Annual floods in southwestern British Columbia, Canada

In southwestern British Columbia there exists a transition from rain-generated floods in the west to snowmelt floods in the east. When a single process dominates, the Gumbel distribution provides an adequate fit to observed flood exceedance probabilities. The two processes produce distinct timing and magnitude characterisations in the resultant floods. The area of transition between these two sets of conditions displays varying mixtures of both characteristics which can best be fitted by a compounded Gumbel distribution. It is proposed that this technique is generally applicable wherever annual flood series are generated by more than one hydrological process.