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- 40 Improving American River Flood Frequency Analyses approach. Estimates of flood frequency distributions for the American River using various combinations of systematic, historical, and paleo data are presented along with a recommended distribution. Finally, evidence suggesting that the recommend- ed distribution should not be extrapolated beyond a return period of 200 years is presented. BULLETIN 17-B Recommended procedures for flood frequency analyses by federal agencies are described in Bulletin 17-B (IACWD, 1982). Thomas (1985) describes the history of the development of the$e procedures. The recommended technique is based on fitting a Pearson type III distribution to the base-10 logarithms of the peak discharges. The flood flow Q associated with cumulative probability p is then 10g[Qp] = X + KpS where X and S are the Slllllple mean and standard deviation of the base-1 0 logarithms Xi, and Kp is a frequency factor that depends on the skew coefficient and selected exceedance probability. The mean, standard deviation, and skew coefficient of station data are computed using p, =x= tx/n n , It o-'=S2=~(Xi-X) /n-l) n 3 It y, =G=n~(xi-X) /n-l)(n-2)S2 Estimation oftbe Skew Parameter Because of the variability of at-site Slllllple skew coefficients, Bulletin 17-B recomrnends using a weighted average of the station skew coefficient and a generalized skew coefficient, a regional estimate of the log space skewness. In the absence of detailed studies, the generalized skew coefficient Gg for sites in the United States can be read from Plate I in the Bulletin. Assuming that the generalized skew coefficient is unbiased and independent of statio~ skew coefficient, the mean square error (MSE) of the weighted estimate is minimized by weighting the station and generalized skew coefficients inversely proportional to their individual mean square errors: