Abstract

Ice-push features have been studied along the Mackenzie River through the 1966–1975 period in order to determine the long term stability of the features. The principal field method was to paint a series of parallel lines, across an ice-push feature, and then to measure changes in subsequent years. The ice-push features were: boulder pavements with tightly embedded boulders; loose boulder pavements with the rocks resting on a bouldery matrix; loose boulders; ice-push island buttresses; and rhythmically spaced boulder ridges. The stability of a feature is directly correlated with the embedment depth of the boulders, for the deeper the burial the more stable the boulder. Boulder pavements, the most stable feature, may persist for decades, if not centuries. The overriding action of rock shod river ice tends to align the boulders with their long axes parallel to the river bank, to size sort the pavement boulders so that the sizes decrease downstream, and to abrade and striate the pavement surface. Loose bouldery areas can change from breakup to breakup. The localization of bouldery ice-push features reflects, in general, a nearby source of stony till. Ice-push features may occur high above the flood limit in the form of driftwood, bulldozed heaps, and a trimline. The melting of stranded river ice produces a variety of melt features which help to modify the river banks.

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