from the Klamath south to San Diego (Hoyt and Langbein, 1955; McGlashan and Briggs, 1939). Lynch (1931) concluded that the flood of 1862 was probably the largest in California since the settlement of the Spanish missions in 1769; he had little information for northern California. McGlashan and Briggs (1939) indicated that the floods of 1861-1862 appear to have been the largest in California since at least the early 19th century. The flood is described as covering the entire Sacramento valley with a vast inland sea (Guinn, 1907) except Marysville Buttes (Ellis, 1939). According to Engstrom (1996) the inland sea or lake ranged from 250 to 300 miles long and from 20 to 60 miles wide. Sacramento was submerged and almost ruined by the floods (Guinn, 1907). Bossen (1941) estimated the peak flow on the American River at Fair Oaks to be 265,000 cfs.

The utility of the historical record from about 1848 to 1907 (and perhaps even part of the early systematic gaged record) is questionable because of unknown cumulative effects of land-use changes associated with gold mining. The largest peak flood (1862) in the systematic and historic period occurred during the period of maximum watershed disturbance. Limited precipitation data in Sacramento and Nevada City available during the winter of 1861-1862 suggests that the rainfall and snowmelt contributing to the peak discharge was comparable to the record storms in 1986 and 1997. The estimated peak flood discharge in 1862 was only slightly larger than the floods in 1986 and 1997, suggesting that even with the extensive basin disturbance in the last half of the nineteenth century, basin response may not have been much different from today. One possible explanation is that snowpack covering disturbed surfaces may have masked the potential increase in runoff from mining and vegetation removal. It is also possible that the estimated peak discharge of the 1862 event is low. In any case, it is prudent to cautiously incorporate the historical data in the flood frequency analysis.

As this report was being prepared, the U.S. Bureau of Reclamation (USBR) was concluding a comprehensive paleoflood investigation of the American River and nearby basins. The primary objective of the USBR study was to characterize the probabilities of flood magnitudes greater than those contained in the historical record for use in risk assessment of Folsom Dam. Summarized below are some of the major findings of the paleoflood study provided by Dean Ostenaa (U.S. Bureau of Reclamation, written communication, 1998).

The American River, both upstream and downstream from Folsom Dam, is flanked by a distinct series of stream terraces. These terraces represent abandoned floodplains whose surface morphology and underlying soils accurately record the time since the last major flood. The main objective of the USBR study was to identify and assign ages to terrace surfaces adjacent to the river that serve as limits or paleohydrologic bounds for the stage, and therefore discharge, of past large floods over particular time intervals.

Paleohydrologic records were developed at 12 sites along the American, Consumnes, Mokelumne, and Stanislaus Rivers. Despite the extensive mining activity locally along these rivers, the geologic record of floods remains intact and