CHANNEL CHANGE IN RELATION TO MELTWATER FLOODING, BAS-GLACIER-DAROLLA, SWITZERLAND

Meltwater flooding in the valley train of the Bas Glacier d'Arolla, Sw itzerland (1986 and 1987) produced significant changes in channel form . The general channel response involved switching from a single thread , low sinuosity stream to a wider channel with zones of braiding. Brai ding occurs where the valley sandur is unconfined by valley side bluff s. During floods spatial variations in channel adjustment were conside rable and contrasts in channel change during different floods were ver y marked. In 1986, one flood was characterised by a laterally shifting thalweg and relatively low channel sediment yield (1970 tonnes) whils t in July 1987 a flood of similar magnitude was associated with thalwe g scour and much higher channel sediment yield (4790 tonnes). Meltwate r floods cause periodic sediment flushing events which are not necessa rily related to whether the glacier is advancing or retreating.