Assessment of Bedload Delivery from Tributaries: The Drôme River Case, France

This paper examines the potential to reverse bed degradation trends in the Drôme River (southeast France), a prealpine gravel-bed stream, by assessing coarse sediment supply in tributary subbasins. Two approaches are developed. First, a Principal Component Analysis was performed on 24 subwatersheds to characterize the most productive ones. Second, fieldwork and aerial photographs provide detailed assessment for two mountain streams, one characterized by abundant bedload (the Esconavette Creek), the other characterized by 30 yr of inactivity (the Barnavette Creek). Active tributaries (with developed active gravel bars) have been discriminated according to variables describing watershed sediment storage and production potential. Variables describing human intervention on erosion and vegetation cover appear to exert secondary influences on geomorphic activity in a context of general geomorphic stabilization. Declining activity associates features such as bed incision, channel narrowing, and vegetation development in riparian buffer strips in the two mountain streams studied. They are more frequent on the Barnavette Creek, the inactivity of which is partly explained by smaller lateral inputs from valley-filling.