Appreciated by fish and well-known to anglers, thermal refuges in rivers are precious for aquatic fauna. A large number of thermal refuges have an invisible origin: groundwater.
© BRGM
Thermal refuges: shelter for aquatic fauna during heatwaves
During heatwaves, river water gradually warms up. Like us, aquatic fauna can adapt to variations in the temperature of their surrounding environment, but only up to a certain limit. Beyond that, if they are to survive, they need to find a cooler place to take shelter. A stretch of river where the water temperature is significantly cooler in summer is known as a “thermal refuge”, providing shelter for wildlife to survive and reproduce.
Thermal refuges are often due to visible hydrographic features: riverside vegetation that provides shade, stretches of the watercourse running through gorges that protect it from the sun, or a confluence with a colder tributary from the mountains. However, a large number of thermal refuges have an invisible origin: groundwater.
The occasional influx of groundwater along the river, which generally has a constant temperature throughout the year, helps cool the water in summer (and warm it in winter), creating what hydrologists call a “thermal anomaly”. If it attracts aquatic fauna, it becomes what ecologists call a “thermal refuge”.
The water in our rivers warmed by almost 1°C over the period 2009-2022. It should continue to warm under the impact of climate change, particularly in summer, as a result of the drop in low-water levels. Against this backdrop, will thermal anomalies be able to continue providing a refuge? How might groundwater warming affect this mechanism?
Thermal refuges : Does groundwater play a role?
Three thermal refuges studied in the Rhône basin
BRGM, the French geological survey, studies the subsurface and the groundwater it contains. As part of the ESTHER project, carried out in collaboration with the Rhone-Mediterranean-Corsica Water Agency, an observation and modelling programme for groundwater-dependent thermal refuges was carried out to monitor three thermal refuges located in the Rhône basin:
- the Argens river in the Provence Verte region;
- the Drac, a mountain river with a regime influenced by melting snow, originating in the Écrins Massif;
- the Veyle, a river in the Dombes region.
At the end of this first phase, it is already clear that when groundwater arrives in sufficient quantity in relation to the flow of the watercourse, it acts as a “natural regulator” of temperature variations. The river then maintains a more stable temperature, over a distance of up to several kilometres, neither too hot in summer nor too cold in winter, thus creating an ideal environment for aquatic fauna.
How can we preserve thermal refuges in the face of climate change?
In the second phase of the ESTHER programme, scientists are using numerical modelling to anticipate the impact of climate change on river temperatures and, more specifically, on the functioning of groundwater-dependent thermal refuges.
This methodology explicitly links climate, hydrology, hydrogeology and heat exchanges between the atmosphere, groundwater and water courses to explore how river temperatures will evolve under different climate change scenarios, at the scale of a given river section or thermal refuge.
Once they have been properly calibrated, we will be able to use these models to test solutions for adapting to climate change, such as the development or densification of plant species along watercourses, the reduction of pumping from aquifers to preserve the flow of groundwater, or even developments designed to improve the quantity and quality of watercourses. This will provide managers with an assessment of the vulnerability of thermal refuges and help them to identify ways of reducing it.
