Temperature signal in suspended sediment export from an Alpine catchment
Authors: Costa Anna , Molnar Peter , Stutenbecker Laura , Bakker Maarten , Silva Tiago A. , Schlunegger Fritz , Lane Stuart N. , Loizeau Jean-Luc , Girardclos Stéphanie 
Published: January 2018

Abstract. Suspended sediment export from large Alpine catchments (> 1000 km2)
over decadal timescales is sensitive 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 regulation). 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 water discharge (transport capacity), and the
activation of different 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 surfaces, 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 contribution 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
enhanced 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 recession, 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 suspended
sediment concentration through hillslope erosion by rainfall on snow-free
surfaces, and increased meltwater production 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
understanding of the multitude of sediment sources involved together with the
hydroclimatic conditioning of their activation (e.g. changes in
precipitation, runoff, air temperature). In addition, 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.