Bar evolution and development of stratification in the gravelly, braided, Kicking Horse River, British Columbia
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abstract
The Kicking Horse River is a gravelly, braided stream characterized by very low winter discharge, a peak spring flood (70 m3s−1 in 1973), and summer diurnal discharge fluctuations (from 18 to 44 m3/s−1 in 1973) related to daily melting on the glaciers that supply the river. The reach studied, near Field, B.C., is characterized by abundant braid bars, and was subdivided into three parts.The upstream reach is characterized by only one major channel, with few unit bars within it. Clast movement takes place at peak flood stages, and the clasts move in 'diffuse gravel sheets' on the channel floor. These sheets are only one to two clast diameters in thickness, and when the clasts stop rolling, the sheet becomes a coarse lag. By contrast, in the midstream and downstream reaches, the channel is more anastomosing, and there are many in-channel bars, mostly transverse and diagonal unit bars. The diagonal bars mostly lack foreset slopes at their downstream margin, and it is suggested that massive to crudely horizontally stratified gravels would be deposited. Transverse bars normally have a foreset slope, giving rise to cross-stratified gravels.Several bars were monitored continuously at their active stages during the diurnal rise in discharge in the late afternoon – evening. Measurement of water slope, current velocity, depth, clast sizes, and rate of bar migration were made every 15 minutes at each of the stations on each bar. The bars tended to advance episodically during periods of velocity increase, as recorded at the upstream monitoring station. However, clast size at the bar front does not seem to increase as velocity increases, possibly due to shielding effects of the largest, stationary, clasts on the bed. This effect made existing initiation-of-bedload-movement equations difficult to apply, and of severely limited use with respect to the poorly sorted gravels.Stratification could not be directly observed in the bars, but we suggest that upstream, the diffuse gravel sheets would form crudely horizontally stratified gravels, probably with a long-axis-transverse imbrication. In the midstream and downstream reaches, in finer gravels, more cross-stratification would be present, related to migration of transverse bars.
