Recent morphological channel changes in a deltaı̈c environment. The case of the Rhône River, France

Channel morphological changes were measured at two sites within the Rhône delta along the main channel of the Rhône River over one century. Results show an average erosion rate of 2.8 mm year−1 during the 20th century and decreasing to <1.5 mm year−1 since the 1960s. Erosion affected site 2 more than site 1 but cross-sections can alternately be in erosion or accretion depending on bed topography and/or floor lithology. Riverbed adjustments are partly explained by the boundary shear stress and specific stream power but local factors also influence the Rhône River evolution. Results compared with those obtained upstream or in other European rivers suggest that the lower Rhône River has developed an original response to external stresses: the first part of the 20th century is characterised by an important degradation followed by a decrease in the incision rate since the 1960s. The initiation of the erosion phase is assumed to be linked to the end of the Little Ice Age, to the reforestation of the alpine hillsides, to the earlier management and to the reduction of the sedimentary yield (according to the literature). The second phase possibly corresponds to the relaxation period. In this perspective, the effects of dams are probably less significant than we would suppose up to now. It implies that such a large river records morphological responses which may vary all along the fluvial system in relation to the time and space scales involved.