Sediment loading of alpine debris-flow channels by snow avalanches, Canadian Rocky Mountains

Abstract

ABSTRACTMeasurements of sediment deposition by snow avalanches were carried out in five alpine debris-flow channels in the Canadian Rocky Mountains after five or six winters by employing on average 15–29 sampling polygons in each channel. Following the melting of the avalanche snow each summer, the sediment deposited inside each polygon was collected, weighed, and converted to a depth of deposition. The results show that deposition in the channels averages 0.09–0.38 cm a−1, with an overall average of 0.19 cm a−1 for the five channels. Most of this deposition is contributed by weak clastic rocks in the basins but the largest particles are derived from much stronger carbonate rocks. Taking into account the channel area over which the measurements were made and the basin area, the measured sediment depths are equivalent to 8.3–36.4 m3 a−1 of deposition per channel or 10.9–311.7 t km−2 a−1 basin sediment yield. For the five channels, the overall averages are 16.4 m3 a−1 and 95.8 t km−2 a−1, respectively. On average 8–38% of polygons in each channel had no sediment deposition after a winter, reflecting the high spatial variability of avalanche sediment deposition. Comparison of debris-flow volumes and return periods with the measured annual volumes of sediment deposition suggests that avalanches are by themselves incapable of loading these channels for debris flows, and other processes such as rockfall, creep and streamflow also play a role. The most important role of avalanches may be as a hillslope-channel coupling mechanism in the sediment cascade and to assist in debris-flow bulking on the fan.