Sediment budget of the Rhône delta shoreface since the middle of the 19th century

This study analyses long-term (1841–1872–1895–1974) changes in the Grand Rhône prodeltaic lobe to (1) quantify the accumulation, (2) determine the evolution of relict prodeltaic lobes and (3) establish long-term relationships between river sediment discharge, the shoreface and the continental shelf. Our results show a reduction of the sedimentation of the prodeltaic lobe at the main river mouth, since 150 yr by a factor of 3.7 (12.63 to 3.41 × 10 6 m 3 yr − 1 ). At the minor mouth of the river, erosion dominates and speeds up during the same period (− 0.53 to − 1.34 × 10 6 m 3 yr − 1 ). These changes are found to directly result from the river sediment input decrease related to the natural decreasing of the frequency of major floods (end of the Little Ice Age), the reforestation in the catchment area, the dam construction and the dredging activities (since the 1950s). Our results indicate that while there is large sediment accumulation in the area around a growing prodeltaic lobe, there is also a reduced contribution of the river sediments to the non-adjacent beaches of the mouth. Following a shift in the river channel and mouth, the relict prodeltaic lobes (Petit Rhône–St Férreol, Bras de Fer and Pégoulier) are reworked by waves and their sediments contribute partially to the growth of the spits (Espiguette, Beauduc and Gracieuse). This suggests that there is a “time-shift” between the input of river sediment to the sea and the build up of a beach. The chronic erosion of the coastline is likely to continue in the future since (1) a river shift is not possible, because the river channels are controlled by dykes and human intervention, (2) the decrease of river sediment input and (3) the relict prodeltaic lobes constitute sedimentary reservoirs that are gradually being used up.