Rainfall-induced high-magnitude runoff events in highly-glacierized Alpinebasins

Several major rainstorms produced floods in the highly-glacierized basins c ontaining Findelengletscher and Gornergletscher, Pennine Alps, Switzerland, in the ablation season of 1987. Runoff response to the sustained rainfall events varied according to timing within the season. The transient snow lin e partitions a glacierized basin into snow-covered and snow-free portions w hich respectively retain or return rapidly to runoff both meltwater and rai n. Elevation of the freezing level in the atmosphere interacts with basin h ypsometry to determine the proportion of the catchment area that receives r ain. Progressive rise of the transient snow line in summer increases the sn ow-free area, and hence the area of basin which rapidly responds to rainfal l. Rainfall-induced floods are therefore most likely to be largest between mid-August and mid-September, terminating periods of elevated air temperatu res, when thermally-induced runoff is at a maximum and subglacial drainage pathways are well-developed.