Proglacial Aggradation and Changes in Braided Channel Patterns During a Period of Glacier Advance: An Alpine Example

ABSTRACT

Studies of the morphological changes exhibited by an Alpine proglacial valley-train over a six-year period of glacier advance have provided quantitative evidence of initial degradation and subsequent extensive aggradation, the latteraver- aging ca. 1300m³/yr. Valley-train gradients have been consistently steepened as maximum amounts of coarse debris accumulated on the proximal slope of the valley-train, while localized aggradation has caused progressive burial by outwash deposits of a former terminal moraine. Aggradation of fluviogla- cial sediments occurred as sediment loads exceeded the competence of the meltwater streams; the streams created an increasingly dense network of braided courses in order to accommodate the relative increase in sediment loads, a stream pattern that was encouraged by the series of coarse, unstable bank deposits, frequent and large variations in meltwater discharge. a sudden downstream decrease in gradient, and the absence of vegetation. Mean channel sinuosity, bar shape, and log-normal size frequency distribution of mid-channel bars remained the same from year to year, while the actual size of bars exhibited a marked decrease; the mean orientation of mid-channel bars moved systematically westwards by over 10 degrees, as deposition of proximal outwash was extended eastwards along the ice-marginal zone. A simple model is proposed to illustrate the possibility of net aggradation and degradation occurring during a glacier advance and/or retreat, according to the balance between meltwater competence and sediment loads within the proglacial zone.