POSTGLACIAL HYDROGEOMORPHOLOGICAL CHANGES AND RATES OF SURFACE LOWERING IN A SMALL APPALACHIAN CATCHMENT NEAR RIMOUSKI (BAS-SAINT-LAURENT, QUEBEC)

Postglacial hydrogeomorphological changes and rates of surface lowerin g in a small Appalachian catchment near Rimouski (Bas-Saint-Laurent, Q uebec). Stratigraphical analysis of a small alluvial fan (radius: 60 m ) located near Rimouski in a forest environnement allows to reconstruc t paleohydrological change since 11 000 yr BP in relation with coeval environmental changes. The fan materials are well exposed in a long tr ench across the distal part of the fan. The basal unit (mean thickness : 93 cm) is mainly composed of locally derived angular and subangular chips (shale, siltstone) of gravel-size with only crude stratification locally. This coarse unit is overlain by an unstratified silty sand ( mean thickness: 66 cm) containing few scattered lenses of well-rounded gravel, pieces of wood, charcoal, buried organic soil and one layer o f pure organic debris. The transition from basal gravel to superficial organic-rich silty sand by 7200 yr HP is synchronous with the definit ive closure of the regional forest cover as established by pollen anal ysis. This suggests that hydrological changes were closely related to the postglacial vegetation history. Fan volume estimates allows to cal culate the postglacial evolution of the rate of surface lowering in th e related feeding basin (0.145 km(2)). The rate of lowering changed ra dically from 0.0063 mm/y (or 6.3 B) between 11 000 and 7200 yr BP to 0 .0023 mm/y (or 2.3 B) since 7200 yr BP.