approach. Estimates of flood frequency distributions for the American River using various combinations of systematic, historical, and paleodata are presented along with a recommended distribution. Finally, evidence suggesting that the recommended 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 these 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

where and S are the sample mean and standard deviation of the base-10 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

Estimation of the Skew Parameter

Because of the variability of at-site sample skew coefficients, Bulletin 17-B recommends 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 station 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:

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