Impact of dam flushing operations on sediment dynamics and quality in the upper Rhône River, France

International audienceThe Rhône River (France) has been developed for energy production and 21 dams have been built. To avoid problems due to sediment storage, dam flushing operations are periodically organized. In order to investigate the impact of such operations on suspended particulate matter (SPM) dynamics (resuspension and fluxes) and quality (physico-chemical characteristics and contamination), we monitored flushing operations performed in June 2012 on 3 major dams on the Upper Rhône River. Samples were collected from 2011 to 2016 at a permanent monitoring station, located 150 km downstream. At equal water discharge, mean SPM concentrations during flushing operations were on average 6-8 times higher than during flood events routinely recorded at the permanent station from 2011 to 2016. While of short duration (19 days), the flushing operations led to the resuspension of SPM and contributed to a third of the mean annual SPM flux. Moreover, physico-chemical characteristics of transported SPM differed from those measured during usual baseflow and flood regimes. Variation of particulate organic carbon and contaminant concentrations (trace metals elements (TME) and organic compounds) were mostly driven by particle size and origin. SPM contamination was generally lower than during baseflow and flood regimes. However, flushing operations led to the resuspension of contaminated sediment trapped behind dams and to an increase of polycyclic aromatic hydrocarbons (PAHs) and Bis(2-ethylhexyl) phthalate (DEHP) concentrations. Finally, we propose a list of recommendations for the design of an adequate monitoring network to evaluate the impact of dam flushing operations on large river systems