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ABSTRACT
Thresholds for channel change were investigated on two proglacial outwash fans, one silty, the other gravelly, over, different temporal and spatial scales. Short-term channel change was monitored in detail over a period of seven weeks from July to August 1988, whilst longer term changes were observed between summer 1986 and 1988. A model predicting channel change under varying threshold conditions was devised.
Input controls showed that base level fluctuations, discharge variations, suspended sediment concentrations, grain size variations and sorting, sediment structures and stream gradients played different roles not only on an inter- and intra- fan basis but also over varying time scales. Both minor and major thresholds influenced channel change processes of sediment transport; channel scour, accretion, division and abandonment; bank collapse; bar formation; fan erosion and deposition.
Model outputs were defined in terms of planimetric change, braiding intensity, fan incision and extension, and cross-sectional change. A special intransitive threshold condition causing abrupt base level increase during the course of a jökulhlaup (R.I. 2–3 years) triggered complete channel system metamorphosis on the gravel fan and significant changes on the alluvial fan.