Ecological restoration of the Selune River

Understanding ecological restoration mechanisms following dam’s removal

Scientific studies
The project

The project

When the contracts for the Vezins and Roche-Qui-Boit hydroelectric dams were not renewed, the French Ministry of the Environment decided in 2012 that the dams would be removed to restore ecological continuity. This decision came about as a result of the EU water framework directive (WFD) and the French Grenelle Environment Forum (i.e., discussion of the “blue corridor” concept).

In collaboration with INRA, a long-term interdisciplinary monitoring program has been set up to track the ecological restoration of the river basin (before and after dam removal) and to characterize the related physical, chemical, and biological changes. Societal impacts are also being studied.

This project is being carried out at multiple scales—over 100 scientists are examining everything from water chemistry to landscape dynamics. The ultimate goal is to understand every facet of the river restoration process and thus provide concrete recommendations to the agencies tasked with such work.


Within landscapes, dams represent breaks in ecological continuity. They result in major changes to hydrological regimes, water temperatures, species migration, and biogeochemical cycles. They also cause socioeconomic concerns because many of the hydroelectric dams built in the early 1910s and 1920s have aged, resulting in greater security risks and diminished economic returns. Across the globe, more and more dams are being removed, and projects are underway to restore the ecology of watercourses. In order to gauge the success of these efforts, international research is on the rise.

To date, the US has been the most proactive in dismantling dams. However, few dam removal projects have been the object of scientific monitoring, and most of the dams concerned have been small (<15 m). The studies that have taken place have focused on restoring populations of diadromous fish, which are a societal and cultural resource and bioindicators of the impacts of climate change on watercourses.

In France, three large dams have been removed to date (between 1996 and 1998 in Brittany—on the Léguer and the Loire). However, none of the operations were the object of scientific study. Following the 2007 Grenelle Environment Forum, the national government proposed the removal of dams on two rivers—the Allier (dam at Poutés) and the Sélune (91 km, 1,040 km²). The Sélune is one of the four watercourses that flows into the bay of Mont Saint-Michel. It is home to a diversity of diadromous species (Atlantic salmon, trout, eel, lamprey, and shad), which are restricted to a 14-km stretch of the river because of the two hydroelectric dams found on the lower third of the Sélune: La Roche Qui Boit (16 m high) and Vezins (36 m high) (Photo 1), which began operating in the 1920s and 1930s, respectively.

The dams are aging; their reservoirs contain large quantities of sediment and harbour toxic cyanobacteria in the summer. For economic reasons (i.e., feeble returns and the risk of WFD-related sanctions), the contracts for these hydroelectric dams were not renewed. Then, spurred by the WFD, the French Ministry of the Environment stated its intention to remove the dams (first in 2009, then again in 2012) with a view to restoring the ecological continuity. Ultimately, the final decision on the dams’ fate has been delayed. The only action being carried out is the drainage of the reservoir(s). In tandem, regional authorities want to boost socioeconomic conditions in the river valley.

In collaboration with INRA, a long-term interdisciplinary scientific monitoring program has been established to observe conditions before and after dam removal. Within Europe, this program is unparalleled in its scope and aims to fully evaluate the ecological restoration of the river system. Several research teams from various scientific institutes will be working on this project, which is jointly headed by INRA and the newly created French Agency for Biodiversity (AFB), headquartered in Rennes. The ultimate goal is to use this opportunity to observe the ecological and societal implications of dam removal from a scientific perspective, with a view to better understanding the issues related to this type of increasingly common large-scale operation.

Until the final decision is made on the dams’ removal, the project is focusing on two more proximate objectives: (1) characterizing the watercourse’s current state given the presence of the two dams and (2) analyzing the ecological and societal impacts of dams’ removals.

The challenges

In Europe, the Sélune’s situation is one of a kind, because of the height of the dams (16 m and 36 m), their proximity to the ocean, and their relative impact on the watercourse (20 km, or 25% of the river’s length, is submerged). The river basin is also characterized by intensive agricultural activity (mixed crop-livestock systems, mostly dedicated to dairy production). This joint INRA-AFB scientific monitoring program has the following aims :

  1. to evaluate mechanisms for ecologically restoring the river basin and boosting its socioeconomic value
  2. to gather practical knowledge that will inform future decisions regarding the fate of aging dams across the world (i.e., maintain, update, or remove)

The project is multidisciplinary and focuses on four thematic areas: landscapes, humans, and usage; agricultural and riparian landscapes; river dynamics; and aquatic ecological communities. It has brought together around 100 scientists from 19 research labs associated with a variety of scientific institutes (INRA, CNRS, MNHN, IFREMER, and various universities). Two phases are planned: the pre-removal phase (2012-2018, preliminary assessments and evaluation of reservoir draining) and the post-removal phase (ecological restoration). In addition to the funding provided by the project’s scientific partners, financial support also comes from the Seine-Normandy Water Agency (AESN) and the AFB. The project is taking place in the French administrative department of La Manche: the Departmental Directorate of Land and Sea (DDTM) is in charge of dismantling the dams and the department’s prefect has jurisdiction over the project’s administrative facets. The sustainable development plan for the valley has two parts. The socioeconomic part is being directed by the Syndicat Mixte des Pays de la Baie (a mixed association of governmental bodies), and the environmental part is being directed by the Syndicat Mixte du Bassin de la Sélune, which is responsible for the basin’s water use planning and management scheme (SAGE).

The challenges

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