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hydraulic conditions are definable in localized reaches conducive to paleoflood reconstructions. Chronology for paleohydrologic bounds was established by 60 radiocarbon ages, 21 archaeological sites, published soil surveys, and 39 soillstratigraphic sections. Paleohydrologic discharge estimates were established by a variety of hydraulic modeling techniques. For some sites, discharge estimates were obtained by comparison to measured and estimated discharges at nearby gaging stations. For other sites, detailed topographic surveys provided the basis for two- dimensional flow modeling of study reaches up to 12 miles in length. Paleoflood sites were located in bedrock-controlled reaches; channel geometry for the reach near Fair Oaks, which has changed substantially in the 20th century, was reconstructed from topographic surveys made in 1907. USBR study results indicate that the flood experience in the American River over the last 50 years is not anomalous. Floods of a magnitude similar to the January 1997 flood have occurred during the past few hundred to several thousand years. Geomorphic and stratigraphic evidence also indicates that there have been floods somewhat larger than the January 1997 flood, but there is no evidence of floods with peak discharges substantially larger than that of January 1997. Peak stage indicators consisting of fme-grained flood sediments, which included mining debris, were used to estimate the peak stage of the largest flood, probably the flood of 1862. The estimated stage was slightly higher than the 1997 peak stage. The peak estimated discharge at Fair Oaks was 260,000 cfs, which is close to the estimate of Bossen (1941). Paleoflood data for the lower American River indicate that a peak discharge of about 300,000 cfs to 400,000 cfs has not been exceeded in the past 1,500 to 3,500 years. These results are consistent with paleoflood data at sites upstream of Folsom Dam and at sites on other rivers in the region. The quality of the USBR data and analysis is excellent. The committee fmds no reasons to disagree with the paleoflood information that the USBR has assembled. As discussed previously, the committee has serious doubts about the assumption that flood magnitudes have been completely independent and identically distributed in time during the period represented by the paleoflood information. Although paleoflood chronologies have not been well documented in the Sierra Nevada (the USBR study is the frrst systematic attempt to document paleofloods in the region), other paleoclimatic studies have indicated systematic variations in climate there that are consistent ....ith regional and global patterns. For example, paleoecological data (Woolf ender, 1995) indicate that the Sierras experienced persistent above-average temperatures during the Medieval Warm Period (approximately A.D. 950-1350) and persistent below-average temperatures during the Little Ice Age (about A.D. 1350- 1850). During the latter period, the Sierras experienced multiple advances of alpine glaciers and a decrease in the number of fire events (Birman, 1964; Burke and Birkland, 1983; Curry, 1969; Gillespie, 1982; Scuderi, 1984, 1987; Swetnam, 1993). Given that extreme floods on the American River occur in winter storms that mainly produce rain rather than snow, it is possible that the frequency of extreme floods would have been lower during the Little Ice Age. The key issue regarding the usefulness of any data on past floods to a particular planning or design problem is the information the data provide on the potential for flooding during the planning horizon. If floods can be assumed to be independent and identically distributed in time, then all past information is equally 34 Improving American River Flood Frequency Analyses -