Thermal regime of a heavily regulated Alpine river at multiple spatial scales

The present study aims to quantify the thermal regime of a highly regulated Alpine river at the catchment scale, some of its ecological implications, to support decision making in water management, particularly in relation to revising existing ecological flows protocols. The chosen case study is  the Sarca river (NorthEastern Italian Alps), where a complex hydropower diversion scheme, consisting of more than 60 km of tunnels and penstocks, two large storage reservoirs and two major hydropower plants, were constructed in the 1950s.

The Sarca River originates from the Adamello glacier, flowing for approximately 80 km to Lake Garda. The study spans from Spiazzo (1100m asl)  to Sarche (150m asl), including some major lateral tributaries such as the glacier-fed Bedù stream. River water temperature was monitored since May 2025 using 14 continuous HOBO sensors conveniently distributed along the study area, on the basis of criteria that account for existing hydrometric stations, the ability to capture relevant spatial and temporal variability, and physical accessibility. In addition to such year-round catchment-scale distributed monitoring, short-term temperature monitoring at five sites along the middle course of the Sarca River was designed to assess local thermal variability associated with riparian vegetation shading, and valley slopes morphology and exposure comparing sensors placed in locations with different sunlight exposure with a spatially explicit assessment of hourly radiation.

The measured thermal regime reveals that river water temperatures already achieve values of concern for the local fish community, particularly during summer heatwaves. An analysis of continuous event duration under threshold allows to estimate some possible effects on the autochthonous marble trout. Reaches with a North-South alignment and a wider valley floor show higher daily thermal oscillations despite their higher elevation.  The effect of local hydrology, valley morphology, riparian vegetation, in creating local thermal refugia for target species such as brown and marble trout is quantified. By integrating watershed-scale analyses, thermal monitoring, fish thermal requirements, spatially explicit year-round shading models, this study shows the relevance of combining ecological, hydrological, and water management perspectives to understand the thermal effects of water diversions and climate change on Alpine riverine ecosystems, providing the basis to design possible mitigation measures to increased river warming.