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A vital role
Thermal refuges in rivers are areas where the water remains cooler than the rest of the watercourse during the summer. They play a vital role for aquatic fauna, which can seek shelter there when temperatures rise above their tolerance limits. Often well known to anglers, these refuges are visible to the naked eye when they are associated with the shade of riparian vegetation, a steep-walled valley, or the confluence of a cooler tributary. But many of them also depend on invisible phenomena: groundwater inflows.
An important issue
It is precisely this mechanism that the BRGM and its partners have studied in the Rhône basin, through three thermal refuges fed by groundwater. This is a critical issue because in the summer, after several days without rain, rivers rely more heavily on groundwater to maintain their low-flow rates. However, this groundwater, which generally maintains a stable temperature, helps to cool the river locally and create a thermal anomaly that supports aquatic life.
These thermal anomalies act as natural regulators. When the underground inflow is sufficient relative to the river?s flow rate, the temperature becomes more stable, sometimes over several kilometers. Wildlife finds a more suitable environment there to survive, feed, and reproduce. Researchers are particularly interested in indicators such as the number of days when the water temperature exceeds 20 °C, a threshold considered biologically sensitive for many species.
But climate change is undermining this balance. Heat waves are more frequent, air temperatures are rising, rivers are warming, and low-flow rates are decreasing. The question raised by the study is therefore crucial: Will groundwater still be able to serve as a refuge in the context of sustained warming? And what will happen if the groundwater itself warms up? For rivers?and for all the life they support?the stakes go far beyond the issue of temperature alone.
