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Patterns and controls of sediment production, transfer and yield in the Illgraben

Bennett, G.L., Molnar, P., McArdell, B.W., Schlunegger, F., Burlando, P.
Geomorphology 2013 v.188 pp. 68-82
air temperature, basins, data collection, landscapes, rain, runoff, sediment yield, sediments, snowpack, watersheds, weathering, Alps region
Quantification of the volumes of sediment removed by rock–slope failure and debris flows and identification of their coupling and controls are pertinent to understanding mountain basin sediment yield and landscape evolution. This study captures a multi-decadal period of hillslope erosion and channel change following an extreme rock avalanche in 1961 in the Illgraben, a catchment prone to debris flows in the Swiss Alps. We analyzed photogrammetrically-derived datasets of hillslope and channel erosion and deposition along with climatic and seismic variables for a 43year period from 1963 to 2005. Based on these analyses we identify and discuss (1) patterns of hillslope production, channel transfer and catchment sediment yield, (2) their dominant interactions with climatic and seismic variables, and (3) the nature of hillslope–channel coupling and implications for sediment yield and landscape evolution in this mountain basin. Our results show an increase in the mean hillslope erosion rate in the 1980s from 0.24±0.01myr⁻¹ to 0.42±0.03myr⁻¹ that coincided with a significant increase in air temperature and decrease in snow cover depth and duration, which we presume led to an increase in the exposure of the slopes to thermal weathering processes. The combination of highly fractured slopes close to the threshold angle for failure, and multiple potential triggering mechanisms, means that it is difficult to identify an individual control on slope failure. On the other hand, the rate of channel change was strongly related to variables influencing runoff. A period of particularly high channel erosion rate of 0.74±0.02myr⁻¹ (1992–1998) coincided with an increase in the frequency and magnitude of intense rainfall events. Hillslope erosion exceeded channel erosion on average, indicative of a downslope-directed coupling relationship between hillslope and channel, and demonstrating the first order control of rock–slope failure on catchment sediment yield and landscape evolution.