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4 Improving American River Flood Frequency Analyses space method of moments method. The EMA (Cohn, Lane, and Baier, 1997), developed well after the publication of Bulletin 17-B, makes more effective use of historical and paleoflood information than does the weighted-moments method recommended in Bulletin 17-B for use with historical information. The committee explored alternative estimates of the flood frequency distributions for the American River using various combinations of systematic, historical, and paleoflood data and selected a recommended distribution, shown in Table ES.l and Figure ES. L The recommended distribution is based on the systematic record of three- day rain-flood flows estimated by the USACE from the U.s. Geological Survey flow record for Fair Oaks, and upon the historical record for 1848-1904 which included an estimated large three-day flow associated with the 1862 historic flood. Based on several independent analyses conducted by the committee and the USACE, the committee concludes that the three-day rain flood record is an accurate representation of the magnitude of the flood flows over the period of record, and that the observed increase in the frequency ofIarge floods since 1950 is not an artifact of the method by which flood peaks were computed. The committee's estimate of the three-day flow associated with the 1862 flood is based on a regression model developed by the committee. In its frequency analysis the committee assumes that this flow was the largest three-day flow in the historical period from 1848 to 1904. The recommended frequency distribution assumes a log-skew of -0. L This skew is based on a weighted average of a regional skew (-0.1) and the sample skew (-0.06). The committee estimated the regional skew by averaging the sample log- skew of three-day flow series from seven rivers on the west slope of the central Sierra Nevada. Sensitivity analysis using the committee's recommended approach indicates that censoring below various flows with exceedance probabilities ranging from about 0.94 to 0.31 does not significantly affect the estimated distribution. In developing its recommended flood frequency distribution, the committee chose not to use the paleoflood information to compute a frequency curve for the American River. When the paleoflood data are used in conjunction with the systematic and historical data in an estimation framework consistent with the spirit of Bulletin 17-B, the resulting log-Pearson type ill distribution provides a poor fit to the systematic data (Figure 3.3). While it might be possible to improve the fit by using a method outside the framework of Bulletin 17-B (e.g., censoring the systematic data at a vel)' high threshold), the committee chose not to take this approach for several reasons. First, the committee was committed to following the spirit of Bulletin 17-B. Second, the committee was uneasy about using the paleoflood data because of questions about climatic variability during the 3,500-year period represented by this information. In particular, given present understanding of global climate variations during the past 10,000 years, the committee questions whether it is prudent to assume that flood magnitudes during this period are independent and identically distributed. While the committee's preferred estimate of the frequency distribution of three-day rain flood flows on the American River is consistent with the systematic and historic data, the committee cautions against extrapolating much beyond these data. Frequency analysis of basin average precipitation data (as well as the paleoflood information) indicates that the upper tail of the "true" distribution flattens for vel)' large flows. -