Extreme flood events in partially-glacierised high-mountain basins appear to have become more frequent in the warm summers of the 1980s and 1990s, in both European and Asian settings. Dramatic floods occurred in the Karakoram over a wide area in September 1992 for example, whilst in Europe serious floods often accompanied by mudflows and debris torrents were much localised in the French, Italian and Swiss Alps in 1987, again in 1993 in the Rhone basin in Wallis, Switzerland, and in 1996 in Chamonix in France.

Largest rain-induced flows occur in late July and late August (1987) and in September (1993), usually when the thermal component of runoff generated by icemelt is relatively high, and the area of bare ice exposed on glaciers by the rise of the transient snowline at a maximum. In streams draining glaciers, large quantities of sediment, which appear to be subglacial in origin, are evacuated during such events. In the 1993 event, the area from which Findelengletscher (Wallis, Switzerland) has retreated since the late 1980s was also an important sediment source.

Records of discharge of the portal melt-streams draining Findelengletscher and Gornergletscher, Kanton Wallis, Switzerland, for the period 1970-1993, have been examined with a view to determining the incidence and recurrence of high-magnitude discharge events which interrupt the usual ablation season diurnal rhythmic flow regime in these highly-glacierised basins. Sudden drainage of the ice-dammed Gornersee beneath Gornergletscher, usually once each summer, and periods of intense rainfall in both basins translate into peak flows. A model is developed which integrates effects of elevation of the transient snowline, hypsometry of the ice-covered and ice-free areas of a basin, and elevation of the zero degree isotherm in determining the effective catchment area from which runoff is rapidly formed in storms. Probability of high rainfall is also assessed. Concepts of recurrence interval have to be refined for basins with permanent ice and variable snow-cover.