Assessment of fine sediment river bed stocks in seven Alpine catchments

International audience; While the finest fraction of suspension has long been considered to have limited interactions with the river bed, several recent studies based on flume and field observations raise questions about this hypothesis which is fundamental for suspended load modeling and river management. In this study, we report a large field campaign in which we quantify the river bed stocks of fine particles in 7 contrasted Alpine catchments. Using a simple protocol, we performed more than 300 riverbed measurements of the local surface and subsurface stocks. Results indicate that even when the river bed surface contains no fine particles, significant quantities can be found in the subsurface layer which is in most cases the layer having the higher stocks. We also observed that stocks highly depend on the facies considered suggesting that storage processes are strongly driven by the local hydraulics and river bed characteristics. By integrating these local stocks at the catchment scale, we estimated that they could represent more than 50% of the mean annual suspended load in catchments having large alluvial braided sections. On the opposite, these stocks could be as small as 1% in highly eroded head water catchments. This suggests that the bed of large alluvial Alpine rivers can be considered as a significant source of fine particles. These observations were confirmed by using a simplified vertical scouring model to estimate conditions for these stocks to be released in the flow. However, the use of this model suggests that other bed reworking processes (channel widening and migration) have to be considered. Finally, these observations suggest that interactions between particles transported as suspension and gravel beds are far from being negligible processes in catchments having large alluvial sections typically found in Alpine environments.