all wells. Orthophosphorus concentrations ranged from 0.010 to 0.067 mg/L, with 75 percent below 0.032 mg/L. Bioreactive iron (Fe) concentrations ranged from 4.3 to 8,800 µg/L, with 75 percent of samples having concentrations below 32 µg/L. The highest values of the ammonia species of nitrogen nutrients occurred in one shallow observation well near the Truckee Marsh (well 71). Ground water from this well also had high concentrations of ammonia in the 1995 and 1996 water years (Bauer and others, 1996; Bostic and others, 1997). These high values probably are due to decomposition of organic material from the wetland.