Bedload transport histories in heterogeneous Alpine glaciated catchments

The rapid retreat of Alpine glaciers in recent decades has a direct impact on glacier runoff and sediment transport. As deglaciation begins, runoff increases to a maximum (“peak water”), after which runoff gradually decreases. Sediment transport is expected to vary as well over the time, but despite the significant environmental relevance of sediment transport for the ecological functioning of Alpine streams and its potential hazard to Alpine communities, little is known about how sediment transport (and most the notably bedload fraction) changes with deglaciation. The difficulty of measuring bedload transport in the field determines the paucity of measured time-series extending back in time for more than a couple of decades, and none in deglaciating river basins.

One method of overcoming this problem is to use data collected from water intakes of hydropower plants, widespread in Alpine streams and rivers. Many intakes were built in the 1960s, and mostly at high elevations, in glaciated basins. Most of them have sediment traps where water and sediment are separated before water can be transferred to storage or to the turbines. Such traps need to be flushed when bedload deposits reach a certain level. By knowing the volume of the sediment traps and the packing density of the sediment, it is possible to reconstruct the bedload transport history of a given catchment, by analysis the flushing operation. For regulatory reasons, such records are commonly also complemented by very high-quality water discharge records that can be used to reconstruct the associated bedload transport capacity and thus determine the extent to which bedload has been supply or transport limited through different hydrological periods.

In this work, we present the reconstruction of the volume of bedload exported over the last 60 years for more than 20 Alpine catchments located in the southwest of Switzerland. These basins are heterogeneous with different extents of contemporary glaciation and different climatic and geographical characteristics. Data suggest an upward trend in sediment transport since the late 1980s for most of the catchments analysed, coinciding with the onset of rapid Alpine warming in the 1980s. Bedload transport slowed in the 1990s, seemingly associated with a series of years with reduced up-glacier snowline recession, before accelerating again in the early 2000s, with some evidence of a peak sediment export in the 2010s. The snowline recession effect is interesting because it is supported by recent process-based studies which suggest that the ability of glaciers to evacuate bedload-sized sediment is constrained by up-glacier extension of the subglacial drainage network during a melt-season, itself controlled by snowline recession. However, some glaciers show anomalous behaviour which are possibly caused by the direct effects and legacy of glacial overdeepenings. This can lead to site-specific, geomorphologically-influenced responses of bedload transport on top of the underlying regional-scale trend of climate warming.