Sediment connectivity in the high-alpine valley of Val Müschauns, Swiss National Park — linking geomorphic field mapping with geomorphometric modelling

Abstract

The efficiency of sediment routing through mountain sediment cascades is controlled by the connectivity of hillslopes to the main river system. A lack of connectivity may cause long-term sediment storage and exclude large fractions of a basin from the sediment routing for several thousand years. Here, we studied sediment dynamics in a small, formerly glaciated valley in the Swiss Alps. To characterise the sediment connectivity to the stream, we calculated a morphometric index using a GIS algorithm. The modelling results were tested against a field based geomorphic mapping of sediment storages, which were evaluated with respect to their state of (de)coupling. In accordance to the field diagnostics, the modelling results indicate very well that the present-day sediment flux is conditioned by the glacial valley morphometry inherited through Pleistocene glaciation. Especially in the upper hanging valleys, the connectivity index is reduced noticeably due to the glacial cirque morphology. Based on the field mapping, 30% of the hillslope sediment cascades are interrupted and 20% of the storage boundaries are currently affected by a lack of material transfer. As a consequence, ~29% of the basin surface is currently disconnected from the main river. Nevertheless, the GIS algorithm overestimates the connectivity within the basin, because it fails to calculate decoupling between neighbouring pixels in digital terrain models (DTMs). Around 35% of the basin surface, which has been mapped in the field as being decoupled, is related to relative high connectivity. Our study highlights the potential of morphometric GIS modelling for studying sediment connectivity, but additionally emphasises the indispensability of geomorphic field mapping for a holistic understanding of mountain cascading systems.