RELATION BETWEEN PEAK FLOWS, PRECIPITATION, AND PHYSIOGRAPHY FOR 5 MOUNTAINOUS REGIONS IN THE WESTERN USA

Flood frequency curves are presented for five regions in the western USA characterized by diverse climate but similar physiography. The regional curves are obtained via an index flood method based on probability-weighted moments of the generalized extreme value distribution. The curves differ markedly in their slopes, reflecting differences in mechanisms that produce floods. The Foothills region of Colorado is typical of semi-arid areas where floods are produced by intense thunderstorms. In this region, the 100-year flood may be more than ten times the mean annual flood. The Alpine region of Colorado is typical of areas where runoff is generated by snowmelt. In this region, the 100-year flood is less than two times the mean annual flood. The Sierra Nevada, Coast Range and Klamath Mountain regions of California typify areas where floods are produced by large-scale frontal storms. In these regions, the 100-year flood may be three to six times the mean annual flood. In all regions the mean annual flood is most highly correlated with drainage area and mean annual precipitation. No correlation is found between the standard deviation of annual maxima and various measures of basin physiography or drainage area. It is suggested that the variation in flood frequency distributions reflects largely the variability in precipitation amount and intensity rather than differences in physiography.