Fact Sheet 035-02

Estimated Flood Flows in the Lake Tahoe Basin, California and Nevada

E.J. Crompton, G.W. Hess, and R.P. Williams

Introduction

Lake Tahoe, the largest alpine lake in North America, covers about 192 square miles (mi²) of the 506-mi² Lake Tahoe Basin, which straddles the border between California and Nevada (Fig. 1). In cooperation with the Nevada Department of Transportation (NDOT), the U.S. Geological Survey (USGS) estimates the flood frequencies of the streams that enter the lake. Information about potential flooding of these streams is used by NDOT in the design and construction of roads and highways in the Nevada portion of the basin. The stream-monitoring network in the Lake Tahoe Basin is part of the Lake Tahoe Interagency Monitoring Program (LTIMP), which combines the monitoring and research efforts of various Federal, State, and regional agencies, including both USGS and NDOT.

The altitude in the basin varies from 6,223 feet (ft) at the lake's natural rim to over 10,000 ft along the basin's crest. Precipitation ranges from 40 inches per year (in/yr) on the eastern side to 90 in/yr on the western side (Crippen and Pavelka, 1970). Most of the precipitation comes during the winter months as snow. Precipitation that falls from June through September accounts for less than 20 percent of the annual total.

Methods

The estimated magnitude of peak flows at 50- and 100-year recurrence intervals was determined for 46 sites in 21 watersheds that cover about 177 mi² of the Lake Tahoe Basin (Fig. 1; Table 1). The drainage areas of the 46 sites range in size from 0.63 mi² at Eagle Rock Creek near Stateline, Nev., (site 40) to 54.0 mi² at Upper Truckee River at South Lake Tahoe, Calif. (site 4). Expected peak flows were determined using five methods (refer to Table 1 footnotes for the method used to estimate discharge at each site).

Method 1 uses a USGS computer model (W.O. Thomas and others, U.S. Geological Survey, written commun., 1998) that is based on statistical flood-frequency analysis of annual peak-flow records according to the U.S. Interagency Advisory Committee on Water Data (1982) guidelines. This method employs the Pearson Type III distribution with log transformation of the data as the base method for flood-flow frequency. Method 2 is a variation of method 1 in which historic peaks are considered (W.O. Thomas and others, U.S. Geological Survey, written commun., 1998). This option modifies the length of the historic return period. Method 3 is a two-station comparison as presented in U.S. Interagency Advisory Committee on Water Data (1982) guidelines. This method is used to adjust the logarithmic mean and standard deviation of a short record on the basis of a regression analysis with a nearby long-term record.

Because methods 1-3 use annual peaks to determine the estimated peak discharge, better estimates are obtained by incorporating more years of data. The range in the period of record for the 20 sites at which these methods were used ranged from 10 years at seven sites to 40 years at two sites, with an average period of record of 18 years. The years for which data were collected at each site are listed in Table 1.

Method 4 estimates the magnitude of peak-flood discharge as a generalized least-square regression equation determined for the eastern Sierra (Thomas and others, 1997, p. 45). This equation uses drainage area, mean basin elevation, and latitude of the site in decimal degrees. In this method the period of record is not important.

Method 5 is an equation presented in Thomas and others (1997, p. 14) that was used to determine flood-frequency relations at sites near gaged sites on the same stream. This equation is based on the relation of the discharge at the gaged site and the drainage area of the two sites. Ideally, the drainage-area ratio should be approximately between 0.5 and 1.5. In methods 4 and 5 the period of record for the 24 sites ranged from 2 years at four sites to 15 years at one site, with an average period of record of 7 years. At sites 41 and 42 peak flows were not determined because these sites are affected by regulation.

The peak flows published in this report vary from -24 to 9 percent of the values published in Rowe and others (1998). These differences in estimated peak values can be explained either by the additional three years of data or by the methods used for this analysis.

Observed Floods

Table 1 also provides the largest recorded flood peak at each site. During 1997, 31 stream-monitoring sites were in operation, 21 of these sites recorded their largest flood peak as a result of the widespread flooding that occurred on January 1 and 2, 1997, the result of a rain-on-snow event (Hunrichs and others, 1998; and Rowe and others, 1998). For many of the other sites, the largest recorded flood was the result of either convective or frontal rainstorms that occurred during the spring or summer months.

References Cited

Crippen, J.R., and Pavelka, B.R., 1970, The Lake Tahoe Basin, California-Nevada: U.S. Geological Survey Water-Supply Paper 1972, 56 p.

Hunrichs, R.A., Pratt, D.A., and Meyer, R.W., 1998, Magnitude and frequency of the floods of January 1997 in Northern and Central California-Preliminary determinations: U.S. Geological Survey Open-File Report 98-626, 120 p.

Rowe, T.G., Rockwell, G.L., and Hess, G.W., 1998, Flood of January 1997 in the Lake Tahoe Basin, California and Nevada: U.S. Geological Survey Fact Sheet FS-005-98, 2 p.

Thomas, B.E., Hjalmarson, H.W., and Waltemeyer, S.D., 1997, Methods for estimating magnitude and frequency of floods in the southwestern United States: U.S. Geological Survey Water-Supply Paper 2433, 195 p.

U.S. Interagency Advisory Committee on Water Data, 1982, Guidelines for determining flood flow frequency, Bulletin 17-B of the Hydrology Subcommittee: Reston, Va., U.S. Geological Survey, Office of Water Data Coordination, 183 p.

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