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Auburn On North Fork near Auburn damsite 1,295 Placerville On South Fork 1,890 Nevada City North catchment divide 2,600 Lake Spaulding North catchment divide 5,153 Tahoe City East of divide by Lake Tahoe 6,230 a Used to estimate basin average precipitation. .093 TABLE 2.1 Weather Stations Used to Estimate Basin Average Precipitationa Gauge Location Elevation (ft MSL) Represa Near Folsom Dam 295 Weighta .040 .090 .204 .261 .312 River flood volumes in a physically plausible manner, and then add to that model an indicator ("dummy") variable having a value of 1 for the period 1956-1997-0 otherwise. If the value of the estimated linear coefficient on this indicator, equal to the intercept shift or shift in mean during this period, is statistically significant, it would be interpreted as evidence for a shift in hydrologic regime. The basic model estimation results can be interpreted to indicate that variation in three-day event precipitation accounts for 75% of the interannual variation in three-day flood volumes. The addition of three-day storm temperature at higher elevations increases explanatory power by an additional 8.3% to 83.3%. Addition of variables for date of occurrence and antecedent precipitation does not significantly improve the model. When the period indicator is added to the model, explanatory power is increased by only 0.3%, and the t statistic for the period indicator in this equation, which has already accounted for the influence of precipitation and temperature, has a P value somewhat above 0.10. This suggests that any residual variation in flood volume magnitudes "explained" by period effects . in a linear model already accounting for the influences of event precipitation and temperature, albeit crudely specified, is not statistically significant. Historical Activities in the American River Basin Before considering the historical and paleoflood data for the American River, it is instructive first to review the history of land use practices in the American River basin. Of particular impottance are the activities associated with gold mining in the region, which began with the discovery of gold in 1848. Although the impacts of these activities on flood hydrology are not well known, it is impottant to consider them when evaluating the relevance of historical and paleoflood data. Initial gold mining activities involved small placer claims along Sierra streams and probably had relatively minor effects. Hydraulic mining began in California in 1853, and by the mid-I 860s giant hydraulic mines were in place. These mines had enormous impacts on the streams, particularly with respect to sediment - 30 Improving American River Flood Frequency Analyses