Temperature signal in suspended sediment export from an Alpine catchment

Costa, Anna; Molnar, Peter; Stutenbecker, Laura Antonia; Bakker, Maarten; Silva, Tiago A.; Schlunegger, Fritz; Lane, Stuart N.; Loizeau, Jean-Luc; Girardclos, Stéphanie (2018). Temperature signal in suspended sediment export from an Alpine catchment. Hydrology and earth system sciences, 22(1), pp. 509-528. European Geosciences Union EGU 10.5194/hess-22-509-2018

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Suspended sediment export from large Alpine catchments ( > 1000 km2) over decadal timescales is sensi- tive to a number of factors, including long-term variations in climate, the activation–deactivation of different sediment sources (proglacial areas, hillslopes, etc.), transport through the fluvial system, and potential anthropogenic impacts on the sediment flux (e.g. through impoundments and flow reg- ulation). Here, we report on a marked increase in suspended sediment concentrations observed near the outlet of the upper Rhône River Basin in the mid-1980s. This increase coincides with a statistically significant step-like increase in basin-wide mean air temperature. We explore the possible explanations of the suspended sediment rise in terms of changes in wa- ter discharge (transport capacity), and the activation of dif- ferent potential sources of fine sediment (sediment supply) in the catchment by hydroclimatic forcing. Time series of precipitation and temperature-driven snowmelt, snow cover, and ice melt simulated with a spatially distributed degree- day model, together with erosive rainfall on snow-free sur- faces, are tested to explore possible reasons for the rise in suspended sediment concentration. We show that the abrupt change in air temperature reduced snow cover and the con- tribution of snowmelt, and enhanced ice melt. The results of statistical tests show that the onset of increased ice melt was likely to play a dominant role in the suspended sediment concentration rise in the mid-1980s. Temperature-driven en- hanced melting of glaciers, which cover about 10 % of the
catchment surface, can increase suspended sediment yields through an increased contribution of sediment-rich glacial meltwater, increased sediment availability due to glacier re- cession, and increased runoff from sediment-rich proglacial areas. The reduced extent and duration of snow cover in the catchment are also potential contributors to the rise in sus- pended sediment concentration through hillslope erosion by rainfall on snow-free surfaces, and increased meltwater pro- duction on snow-free glacier surfaces. Despite the rise in air temperature, changes in mean discharge in the mid-1980s were not statistically significant, and their interpretation is complicated by hydropower reservoir management and the flushing operations at intakes. Overall, the results show that to explain changes in suspended sediment transport from large Alpine catchments it is necessary to include an under- standing of the multitude of sediment sources involved to- gether with the hydroclimatic conditioning of their activation (e.g. changes in precipitation, runoff, air temperature). In ad- dition, this study points out that climate signals in suspended sediment dynamics may be visible even in highly regulated and human-impacted systems. This is particularly relevant for quantifying climate change and hydropower impacts on streamflow and sediment budgets in Alpine catchments.

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