Sediment production and transport in a proglacial stream: Hilda Glacier, Alberta, Canada

Abstract

Hilda Glacier, a small cirque glacier in the Canadian Rocky Mountains, yields two principal types of sediment: ablation till, deficient in fine material and produced by rockfalls and avalanches falling on to the glacier surface, and basal lodgement till, rich in fines and formed mainly by subglacial erosion. Recent recession from its Neoglacial maximum has exposed large areas of basal till with thin veneers of ablation till which, when combined with present subglacial and supraglacial debris, provide abundant material for erosion and transport by the mcltwatcr stream. Sediment transport measurements over two summers (1977–1978) showed that bed load and suspended load occur in approximately equal proportions and that dissolved loads are minor. Local source variations, especially bank slumps, are a major cause of scatter in sediment rating curves. Suspended‐sediment concentrations are greater early in the melt season due to availability of loose sediment produced by freezing and thawing. Other contributors to scatter in suspended‐sediment rating curves include rain showers and diurnal hysteretic effects. Although the distinction between bed load and suspended load is never sharp, available data suggest that the sand/ gravel grain‐size boundary (‐1ø) approximates the suspendcd‐load/bed‐load division for characteristic Hilda flows transporting gravel. This approximation, combined with till grain‐size analyses, suspended‐sediment measurements, and spatial distributions of till types, leads to the following computations of fluvial sediment sources: for suspended load ‐ 6% supraglacial, 47% subglacial, 47% channel banks; for bed load ‐ 46% supraglacial, 27% each subglacial and channel banks. Supraglacial debris provides only about one‐fourth of all fluvial sediment, but nearly half of the bed load.