Suspended sediment dynamics in Alpine rivers: from annual regimes to short-term extremes

Suspended sediment is a natural component of rivers, but extreme concentrations can have substantial impacts on water quality, aquatic ecosystems, floods, hydropower production, etc. In mountain environments, sediment availability and transport are modified by a changing climate through changes in erosive precipitation, snow cover and glacier retreat. As it is well known that the majority of suspended sediment load is transported during a few extreme events, it is essential to better understand the spatial and temporal dynamics of suspended sediment concentration (SSC) during extreme events, now and in the future. To date, most studies have attempted to predict SSC dynamics based on catchment characteristics and hydroclimatic factors, however, mostly for individual catchments or specific events, which limits our understanding of SSC dynamics at larger spatial scales. This research aims to identify the main factors that influence the spatio-temporal variability of SSC and the occurrence of SSC extremes in the Alps.

We use 10 years of observed subdaily SSC data from 38 gauging stations in Switzerland and Austria to study the temporal and spatial variability of SSC. First, we examine spatial patterns in the annual SSC regime. We identify three main types of annual SSC regimes after applying hierarchical clustering based on regime differences in magnitude, timing and shape. Our results show that snow and ice significantly influence the annual SSC regime in small mountainous catchments, in contrast to low-elevation and larger catchments where rainfall is more important. The presence of glaciers and the timing and amount of snowmelt play a crucial role in shaping the annual SSC regime and determining when peak SSC occurs, whereas geological and soil characteristics and the annual runoff regime have a smaller influence.

Second, we move from the annual to the event scale at a subdaily time step by analyzing extreme events. We introduce a new classification scheme to categorize the 2,398 extreme SSC events into nine distinct types, based on their dominant transport processes. Our study reveals that rainfall is the main cause of these extremes, responsible for 80% of the events. However, in high-altitude and partially glaciated catchments, up to 40% of the events are driven by snow and glacial melt. Events triggered by both glacial melt and intense rainfall produce the highest sediment concentrations and area-specific yields. These insights into the large-scale and catchment-specific variations in SSC and their extremes are valuable for improving our understanding of the complex hydrology-sediment system response.