Drought: when our rivers lose their refuge-like qualities
The images taken in recent days on the Dournaujou, the Manet, the Dronne and the Malincourie are striking. They show waterways at exceptionally low levels, sections of dry riverbeds and, in places, freshwater pearl mussels that are no longer covered by water.
Rivers at their lowest levels
In mid-August, Météo-France observed an unprecedented soil drought on a national scale. On the Upper Dronne River, the flow rate recorded on August 14th at the lowest point of the low water period* was 0,065 m³/s. For comparison, at the Manet gauging station, the average annual flow rate of the Dronne in August is approximately 0,61 m³/s. The observed flow rate thus represented barely more than 10% of this seasonal average.
Sections of the riverbed, which until now served as refuges for biodiversity during the summer months, are now either dry or significantly reduced in size. For aquatic species, this means less available space, higher temperatures, less oxygen, and, for organisms that cannot move quickly, such as freshwater pearl mussels, a direct risk of mortality.
This situation occurred during an exceptional summer of 2026. After the hottest June ever recorded in France, July 2026 became the hottest month ever observed in France, across all months.
The freshwater pearl mussel, an endangered symbol
In Périgord-Limousin, this situation directly impacts one of the treasures of local biodiversity. The Haute-Dronne is home to one of the main French populations of freshwater pearl mussel ( Margaritifera margaritifera ), a species that is particularly demanding in terms of water quality, oxygenation and freshness.
Its conservation has justified a considerable collective investment. The European LIFE Haute-Dronne program, led by the Périgord-Limousin Regional Natural Park between 2014 and 2020, mobilized nearly 6 million euros . It made it possible to restore ecological continuity on numerous sites and to produce and then reintroduce more than one million young freshwater mussels into the Dronne and Dournaujou rivers.
These efforts remain essential. But climate change is now increasing the pressure on these environments and makes reducing other factors of vulnerability all the more necessary.
Water bodies and reservoirs: possible effects on water temperature and flow rates
Not all ponds have the same characteristics or the same impacts. Their effect depends in particular on their size, their depth, their method of water supply and release, and above all their connection to the hydrographic network.
When a pond or reservoir is located directly on a small stream, particularly at the head of a watershed, it locally transforms flowing water into a slower-moving mass of water . The water can then warm up more before being returned to the stream downstream.
This thermal effect is well documented . On the Haute-Dronne itself, monitoring carried out within the framework of the LIFE project had shown that some structures caused temperature increases of up to + 3,5°C before their restoration.
However, when the flow decreases, a few extra degrees become crucial: warmer water contains less oxygen and further reduces the possibilities of refuge for cold water species.
Water bodies also constitute open water surfaces subject to evaporation , which is particularly significant during warm periods. Their accumulation in a watershed can therefore alter its hydrological functioning and contribute, along with other pressures, to weakening summer flows.
But the issue is not simply a matter of comparing the evaporation of a pond and the evapotranspiration of a wetland.
Some bodies of water have indeed been created in areas that were previously wetlands or wet valley bottoms. Their creation then profoundly transforms the environment and can cause the disappearance or alteration of the functions provided by the pre-existing wetland.
However, a functional wetland is not simply an area where water "evaporates less". Depending on its nature and position in the watershed, it can temporarily store water in the soil, slow down flows, promote exchanges with groundwater, contribute to maintaining flows and offer essential habitats to many species.
It would therefore be too simplistic to contrast the evaporation of a pond with the evapotranspiration of a wetland. The real issue lies in the overall functioning of the watershed and the hydrological and ecological services provided by wetlands.
The Park, a key player in preserving water resources
Faced with these upheavals, preserving and restoring wetlands, maintaining riparian zones and the shade provided by waterways, restoring their ecological continuity, and reducing the impacts of the most problematic bodies of water are complementary levers for strengthening the resilience of our rivers and conserving their biodiversity. This is precisely the work the Park has been carrying out for many years, alongside all stakeholders in the water sector. This action is based on a win-win approach: by acting in favor of aquatic environments and their biodiversity, the Park also contributes to preserving a resource essential to the inhabitants of the region.
Indeed, the actions undertaken to preserve water resources have direct qualitative and quantitative effects and contribute to maintaining their potability . For example, in the Valouze basin, the removal of five artificial ponds has helped improve water quality. The elimination of these ponds has notably limited the proliferation of cyanobacteria in this waterway, which supplies the drinking water intake for La Coquille.
Photographs from this summer show us that climate change is no longer a distant prospect for the aquatic biodiversity of the Périgord-Limousin region. It is already visible in our streams. Faced with this reality, the Park's action in support of water resources is more essential than ever, both for biodiversity and for the citizens.
*Low water level refers to the minimum flow rate or lowest level of a watercourse or groundwater.
