Hydrothermal Resilience of Quebec Rivers: A 3D Modeling Approach to Groundwater’s Cooling Effect During Heat Waves

Exchanges between ground and surface water strongly influence how rivers thermally respond. Ground-to-surface water connections are particularly important during periods of intense atmospheric heat waves. In salmonid-rich rivers of Quebec, elevated summer temperatures can induce thermal stresses, threatening aquatic ecosystems. This study’s objective was to evaluate the influence of groundwater discharge on river water temperature, using a 3D coupled flow and heat transfer model calibrated with one year of field data. The results show that groundwater inflow reduced the peak river temperatures by 1.5–3.2 °C during heat waves, representing up to 40% of the river’s thermal budget under low-flow conditions. In both rivers, groundwater prevented the temperatures from exceeding the 20–22 °C threshold critical for salmonid survival. These findings underscore the importance of integrated hydrothermal modeling for predicting ecological vulnerability under climate change.