Sediment Production in French Alpine Rivers

Global compilations of river sediment loads show that mountainous areas produce a high proportion of the sediment transported to the oceans. However, because of the effort involved in measuring sediment fluxes in mountain river systems, the loads of these rivers are generally unknown. Here, we present estimates of contemporary sediment loads of 16 gravel bed rivers draining the Ecrins-Pelvoux Massif in southeast France. Sediment production in this part of the Alps is relatively high and many river segments are either wandering or braided. We model sediment fluxes and annual sediment yields by coupling hydraulic-based relations for sediment transport with hydrologic-based relations for flow frequency. Bed load fluxes are modeled for a range of discharges using a function that relates transport rates to excess shear stress. Fluxes are then weighted by the frequency of individual discharges and summed to get the annual bed load for each site. The suspended load is estimated empirically as a fraction of the bed load. Results suggest that bed load fluxes at channel-forming flows scale almost linearly with downstream increases in discharge. In addition, it appears that annual sediment loads (bed load + suspended load) scale linearly with drainage area. A complementary relation for specific sediment yield suggests that the load per unit drainage area is constant across the range of basins studied. The modeled sediment yields are comparable to previous field-based estimates of modern sediment yields, and generally lower than estimates developed from analyses of cosmogenic radionuclides.

Automated summary

Global compilations of river sediment loads show that mountainous areas contribute significantly to the sediment transported to the oceans. This study focused on estimating the contemporary sediment loads of 16 gravel bed rivers draining the Ecrins-Pelvoux Massif in southeast France. The results indicate a linear relationship between bed load fluxes at channel-forming flows and downstream increases in discharge, as well as a linear scaling of annual sediment loads with drainage area.