Contribution of Flooded Soils to Sediment and Nutrient Fluxes in a Hydropower Reservoir (Sarrans, Central France)

When a water reservoir is created, the pre-existing soils and vegetation are flooded. Here, we took advantage of the complete emptying of the Sarrans Reservoir, which was flooded 80years ago, to study the contribution of soil flooding to sediment and nutrient fluxes in relation to water management. On the slopes of the annual drawdown zone, soils were eroded; in contrast, soils in the permanently flooded area were covered by silty sediments. A mass budget suggested that approximately one-third of the silty sediment cover that had accumulated on the lower slopes and the bottom of the reservoir had originated from this erosion. Averaged over the first 25years after impoundment, the flux of the soil carbon that was redistributed from the annual drawdown zone was about one-fourth of the flux of the suspended matter that entered the reservoir from the catchment. The flux of eroded phosphorus was approximately two-fifths of that entering Sarrans from the catchment. On the other hand, the fluxes of dissolved organic carbon and N-NO3 entering the reservoir were approximately one order of magnitude higher than those redistributed by soil erosion; however, most of these fluxes left the reservoir through discharge. The comparison between flooded and unflooded grassland soils indicated a major loss of carbon (approximately 50%) since the impoundment, which was most likely due to mineralisation. These additional carbon and phosphorus fluxes may contribute significantly to the trophic upsurge process, as well as to the peak in GHG emissions that followed the reservoir impoundment.