Sedimentology and Bar Formation in the Upper Kicking Horse River, a Braided Outwash Stream

Abstract

Bar formation and sediment distribution patterns were examined in a 4-mile braided reach of the upper Kicking Horse River at Field, British Columbia, which is mainly supplied by meltwater from icefields straddling the continental divide at the British Columbia-Alberta boundary in the southeastern Canadian Cordillera. Marked diurnal variations in discharge, suspended sediment concentration, and water temperature occur during peak summer melting periods when rates of sediment transport and bar formation are greatest. Bed material is mostly limestone and dolomite gravel which undergoes rapid fining in the downstream direction. Gravel bars occurring in a wide variety of shapes and sizes comprise the dominant bed forms. Most exposed braid bars have undergone complex depositional and erosional histories and rarely show simple or consistent patterns of grain size or structures, either internal or superficial. Active bars with simple histories and predominantly depositional morphologies are termed "unit bars." Four general unit bar forms are recognized in the Kicking Horse, with a variety of intermediate forms existing among them: longitudinal, transverse, point, and diagonal. The first three generally form parallel to channel flow; they tend to fine downstream, fine upward, and consist of finer-grained material than adjacent channels. Growth may be downstream or sideways, depending on characteristics of local flow and channel morphology. Diagonal bars form with long axes oblique to flow and appear to lack the grain-size trends of the other unit bar forms. Gravels comprise over 90% of the sediment exposed in shallow trenches. Stratification is mostly massive or horizontal, with subordinate low- and high-angle cross-stratification resulting from migration of bar margins ranging from low-angle riffles to high-angle slip faces. Minor but persistent facies include fine to very coarse cross-stratified sand, very fine to medium rippled sand, horizontally laminated mud and silt, and root-mottled mud, silt, and sand in local overbank areas.