River channel changes in the Rhone Delta (France) since the end of the Little Ice Age: geomorphological adjustment to hydroclimatic change and natural resource management

This study provides a representative example of a river affected by artificial width contraction and consequent bed incision in a context of sea-level rise. Geomorphological adjustment in the Rhone catchment since the final stage of the Little Ice Age has been induced by hydroclimatic change and human disturbances. These adjustments are examined to highlight the response of two sand-bed distributary channels of the Rhone Delta. By using methods such as sedimentological evidence, historical sources, planimetric resurvey, repeated longitudinal and cross-profiling and hydraulic data, the geomorphological and hydraulic responses to channel changes are characterised and their consequences assessed. It is shown that the reduction of sediment yield is a combined effect of a decrease of flood frequency, sediment dredging and building of dams on the Rhone River and its tributaries. The combination of decreased catchment sediment yield, artificial width contraction, increased stream power and boundary shear stress is the cause of channel incision of the Rhone River. In the coastal zone, the increase of the Rhone River's stream power has facilitated the transport of sediment down to the subdeltas. Engineering works, designed to stabilise the planform and to limit overbank flooding, caused (1) channel entrenchment, thus (2) the destabilization of infrastructure (e.g. bridges, quays) along the river, (3) the lowering of the phreatic water table in the floodplain, which in turn induced (4) soil salinization (change in salt wedge position); eventually, the channelization led to (5) an increase of the flood hazard and risks because, although more rare, the inundations have become more destructive. However, the contribution of the large flood events is negligible in terms of progressive channel in-filling because of their low frequency. Finally, no relation exists between the vertical adjustment of the Rhone channel through entrenchment and relative sea level, which rose at a rate of up to 2 mm year(-1) during the 20th century. Clearly, natural resource management, in particular engineering activities, were the main cause of the river channel changes that occurred during the last century. (C) 2002 Elsevier Science B.V All rights reserved.