Erosion assessment in the middle Kali Gandaki (Nepal ) : A sediment budget approach

Résumé En

Active mountains supply the largest sediment fl uxes experienced on earth. At mountain range scale, remote sensingapproaches, sediments provenance or stream power law analyses, collectively provide rough long-term estimatesof total erosion. Erosion is indeed controlled by rock uplift and climate, hence by a wide range of processes(detachment, transport and deposition), all operating within drainage basin units, yet with time and spatial patternsthat are quite complex at local scale. We focus on the Kali Gandaki valley, along the gorge section across theHigher Himalaya (e.g. from Kagbeni down to Tatopani). Along this reach, we identify sediment sources, storesand sinks, and consider hillslope interactions with valley floor, in particular valley damming at short and longertime scales, and their impact on sediment budgets and fluxes. A detailed sediment budget is presented, constrainedby available dates and/or relative chronology, ranging from several 10 kyr to a few decades. Obtained resultsspan over two orders of magnitude that can best be explained by the type and magnitude of erosional processesinvolved. We show that if large landslides contribute signifi cantly to the denudation history of active mountainrange, more frequent, medium to small scales landslides are in fact of primary concern for Himalayan population.