Erosion in cultivated soils and river morphodynamics:: the example of the Tortue Basin (Sarthe, France)

An analysis of the functioning of the valley slope/ stream system is carried out in the catchment area of the Tortue, a hilly, sandy region, east of Le Mans. The swift restructuring of plots is responsible for two types of morphogenesis: Chronic erosion accounts for the slow transfer of sediments towards the lower part of plots and slopes. The overland flow caused by the restructuring of surface soil erodes some 150 to 200 kg ha(-1) during the month following sowing. But the limited amount of deposits (the Royeau brook carries away less than 50 kg ha(-1) per month) proves the existence of numerous relays. Catastrophic erosion, triggered by storm showers falling on dry soils with sparse vegetation led, in 1988 and 1990, to extensive gullying and longitudinal transportation that modified stream dynamics. Taken away partly from the stocks accumulated through chronic erosion, the: sand flow progresses slowly downstream with the floods. (C) Elsevier, Paris.