Background

The Tahoe Regional Planning Agency (TRPA) is a bi-state resource management agency that has primary responsibility for the environmental protection of Lake Tahoe. TRPA's principal mission is to reduce the loss of clarity in Lake Tahoe. TRPA oversees the monitoring of existing environmental conditions in the basin through a number of programs. The U.S. Geological Survey (USGS) began a tributary discharge and water-quality monitoring study in 1988 in cooperation with TRPA. TRPA and USGS also instituted a cooperative ground-water monitoring study during 1990-92. A revised ground-water study was reinstated in 1995. Both of these ongoing water-quality data-collection efforts include the involvement of the University of California-Davis, Tahoe Research Group (TRG) and are included in the Lake Tahoe Interagency Monitoring Program (LTIMP). LTIMP was formed in 1978 with 12 State and Federal agencies and TRG (Goldman and others, 1986). Agencies currently participating in LTIMP include TRG; USGS; TRPA; U.S. Forest Service (USFS); U.S. Natural Resources Conservation Service (NRCS); U.S. Environmental Protection Agency (USEPA); Lahontan Regional Water Quality Control Board; California Department of Parks and Recreation; California Department of Fish and Game; California Tahoe Conservancy; Nevada Department of Environmental Protection; University of Nevada, Reno; Douglas County, Nev.; El Dorado County, Calif.; Washoe County, Nev.; and the City of South Lake Tahoe, Calif.

The current USGS-TRPA networks include 32 surface-water sites where suspended sediment, water-quality, and streamflow data are collected, and 32 ground-water sites, where water-quality and water-level data are collected. These surface- and ground-water sites are located throughout the Lake Tahoe Basin. Both of these networks are described in more detail by Boughton and others (1997) and are shown on a map by Rowe and Stone (1997). From these two networks, six surface-water sites and eight ground-water sites were used from within the study area.

In 1996, TRPA developed the Lake Tahoe Federal Legislative Agenda, a public-private partnership of agencies in the Tahoe region (Tahoe Regional Planning Agency, 1996). The plan designated four Lake Tahoe watersheds as high priority for possible watershed restoration projects. TRPA included Third and Incline Creek watersheds in the north, Edgewood Creek watershed in the southeast, and the Upper Truckee River watershed in the south.

In 1996, the Upper Truckee River watershed was chosen for a focused effort to improve water quality within one watershed of the Lake Tahoe Basin. An advisory group, the Upper Truckee River Watershed Focused Group, was formed as a subgroup of LTIMP.

The Upper Truckee River watershed is the largest of the 63 watersheds in the Lake Tahoe Basin. The Upper Truckee River also delivers the largest volume of surface water and may be providing some of the largest nutrient and sediment loads to the lake. Also, the Upper Truckee River watershed has the greatest human population of any watershed in the Lake Tahoe Basin, thus increasing the chances of negative human effects on water quality. The watershed also has several land-uses representative of many water-quality influences that occur throughout the Lake Tahoe Basin. Trout Creek is included in the study area because the watersheds are adjacent to each other and together comprise most of the South Lake Tahoe area.

Purpose and Scope

This report presents a compilation of ground-water and surface-water data collected in the Upper Truckee River and Trout Creek watersheds during baseflow conditions from July to December 1996. The data are used to (1) determine ground-water levels and direction of ground-water flow in the watersheds, (2) determine the interaction between ground water and streamflow, and (3) compare the water quality of the ground- and surface-water systems during baseflow conditions.

USGS, in cooperation with TRPA, began a study in July 1996 to improve the understanding of the surface-water and ground-water systems and their interactions within the Upper Truckee River and Trout Creek watersheds. Principle efforts included (1) making streamflow measurements during baseflow conditions on the Upper Truckee River and Trout Creek and their tributaries; (2) inventorying existing wells on the basis of well drillers' reports and canvassing local residents; (3) determining depth to water in located wells; (4) developing a map showing the altitude of the water table using depth-to-water measurements in wells and results of seepage estimates; and (5) collecting additional water-quality data.

Previous Studies

The USGS has been involved with surface-water studies in the Upper Truckee River and Trout Creek watersheds since 1960, when operation of streamflow-gaging stations and surface-water-quality sampling sites first began. Periods of record of daily streamflow, water-quality, and suspended-sediment data are listed in table 1 for eight current and historical sites. Data from these eight sites have been published previously in USGS annual data reports by California and Nevada. Previous USGS surface-water studies in the Upper Truckee River and Trout Creek watersheds have included sediment discharge (Kroll, 1976); flood and related debris-flow hazards map for the South Lake Tahoe area (Katzer and Glancy, 1978); and suspended-sediment factors for the Lake Tahoe Basin (Hill and Nolan, 1988). Jeton (1999) has constructed a precipitation/runoff model for the Lake Tahoe Basin that includes Upper Truckee River and Trout Creek.

TRG has been involved with several studies and has collected physical and chemical data on Upper Truckee River and Trout Creek since the early 1970's. These data are included in LTIMP annual reports. The most recent LTIMP report is by Byron and others (1989).

TRPA also has published annual water-quality reports since 1990 for the Lake Tahoe Basin. These reports have included USGS and TRG data on Upper Truckee River and Trout Creek. The most recent report is by Hill (1996).

USFS also has been involved with several studies collecting physical and chemical data on Upper Truckee River and Trout Creek. USFS has published a water-quality report on the Santa Fe erosion control project by Hoffman (1991); a water-quality report summarizing five baseline stations by Lowry and Meeker (1993); a monitoring report on Hell Hole Road water-quality improvement project by Norman (1996); a water-quality-monitoring report on spring runoff in the Grass Lake research natural area by Norman and Parsons (1997); and a monitoring report on Pope Marsh burn by Norman (1997).

U.S. Army Corps of Engineers (USCOE) published a report on flood-plain information for the Upper Truckee River, South Lake Tahoe, Calif. (U.S. Army Corps of Engineers, 1969). El Dorado County Department of Transportation has been involved in several studies and data collection efforts on the Upper Truckee River and Trout Creek. A recently published report on the Apache Erosion Control project was done by Robinson (1996).

Within the study area, ground water is the primary source for domestic and public water supplies. Historical wastewater disposal practices and current large municipal withdrawals of ground water within the Upper Truckee River and Trout Creek watersheds are the basis for several studies that focused on water quality and quantity. For example, the California Department of Water Resources has been monitoring water levels since 1958 for selected wells to identify long-term trends, if any, within the Lake Tahoe Basin. Two of these wells are within the study area. Thodal (1997) used data from 32 wells to characterize ground-water quality within the Lake Tahoe Basin. Six of these wells are within the study area. Scott and others (1978), Blum (1979), and Woodling (1987) report the results of hydrogeologic investigations in the study area. Results of investigations of ground-water nutrient flux of the Upper Truckee River and Trout Creek watersheds are included in reports by Loeb and Goldman (1979), Loeb (1987), and Thodal (1997).

Acknowledgments

This work was done in cooperation with the Tahoe Regional Planning Agency. Appreciation is extended to residents in the area who gave permission to access their wells. Many government and private agencies also are acknowledged for providing data and access to their wells: Agra Earth and Environmental; California Department of State Parks and Recreation; California Department of Water Resources; California Tahoe Conservancy; El Dorado County Department of Transportation; Lahontan Water Quality Control Board; South Tahoe Public Utility District; and U.S. Forest Service--Lake Tahoe Basin Management Unit.