Sediment yield and storage variations in the Negron River catchment (South Western Parisian Basin, France) during the holocene period

Abstract : In the Négron River catchment area (162 km2), surface-sediment stores are composed of periglacial calcareous grèze (5 × 106 t) and loess (21 × 106 t), and Holocene alluvium (12*6 × 106 t), peat (0*6 × 106 t) and colluvium (18*5 × 106 t). Seventy-five per cent of the Holocene sediments is stored along the thalwegs. Present net sediment yield, calculated from solid discharge at the Négron outlet, is low (0*6 t km-2 a-1) due to the dominance of carbonate rocks in the catchment. Mean sediment yield during the Holocene period is 7*0 t km-2 a-1 from alluvium stores and 7*6 t km-2 a-1 from colluvium stores. Thus, the gross sediment yield during the Holocene period is about 18*7 t km-2 a-1 and the sediment delivery ratio 3 per cent. The yield considerably varies from one sub-basin to another (3*9 to 24*5 t km-2 a-1) according to lithology: about 25 per cent and 50 per cent of initial stores of periglacial grèze and loess respectively were reworked during the Holocene period. Sediment yield has increased by a factor of 6 in the last 1000 years, due to the development of agriculture. The very high rate of sediment storage on the slope during that period (88 per cent of the yield) can be accounted for by the formation of cultivation steps (rideaux). It is predicted that the current destruction of these steps will result in a sediment wave reaching the valley floors in the coming decades. Subboreal and Subatlantic sediments and pollen assemblages in the Taligny marsh, where one-third of the alluvium is stored, show the predominant influence of human activity during these periods in the Négron catchment.