Evaluating options to mitigate hydropower impacts on river thermal regimes in a changing climate

Storage hydropower alters the natural flow regime of rivers through reservoir impoundment, modified residual flows, and hydropeaking operations. These hydrological changes may also affect river temperatures and so aquatic ecosystems. Hydropower mitigation has primarily focused on hydrological impacts; thermal consequences and interactions with climate change remain poorly understood. To address this gap, this study evaluates the thermal effects of three hydropeaking mitigation strategies for a peri-Alpine river strongly affected by hydropower regulation; regulation basin, diversion tunnel, and residual flow increase; under current and future climate conditions. Under current climate conditions, both regulation basins and residual flow increases significantly reduce short-term thermal rates of change (from approximately 7 to 4 °C/h), though values remain above those observed in natural rivers (around 1.5 °C/h). Only residual flow increases markedly reduce the number of days exceeding critical thermal thresholds (15 °C here), by up to 35 days. In contrast, diversion tunnels show negligible effect on thermal indicators and may increase vulnerability to temperature extremes. Under future climate scenarios, mitigation strategies maintain their relative effectiveness in limiting thermal gradients, but their ability to reduce threshold exceedances declines under the most severe climate scenarios, due to rising reservoir temperatures. These findings illustrate how ecologically relevant thermal indicators can be integrated into mitigation assessment within a modelling framework that accounts for the combined effects of hydropower operations and climate change under current and future conditions. The work shows the limitations of current approaches to assessment, which are often restricted to short-term thermal indicators or rely on idealized reference conditions.