Restoration of the ecological continuity of the Ressegue River

The project aims to restore ecological continuity on the Ressegue and therefore to open up 96% of the river's length.

Introduction

The Ressègue River is located in the Cère catchment area on the downstream part. The Ressègue is 12.77 km long and follows a South-East/North-West axis. It is a tributary of the Escalmels stream, which is itself a left bank tributary of the Cère.

The Ressègue is classified as a first category river for fish and is listed in List 1 on the entire and List 2, downstream of the Cros bridge. List 1 characterises the watercourses that are in very good ecological condition or identified by the master plans as playing a role of biological reservoir. List 2 aims to restore ecological continuity and thus imposes an obligation to restore the circulation of migratory fish and the adequate transport of sediments. The project area is on list 2 and is also in a Natura 2000 zone.

This site has species of heritage interest, protected and listed by the IUCN. These include the pearl mussel, classified as critically endangered at the European level, the European otter, which has near-threatened status at the European level, and the brown trout, a heritage species and host in the pearl mussel’s reproduction cycle.

Currently, this abandoned weir causes a break in ecological continuity, and thus limits the movement and reproduction of local species essential to the development of the pearl mussel population. Sediment continuity has also been broken, and the quality of the substrate is essential for the settlement of mussels as well as for the creation of breeding grounds for trout. As the watercourse is bordered by agricultural plots, there is no constraint to the removal of the structure. Nevertheless, it is necessary to ensure that the impact on the Natura 2000 area and on the wetland accompanying the watercourse is limited.

At a glance

Project typeDam removal: pre-demolition
Project statusComplete
CountryFrance
RiverThe Ressegue
Potential km to be opened13 km
Focal speciesBrown trout (Salmo trutta), Pearl mussel (Margaritifera margaritifera)
Websitehttps://www.smdmca.fr/

Project context and opportunity

On 1 January 2020, the Syndicat Mixte de la Dordogne Moyenne et de la Cere Aval (SMDMCA) was created and the Chataigneraie Cantalienne community of communes joined. The Syndicate therefore now has the following competence: management of aquatic environments and flood prevention on this territory and therefore on the Escalmels watershed and on this tributaries including the Ressegue.

In 2021 the SMDMCA has launched a global operation to restore ecological continuity on its territory. This operation was initiated by the projects on the river Souvigne. In a context of homogenisation of actions and cost limitation, it is preferable to carry out this restoration action from a common block.

Meetings with the owners and the probable financiers have therefore taken place. An agreement in principle was reached concerning the levelling of the structure. Unfortunately, following the updating of the French Climate Law, public funding of operations to remove a List 2 mill weir is now prohibited, thus bringing to a halt the restoration project orchestrated by the Syndicat.

The OPEN RIVERS programme was therefore an opportunity for the owners and the SMDMCA, since it allows the financing, via philanthropic funds, of studies and works for the removal of structures of less than 2 metres. In this sense, an application was submitted for the Pont de Rhodes weir and was accepted in 2022.

This action is part of the future multi-year management plan led by the SMDMCA on the Cère downstream catchment area, which consists of improving the quality of the environment by restoring wetlands, restoring ecological and sedimentary continuity, protecting the banks, managing the riparian zone, biological monitoring, etc.

Project aims

The SMDMCA’s intention is to restore ecological continuity on the Ressegue and therefore to open up 12.3 km, i.e. 96% of the river’s length. If we take into account the tributaries of the Ressegue, that’s almost 23 km of open watercourse.

These restorations actions include :

  • Reducing river fragmentation

The Ressegue is home to a population of Fario trout, a heritage species. Thus the new access of its environment will be favourable to this local population.

  • Restore the hydromorphological functioning of the Souvigne

The Ressegue carries large quantities of fine sediments, which are blocked and accumulate at this obstacle. Sediment transit is therefore partially interrupted. This results in river bed incision at the foot of the weir, various erosion problems and an unsuitable granulometry of the river bed. This obstacle therefore lead to hydromorphological dysfunctions in the Ressegue. At least 500 meters upstream of weir is clogged.

  • Improving water quality

The accumulation of materials at the level of the obstacle induces a uniformity of the granulometry (important presence of sand and silt) as well as a homogeneity of the flow speeds of the river which, moreover, will tend to decrease. These effects lead to an overall deterioration in the quality of the environment: less oxygenated water, increased temperature, loss of habitats, clogging of the environment and possible breeding grounds.

  • Restore habitats of target species

The current clogging reduces the areas of habitat favourable to the pearl mussel; individuals can be found in the rapids at the foot of the weir, which is not clogged, but very few individuals have been identified upstream from the weir. Removing the weir would thus limit this clogging pressure and provide a larger surface area of habitats favourable to mussels.

Conclusion

The project led by the European Rivers Network embarked on a comprehensive study of the Pont de Rhodes weir, focusing on its removal and the reinforcement of weakened banks using plant engineering techniques. The project team navigated challenges and incorporated valuable feedback from technical partners. The project adheres to the European Water Framework Directive and aligns with master plans, demonstrating a commitment to environmental compliance. The team’s meticulous studies, hydrological surveys, and topographical analyses laid the groundwork for a well-defined project. With a clear administrative context and cost breakdown, the dam removal project is set to commence in September 2024, encompassing vital restoration measures for the site.

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