Les réponses des systèmes fluviaux à une réduction des flux d'eau et de sédiments sous l'effet du reboisement en montagne

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Reafforestation was promoted in the XIX^th c. in order to reduce slope erosion in mountain areas. The purpase of this paper is to examine the impact of reafforestation on rivers flowing downstream from such reafforested areas in the Alps and in the Southern Pyrenees. First, the clearance of forests for agriculture in the XIX^th c. lead to strong links between slopes and aggrading rivers, but alternating periods of very active and relatively calm hydrology have induced a complex response of river beds on the level of decades in upstream valleys. Since the abandonment of slopes and the reafforestation promoted by State Services, a strong decrease in coarse sediment supply and a more moderate reduction in water fluxes during floods resulted in river bed incision in all the studied areas. Lastly, the processes involved in present sediment supply are explored, at both the watershed scale in the valley bottoms, the latter having become the main purveyors of coarse sediment to downstream reaches.