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Baffin Island sandar : a study of arctic fluvial environments

University of British Columbia

Sandur (pl. sandar) is a sand, or gravel plain deposited by a rapidly aggrading, braided river. Valley sandar form the major postglacial physiographic feature in much of the Canadian Arctic. The conditions favorable for sandur development: presence of an abundant supply of detrital material and the occurrence of relatively frequent floods competent to move the material, are characteristic of proglacial and eome periglacial environments. Such conditions occur in central Baffin Island. Nival floods, summer ice-melt floods, and summer storm runoff generate significant sediment transport events. Storm runoff is particularly effective in the Arctic since there is little vegetation and the substrate is frozen, so that a high proportion of rainfall runs off immediately. The distribution of runoff events shows a disproportionate number of moderately high flows. Very active sandar are characterised by occasional extreme floods produced by catastrophic drainage of ponded water from Ice margins. Sediment transport is mainly bedload, though solution and suspended load are considerable. Transport is much greater than the rate of primary detritus production as large volumes of glacially derived material are being redistributed. Movement of extraordinary volumes of material is characteristic of the transition period from one environment (glacial) to another (fluvial). Sandur streams are extremely vide and shallow, a characteristic of bedload transporting channels. The major hydraulic adjustment to increasing discharge is made by stream velocity, indicating a sharp decline in flow resistance associated with the rapid increase of sediment transport and consequent change in the nature of the stream boundary. The long profiles of the streams are approximately parabolic: this form is probably conditioned by the nature of the aggradation on the surface as a whole. The sequence of pools and riffles along the channels constitute a fundamental feature of the river. They appear to function as regulators of energy expenditure in the stream and concomitantly influence flow resistance and the movement and storage of sediment. The sandur surface is a purely aggradational feature. Erosion and deposition occur over wide areas during major floods. Change of stage is rapid so that deposition is often chaotic, and little internal structure occurs in the sediments. Significant pattern is found only in the mean size and variance of coarse materials(cobbles), Indicating deposition under conditions of declining competence downsandur. Study of a former sandur, and of deltaic beds at its distal end, indicate that environmental conditions throughout much of postglacial time have not been dissimilar from the present ones. The more extensive sedimentation that occurred in earlier postglacial tine should be attributed to greater event duration, or to greater availability of detritus, rather than to the occurrence of appreciably greater events.

Text

2010-02-05

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http://hdl.handle.net/2429/19633

Geography

University of British Columbia

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