Restoration of the ecological continuity of the Souvigne, France

Introduction

The Souvigne River flows from Saint-Paul to Argentat-sur-Dordogne along a north/south axis for almost 18 km. The watershed has a surface area of 111 km². The Souvigne River is of major ecological importance, and it is therefore essential to carry out an operation to restore ecological continuity.

In addition, the right bank of the Souvigne is last major tributary of the Dordogne before the Sablier dam. This last dam, which is artificially impassable, is the limit for the upstream migration of large salmonids on the Dordogne. According to a study by ECOGEA for MIGADO, the Souvigne hosted 8.4% of the spawning grounds for large salmonids in the Dordogne basin in 2018. According to the above-mentioned study, “particular attention should be given to the Souvigne, in terms of fish circulation “.

The Dordogne basin has been classified as a Biosphere Reserve by UNESCO since 11 July 2012, making this territory a place of learning about sustainable development, where innovative approaches combining biodiversity conservation, cultural enhancement and economic and social development are being tested.

The project registered by the Syndicat Mixte de la Dordogne Moyenne et de la Cère Aval (SMDMCA) on the Souvigne concerns the “Moulin Bas” weir. The latter is located about 100 meters from the confluence with the Dordogne. It is an old mill weir that is no longer of any use. In the 1990s this weir was equipped with a fish pass which is no longer functional and is therefore subject to compulsory compliance and restoration of ecological continuity by its owners.

Project context and opportunity

On 1 January 2020, the SMDMCA was created and the Xaintrie Val’Dordogne 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 Souvigne watershed.

In 2021, the Nouvelle-Aquitaine region and the Adour Garonne Water Agency are launching a joint call for projects to restore the ecological continuity of the region’s rivers. The Syndicat, aware that this problem is present on the Souvigne, naturally registered, with the agreement of the owners, the 7 blocking obstacles listed on this river. The Syndicat’s proposal was accepted for the 7 structures, including the “Moulin bas” weir, where a removal action was planned.

Unfortunately, the update of the French Climate Law intervenes and prohibits the financing of the removal of mill weirs in list 2 with public funds. The Region therefore decided to remove the Moulin Bas weir from the project (the 6 other obstacles were not mill weirs).

The Moulin Bas weir is the first obstacle encountered on the Souvigne and it is imperative to treat this black spot to justify the efforts made on the 6 other obstacles located upstream from it.

The owners of the mill, who are willing and in agreement with a removal action, are reassured that the studies for the restoration of the site will be taken care of externally.

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

Project aims

The SMDMCA’s intention is to restore ecological continuity on the 7 obstacles present on the Souvigne and therefore to open up the whole of the openable linear section (up to the presence of a natural weir), that is 83% of the linear section (15 km) on the Souvigne River. Taking into account the tributaries of the Souvigne, nearly 25 linear km will be reopened. These restorations have the objective of :

• Facilitating access to an ideal breeding area for large salmonids
• Reducing river fragmentation
• Restoring the hydromorphological functioning of the Souvigne
• Improving water quality

Project achievements

• Installation of a Water Level Probe: Enabled Hydrétudes to define the hydraulic functioning of the Souvigne, contributing to the knowledge of the river’s characteristics and estimating its flow module.
• Unprecedented Study of Souvigne: The river had rarely been studied due to the lack of management before the SMDMCA, making this information highly valuable.
• Water Regulation Establishment: Studies on the Moulin Bas dam and the Laborie dam will facilitate water regulation upstream, ensuring sufficient water flow during low water periods.
• Agreement with Corrèze Departmental Council: The SMDMCA will collaborate with the council to secure the departmental road on the right bank, addressing potential road weakening due to future dam removal work on the Moulin Bas dam.
• Improved Contract Drafting: The SMDMCA learned the importance of specifying needs clearly in public contracts to obtain homogeneous and well-adapted contracts, achieving the best prices and results. This may involve writing separate contracts instead of a single one to ensure quality and cost-effectiveness.

This project was completed in October 2023 and the project team received a follow-on demolition grant from the programme.