Gravel Transport and Stratification Origins, Kicking Horse River, British Columbia

Abstract

<p>In the Kicking Horse River systematic changes occur in bar forms from upstream to downstream reaches. In high gradient coarse-grained upstream localities flow is confined to one or two main channels which have a somewhat meandering to braided form. Bedload is transported only during maximum flow conditions as diffuse gravel sheets. In low gradient fine-grained medial to distal reaches flow is carried down anastomosing braided channel complexes with extensive bar development. These difference in bar-and-channel morphologies can be explained as follows. In upstream reaches bars and channels are relict from flood flows, where lower summer flows are incompetent to rework the diffuse sheets into true bars. In finer-grained reaches bars develop from flood-deposited diffuse sheets, where finer sediment is winnowed out under lower flows and transported as bar lobes. Significant lateral exchange of flow between adjacent channel systems produces a complex braided channel pattern.</p> <p>Diffuse gravel sheets display little sediment sorting and no foreset development. Deposits are massive poorly sorted gravels. Examination of sorting on bar surfaces, suggest that diagonal bar deposits would be lenticular fining-upward or massive gravels with low angle to horizontal bedding. Deposits associated with transverse bar migration would consist of fining-upward gravels with a few lenses of planar cross-stratified sequences.</p> <p>Preservability of stratification types estimated from trench sections in the North Saskatchewan River suggest that horizontally bedded massive gravels are dominant in medial to distal flats. Remaining percentages are largely composed of fining-upward and planar cross-stratified gravels.</p> <p>Proximal sediments would be coarse-grained, poorly sorted massive deposits. Medial to distal deposits would be predominantly massive (due to extensive bar dissection) with subordinant amounts of fining-upward and planar cross-stratified sequences (associated with bar migrations). Systematic transitions from ill-defined bars in proximal areas to true bars in distal reaches, suggest that massive poorly sorted gravels should decrease downvalley, corresponding to an increase in moderately sorted planar cross-stratified sequences.</p>