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ABSTRACT
A detailed field study of sediment transfer processes, sediment yield and channel change was undertaken in the proglacial zone of the Bas Glacier d'Arolla, Val d'Hérens, southern Switzerland, during the 1987 ablation season. Sediment yield estimates were derived from continuous discharge and suspended sediment concentration series combined with bedload transport estimates from sediment traps.
Valley sandur sediment transport dominated proglacial sediment transfers. Tributary streams and hillslopes contributed minor amounts of sediment. Channel change was episodic and channel pattern switched from single-thread to multi-thread to single-thread over the ablation season. Meltwater floods were responsible for catastrophic changes in channel form.
Intensive field measurements (25 May - 30 July) define a proglacial sediment budget which indicated that proglacial sediment sources contributed 23% to the total basin sediment yield. The overwhelming proportion of the proglacial sediment (95%) was eroded from the valley sandur during the brief period of meltwater flooding July 15–18. Expressed in terms of work, the budget suggests that four basic process subsets can be distinguished: (1) channel processes; (2) valley sandur (channel margin) processes; (3) hillslope processes, and (4) slopewash. Large magnitude, infrequent flood events are the dominant control on the release of sediment from proglacial storage.
