Suspended sediment concentrations in Alpine rivers: from annual regimes to sub-daily extreme events

Abstract. The occurrence of extreme suspended sediment concentrations (SSC) can have a detrimental impact on human infrastructure, water use, and the health of aquatic ecosystems. However, the majority of existing studies have focused on the SSC dynamics of individual catchments or single events, with the consequence that large-scale patterns of suspended sediment dynamics remain poorly understood. The objective of this study is to identify the principal factors that influence the spatial and temporal variability of suspended sediment concentration (SSC) and the occurrence of SSC extremes in the Alps. For our analyses, we use 10 years of observed sub-daily SSC data from 38 gauging stations in Switzerland and Austria. First, we examine spatial patterns in the annual median SSC regime, utilising hierarchical clustering based on the differences in magnitude, timing and shape of the annual SSC regime. The clusters are then reconstructed and explained by linking them to a large set of potential static hydro-climatic and catchment-related characteristics. This approach identified three distinct clusters of annual SSC regimes, for which the shape of the regime and the timing of the annual peak SSC are significantly influenced by catchment elevation, the start of the melt season, and the presence of glaciers. Second, we transition from the annual to the event scale at a sub-daily time step by identifying extreme events. We present a novel classification scheme that can be employed to categorise extreme SSC events and differentiate between nine event types based on their dominant transport processes. We examine the spatial and temporal distribution of these nine event types across the Alps, the severity of the events, and the effect of antecedent conditions, such as snow cover, soil moisture, and catchment memory. The analysis of 2398 extreme SSC events across all catchments indicates that rainfall is the primary driver of SSC extremes, responsible for 80 % of all events. Nevertheless, in high-elevation and partially glaciated catchments, up to 40 % of the events are attributed to snow and glacial melt, underscoring the disproportionate impact of meltwater on sediment concentrations in Alpine rivers. The combination of high-intensity rainfall and glacial melt events resulted in the highest SSC and second highest area-specific suspended sediment yields (sSSY) on average among all event types. A notable proportion of the extreme events (24 % of the total) resulted in peak SSC values exceeding 5 g L^-1, highlighting their potential to significantly harm aquatic species and riverine ecosystems. Our findings underscore the importance and impact of extreme SSC events on water quality and sediment transport in Alpine river systems.