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In areas where the annual flood series is generated by more than one distinctive hydrologic process, the Gumbel distribution does not provide a satisfactory fit. Physical evidence is used to identify the flood‐generating processes, which in the Cascade Mountains include snowmelt and rainfall. Individually, the snowmelt and rain generated annual floods are adequately modeled by simple Gumbel distributions. These distributions have to be compounded to provide a good fit to the annual floods which are produced by mixed processes.
Waylen et al. (1982) studied this question.