Medium-term Changes in Suspended Sediment Delivery to the Ocean: Consequences of Catchment Heterogeneity and River Management (Rhône River, France)

The aim of this work was to study the influence of watershed management and catchment heterogeneity on the concentration and mineralogical composition of suspended solids in a large Mediterranean river. The Rhône was chosen as its catchment is characterized by a high geological and climatic heterogeneity. As a consequence, floods can be distinguished according to their geographic origins, i.e. Mediterranean or oceanic. The flow and the suspended solids concentration were measured daily near its mouth, from October 1992 to May 1995, including 10 flood events (return time discharge up to almost 100 years). As shown by the statistical relationships between water discharge and suspended sediment discharge, the solid transport capacity of the river is greatly affected by the catchment heterogeneity. Most of the solid discharge is transported during Mediterranean floods and the mineralogical composition is also highly related to the origin of the water masses. We validated our current non-linear model relating daily solid discharge to mean daily water discharge using an independent data set covering the period 1971–1991. Back-calculations of annual solid discharges during the last 30 years demonstrate the importance of inter-annual variability and the role played by short-term climatic fluctuations. Our mean long-term sediment load (7·4×106tonsy−1) is higher than previously published estimates and demonstrates the consequences of spot sampling. Unpublished data from 1956–58 were used to assess the impacts of dam erection. Our present model underestimates the solid discharge in the 1950s, but by no more than 67%. Although management practices have reduced the flow of sediment to the estuary, long-term variations in river-sediment load since the nineteenth century must also be related to climatic shift.