Glacial drainage development controls temporal and spatial fluctuations of sediment flux in Mont Blanc’s North face.

21 pagesInternational audiencePresent-day global warming raises important issues regarding sediment flux from glaciated catchments. The detrital export from such environments results from erosion processes operating in three geomorphic domains: the supraglacial rockwalls, the ice-covered substratum and the proglacial area, downstream from the glacier. The dominant process controlling present-day sediment export from glaciated catchments remains debated with most studies underlining the paraglacial dynamics in the glacier forefront. This study nuances these observations by quantifying the contribution from each geomorphic domain to the export of a glaciated catchment in the North face of Mont-Blanc (France). High resolution hydro-sedimentary flux data were acquired during eight years in two proglacial streams with contrasting glacio-hydrological characteristics, Bossons and Crosette streams. In the Bossons stream catchment, the sediment response highlights the initiation of the dendritic drainage network beneath the glacier, the short-lived evacuation of an annual storage during the early melt-season and its subsequent steadier regime. In addition, three years exhibit late melt season exports which are uncorrelated with temperature or rainfall. The evolution of the drainage network throughout the melt season explains the evacuation of the annual and pluri-annual subglacial sediment stocks. In addition, glacial retreat in link with higher melt rate allows for exporting a pluri-annual sediment stock stored beneath the glacier. To conclude, the present-day sediment response in Bossons catchment displays distinct components with characteristic timescales and is dampened by intermediate storage controlled by drainage development and extreme events in the glacial and proglacial domains