Hysteretic Bedload-Discharge Dynamics in Austrian Alpine Gravel-Bed Rivers: Evidence from Long-Term Monitoring

Bedload is considered a fundamental aspect of river “health” on a global scale. A comprehensive understanding of bedload transport is crucial for ensuring resilient river function and the implementation of sustainable, long-term basin management strategies. In Austria, integrative monitoring of bedload transport has been conducted for a period exceeding two decades, encompassing numerous monitoring stations and projects. Utilising these extensive datasets, we conducted a comprehensive analysis of bedload-discharge relationships in three alpine gravel-bed rivers (Drau, Isel, and Rofenache) to elucidate hysteretic bedload process behaviour and its underlying drivers. Event-scale and seasonal analyses reveal pronounced hysteresis between bedload flux and discharge, with river-specific dominance of clockwise (CW) versus counterclockwise (CCW) loop patterns. CW hysteresis is associated with unrestricted bedload availability, whereas CCW hysteresis indicates limited bedload supply. The direction of hysteresis and the hystereses loop patterns are related to e.g., seasonality (winter, summer, fall, autumn), anthropogenic influences, like channel modifications, flow regulation, barriers or hydropower, and hydro-climatic factors (temperature and precipitation). In the high-alpine Rofenache catchment, CCW hysteresis is predominant, consistent with temperature-driven melt dynamics, delayed sediment mobilisation relative to peak discharge, and the ongoing influence of glacier retreat on bedload supply timing. Across various melt periods, emerging trends indicate a lengthening and shifting of bedload transport windows, thereby offering insights into future dynamics under the influence of climate change. The Upper Drau exhibits clear counterclockwise hysteresis that points to a bedload deficit, primarily driven by anthropogenic influences such as the residual flow reach downstream of a hydropower plant and insufficient upstream sediment input, both of which cause delayed bedload transport relative to peak discharge. In contrast, the Isel shows CW hysteresis, indicating that no bedload deficits were present at the monitoring station during the study period; here, hysteresis loop patterns are a key component of process understanding. The results obtained can inform the development of evidence-based sediment management strategies, as well as habitat restoration and risk mitigation strategies, which can be tailored to the evolving alpine river systems.