ENVIRONMENTAL SETTING

The elevation of Lake Tahoe, averaging about 6,224 ft above sea level, is greater than any other lake of its size in the United States. The maximum length and width of the lake is about 22 mi by 12 mi. The level of Lake Tahoe is controlled by a 6-ft high dam at the Truckee River outlet. A Federal Court Decree limits the maximum permissible elevation of Lake Tahoe to 6,229.1 ft, U.S. Bureau of Reclamation (BOR) datum, which equals 6,227.96 ft above sea level. The USGS water-level gage at Tahoe City, Calif., records lake- surface elevation; the maximum recorded level was 6,231.26 (BOR datum) in July 1907 and the minimum recorded level was 6,220.26 ft (BOR datum) on November 30, 1992. The average depth of the lake is 1,000 ft and the greatest depth is about 1,636 ft (Smith and others, 1999). The total Lake Tahoe Basin area is 506 mi², consisting of 192 mi² in lake-surface area and 314 mi² in surrounding watershed area, and has a drainage perimeter of 138 mi (Cartier and others, 1995, plate).

The 14 watersheds monitored in this study represent half of the Lake Tahoe Basin watershed land area at 157 mi², with an average area per watershed of 11.2 mi². The remaining 49 unmonitored watersheds also cover 157 mi² with an average area per watershed of 3.2 mi². Specific watershed information for the 14 monitored watersheds derived from Cartier and others (1995) is listed in table 3. The drainage area of the selected watersheds ranges from 1.08 mi² for First Creek to 56.5 mi² for the Upper Truckee River. Drainage perimeters range from 5.60 mi for North Logan House Creek to 53.9 mi for Upper Truckee River. The mainstream channel lengths range from 2.34 mi for First Creek to 21.4 mi for the Upper Truckee River; channel elevations range from 6,229.1 ft (maximum lake level) to 9,170 ft for Third Creek. The maximum basin elevation (10,890 ft) occurs in the Trout Creek watershed. General Creek and North Logan House Creek have the largest areas in the lower elevation groups from 6,229 to 8,000 ft (Jeton, 1999, fig. 5). Third Creek watershed has the largest area in the highest elevation groups from about 9,000 to 11,000 ft.

Subdivided Areas

The 14 watersheds monitored in the LTIMP study were subdivided into 5 areas: Incline Village, Nev. (northeast), Glenbrook, Nev. (east), Stateline, Nev. (southeast), South Lake Tahoe, Calif. (south), and Tahoe City to Meeks Bay, Calif. (west). Monitoring stations in each of the areas are shown in figures 2, 3, 4, 5, and 6.

The Incline Village (northeast) area has 11 sampling stations (2 primary, 2 secondary, and 7 miscellaneous; fig. 2). Features in the Incline Village area that might affect stream water quality include Highways 28 and 431 (Mt. Rose Highway), concentrated development in the lower elevations, two golf courses, and one ski resort. Incline Lake in the Third Creek watershed is the largest impoundment in the area and some diversions from upper Third Creek to Ophir Creek can occur at times. Geology in this area consists mainly of intrusive igneous rocks and unconsolidated sediments with some volcanic rocks (fig. 7; Cartier and others, 1994).

The Glenbrook (east) area has four sampling stations (two primary and two miscellaneous; fig. 3). Features in the Glenbrook area that might affect stream water quality include U.S. Highway 50, an old highway route, previous logging activities, and unpaved logging roads. Within the Glenbrook area, Logan House Creek watershed is largely undeveloped, except for historic logging. Geology in this area consists mainly of intrusive igneous rocks with some volcanic rocks, metamorphic rocks, and unconsolidated sediments (fig. 7).

The Stateline area (southeast) area has eight sampling stations (one primary; two secondary; five miscellaneous which include two urban-runoff stations; fig. 4). Features in the Stateline area that might affect stream water quality include U.S. Highway 50, Nevada State Route 207 (Kingsbury Grade), concentrated development along Route 207, part of one ski resort, and one golf course. Numerous small ponds along Edgewood Creek, including one just above Edgewood Creek at Stateline, can affect quality of water and streamflow. Geology in this area consists mainly of intrusive igneous rocks and unconsolidated sediments (fig. 7).

The South Lake Tahoe (south) area has 13 stations (2 primary, 4 secondary, 1 discontinued gaging station, and 6 miscellaneous which include 5 water-temperature network stations; fig. 5). Features in the South Lake Tahoe area that might affect stream water quality include U.S. Highway 50, California State Highway 89, Pioneer Trail, many unpaved roads and trails, concentrated development in the lower elevations, one ski area, two golf courses, and reclamation of major wetlands. Diversions from the Upper Truckee River watershed are taken from Lower Echo Lake by the El Dorado Irrigation District and are delivered to the South Fork American River via tunnel. Geology in this area consists mainly of intrusive igneous rocks and unconsolidated sediments with some volcanic rocks (fig. 7).

The Tahoe City to Meeks Bay (west) area has five sampling stations (three primary and two secondary; fig. 6). Features in the west shore area that might affect stream water quality include California Highway 89, concentrated shoreline development, and part of one ski resort. Geology in this area consists mainly of intrusive igneous rocks and unconsolidated sediments with some volcanic rocks and metamorphic rocks (fig. 7).

Climate

For the 1915-98 period of record at the National Weather Service (NWS) station at Tahoe City, Calif., the average maximum air temperature was 13.5°C. The warmest months were June-September, which averaged greater than 20.0°C. The average minimum air temperature was -1.0°C. The coldest months were December-March which were recorded as less than -4.0°C. NWS data is available from Western Region Climate Center internet website at http://www.wrcc.dri.edu.

Precipitation in the Lake Tahoe Basin ranges from 30 to 40 in/yr on the eastern side to from 70 to 90 in/yr on the western side (Crippen and Pavelka, 1970, fig. 9). Precipitation occurs mostly as snow, mainly from November through March. Precipitation also occurs during rain events, rain-on-snow events, and summer thunderstorms.

Total annual precipitation for the Tahoe City, Calif., station is plotted in figure 8A for the period of record 1915-98. Total annual precipitation for the period 1988-98 is highlighted in figure 8A. Median total annual precipitation for the 1915-98 period of record was 29.77 in. and for the period 1988-98 was 37.37 in. Total annual precipitation for 1988, 1990-92, and 1994 was below the median; for 1989, 1993, and 1997 was near the median; and for 1995-96 and 1998 was above the median. Years with below median total annual precipitation may be considered drought years.

The years of drought conditions affect the nutrient and suspended-sediment loads in the watersheds as less runoff is generated creating decreased loads. In general, drought years produce less runoff and loads and above average precipitation years produce more runoff and loads.

The greatest total annual precipitation at Tahoe City, Calif. for the period of record was 69.21 in. in 1982 and the least was 8.82 in. in 1977. For the study period, precipitation ranged from a high of 60.87 in. in 1995 to a low of 16.59 in. in 1994. Median monthly precipitation for the 1915-98 period of record and the 1988-98 study period are plotted in figure 8B. Precipitation was highest from November through March and lowest from July through September. Also during the study period, precipitation associated with intense convective summer thunderstorms was less than normal or nearly absent as observed by sampling personnel.