Bedload Fluxes in a Glacier-fed River at Multiple Temporal Scales

In mountain rivers, long-term observations of water and sediment fluxes are crucial for understanding the dynamics of bedload fluctuations. We analyze 7 years of continuous data gathered from eight geophone plates at a monitoring station in the glacier-fed Sulden/Solda River (South Tyrol, Italy) to estimate the bedload flux at 1-min scale. Sixty-five bedload samples were used to derive the calibration equations adopted to quantify the transported bedload mass. The signal power is proposed as a more effective metric than threshold-based impulses for calculating bedload masses. Results show (a) a remarkable variability of bedload rates for the same value of flow discharge, (b) the joint effect of storm-driven flood events and seasonal changes in sediment supply on bedload rates, and (c) the strong impact of climatic factors (i.e., temperature and snow cover) on bedload fluxes. Moderate bedload rates occurring in late spring/early summer are likely related to the mobilization of riverbed sediments, while sustained bedload transport during melt flows in July-August-corresponding to the effective bedload discharge range-is associated with the activation of glacial and proglacial sediment sources. The data set shows a complex climatic control on bedload transport at the basin scale, where precipitation, air temperature, and snow cover determine flow and glacier melting dynamics. These findings suggest how the effects of climate change in the Alps likely will lead to an increase in peak bedload rates in a context of declining annual bedload yields as melt flows will progressively reduce in the next decades.