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 carrie d out in the catchment area of the Tortue, a hilly, sandy region, east of L e Mans. The swift restructuring of plots is responsible for two types of mo rphogenesis: Chronic erosion accounts for the slow transfer of sediments to wards the lower part of plots and slopes. The overland flow caused by the r estructuring of surface soil erodes some 150 to 200 kg ha(-1) during the mo nth 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 nume rous relays. Catastrophic erosion, triggered by storm showers falling on dr y 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.