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<br />Data Sources <br /> <br />- <br />33 <br /> <br /> <br />from the Klamath south to San Diego (Hoyt and Langbein, 1955; McGlashan and <br />Briggs, 1939). Lynch (1931) concluded that the flood of 1862 was probably the <br />largest in Califomia since the settlement of the Spanish missions in 1769; he had <br />little information for northern California. McGlashan and Briggs (1939) indicated <br />that the floods of 1861-1862 appear to have been the largest in California since at <br />least the early 19th century. The flood is described as covering the entire Sacramento <br />valley with a vast inland sea (Guinn, 1907) except Marysvil1e Buttes (Ellis, 1939). <br />According to Engstrorn (1996) the inland sea or lake ranged from 250 to 300 miles <br />long and from 20 to 60 miles wide. Sacramento was submerged and almost ruined <br />by the floods (Guinn, 1907). Bossen (1941) estimated the peak flow on the <br />American River at Fair Oaks to be 265,000 cfs. <br />The utility of the historical record from about 1848 to 1907 (and perhaps <br />even part of the early systematic gaged record) is questionable because of unknown <br />cumulative effects of land-use changes associated with gold mining. The largest <br />peak flood (1862) in the systematic and historic period occurred during the period of <br />maximum watershed disturbance. Limited precipitation data in Sacramento and <br />Nevada City available during the winter of 1861-1862 suggests that the rainfall and <br />snowmelt contributing to the peak discharge was comparable to the record storms in <br />1986 and 1997. The estimated peak flood discharge in 1862 was only slightly larger <br />than the floods in 1986 and 1997, suggesting that even with the extensive basin <br />disturbance in the last half of the nineteenth century, basin response may not have <br />been much different from today. One possible explanation is that snowpack covering <br />disturbed surfaces may have masked the potential increase in runoff from mining and <br />vegetation removal. It is also possible that the estimated peak discharge of the 1862 <br />event is low. In any case, it is prudent to cautiously incorporate the historical data in <br />the flood frequency analysis. <br /> <br />Paleoflood Data <br /> <br />As this report was being prepared, the U.S. Bureau of Reclamation (USBR) <br />was concluding a comprehensive paleoflood investigation of the American River and <br />nearby basins. The primary objective of the USBR study was to characterize the <br />probabilities of flood magnitudes greater than those contained in the historical record <br />for use in risk assessment of Folsom Dam. Summarized below are some of the major <br />fmdings of the paleoflood study provided by Dean Ostenaa (U.S. Bureau of <br />Reclamation, written communication, 1998). <br />The American River, both upstream and downstream from Folsom Dam, is <br />flanked by a distinct series of stream terraces. These terraces represent abandoned <br />floodplains whose surface morphology and underlying soils accurately record the <br />time since the last major flood. The main objective of the USBR study was to <br />identiry and assign ages to terrace surfaces adjacent to the river that serve as limits or <br />paleohydrologic bounds for the stage, and therefore discharge, of past large floods <br />over particular time intervals. <br />Paleohydrologic records were developed at 12 sites along the American, <br />Consumnes, Mokelumne, and Stanislaus Rivers. Despite the extensive mining <br />activity locally along these rivers, the geologic record of floods remains intact and <br />