EXTRAORDINARY HOLOCENE RATES OF RIVER INCISION AND TIMESCALES OF POST-GLACIAL FLUVIAL PROFILE REEQUILIBRATION IN THE FRENCH WESTERN ALPS

The controls on rates of fluvial incision, as well as the timescales required to reach equilibrium in a fluvial profile, are not well understood. We study the late Cenozoic development of drainage basins in the western Alps, which show strongly variable ice occupation during glaciations, in order to understand the processes and rates of fluvial incision and profile reequilibration. Along the border of the glaciated region, damming of ice-free tributary valleys by glacier tongues resulted in massive sediment storage and severe disturbance of fluvial profiles. Downstream of the glacier fronts, and in ice-free catchments, valley bottoms widened under periglacial climates and were subsequently incised during interglacial periods. The inactivated floodplains remain as well-preserved strath terrace treads.

We provide the first absolute ages of alluvial terraces from the Buëch and Drac rivers using in situ cosmogenic ¹⁰Be. In order to overcome problems of inheritance and disparate exposure histories, we employ a profiling technique in which several samples up to a depth of a few meters were analysed. Terrace ages are obtained by fitting theoretical depth-concentration curves to the data.

Terraces from the ice-free Buëch river catchment span several glacial-interglacial cycles, the oldest terraces having an age of ~180 ka. They provide a good estimate for the integrated, long-term incision rate of ~1 mm/y. Within the glaciated area, the Drac river valley is characterised by periodic glacial damming of its outlet, resulting in 800-m variations in the river base level. Cosmogenic dating of post-glacial valley terraces reveals the headward propagation of a well-defined knickpoint over more than 50 km since the inception of incision, soon after final glacier retreat. All terraces are younger than 18 ka, indicating extremely high post-glacial incision rates (up to 30 mm/y) in glacial sediments and bedrock in this river that attempts to restore an equilibrium profile.