Contemporary evolution of forest margins was studied on the middle section of the Ubaye, a gravel bed river of the Southern Alps. A recolonization of the valley flat by Pinus sylvestris which began around 1920-1925 is described. This forest expansion was accompanied by a reduction of active channel area and by local changes in geomorphological pattern, shifting from braided to meandering. This biometamorphosis, explained by the decrease of coarse bedload and peak flows, was primarily related to human activities-watershed reforesting and torrent control, abandonment of agro-pastoral activities-perhaps naturally reinforced by the end of the Little Ice Age climatic degradation. Moreover, spatial variations of forest-channel contact are also shown on the scale of a few decades, for the period of 1945-1990. These variations were also apparently controlled by hydrology and bedload. Management of these floodplains, characterized by complex, dynamic and fragile ecosystems, is discussed.