Sediment regimes of alpine braided rivers (SE France)
Résumé
The sediment regime of alpine braided rivers in SE France was investigated from the 20th century evolution of the channel elevation. Observations are based on 31 sites distributed over the Alps and their piedmont, that cover 129 km of river length, that is to say about 20% of the present-day alpine braided network. For each site, the merging of the present-day long profile (obtained from 2009 terrestrial topographic surveys) with the historical long profile of the "Grandes Forces Hydrauliques" (surveyed between 1894 and 1931 depending on sites) allows reconstructing elevation changes of the water surface at low-flow during the period. Results show that 56% of the river length is in degradation, 20% in aggradation, and 24% stable. The spatial variability of channel responses shows that the most active braided channels are preferentially conserved in the Southern Alps. Statistical links are clearly established between historical elevation changes and forcings of the sediment budget (gravel mining, sediment supply from torrents and lateral shiftening of river channels). We notably show that the persistence of an important sediment recharge from torrents exerts a major control on the sediment regime of braided channels.
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