Les barrages sont à l’origine de modifications significatives du régime hydrologique, de la température de l’eau et des flux biologiques et biogéochimiques. C’est le cas des barrages de Vezins et de la Roche qui Boit, présents sur le cours principal de la Sélune, et d’autres plus petits obstacles qui fragmentent le bassin versant de la Sélune. Cette couche représente les barrages hydroélectriques de la Sélune, avec les batiments associés, en lien avec la production électrique. La géométrie de ces polygones est issue de plusieurs sources : BD Topo, OpenStreetMap (de 2019), complétée par photointerprétation. Des informations extraites du livre « Quand les rivières reprennent leur cours – Notes sur l’effacement de barrages et de seuils, sur la Sélune et ailleurs » ont été ajoutées pour décrire l'état (en service ou arasé), la nature et les caractérisques physiques de ces barrages.

Geodetic markers were installed along the main course of the Sélune. Physically marked out in the field, these markers have been located with a high-precision GPS (planimetric and z-precisions are given in the table). Please note that these markers may move slightly with the wetting/drying cycles of mainly clay soils, and their coordinates may be updated if necessary. Geodetic markers can be used as reference points to define a device or a new study area.

This project examines the life history traits of eels and flounders—two migratory species that live in rivers and spawn in the sea—prior to the restoration of the Sélune River’s connectivity, in order to anticipate the subsequent effects of this restoration. Data: - Catches of eels and flounder (recorded at least within the framework of the Observatory, for eels) - Fat content (measured using a fat meter and chloroform extraction); - Energy content (calorimetric bomb test on muscle tissue) - Position and trophic niche - Age and growth rate - Migration and basin use profile - Prevalence and abundance of parasitic metazoans (overall and for each parasite species), diversity of parasite communities - Diversity and status of the intestinal microbiome determined Sites: 6 sites in total: Sélune (Pêcherie + Roche qui Boit), Oir, Beuvron, Sée, Couesnon, Marais de Dol - Eels: 5 sites (Sélune Pêcherie + Roche qui Boit, Oir, Beuvron, Sée) - Flounders: 4 sites (Sélune, Sée, Couesnon, and Marais de Dol)

In few words : The primary objective of the biomonitoring project “Impact of Dredging on the Animal and Plant Communities of the Sélune” is to establish a baseline assessment of the biological quality of the Sélune upstream and downstream of the two dams, as well as within the reservoirs. It also aims to identify risks (acute exposure) during the work to stabilize contaminated sediment from the Yvrande. It thus seeks to answer the following questions: How does the lotic ecosystem recover after the disappearance of a reservoir, and what new ecological equilibria result? What will be the effects of the dredging work on the quality of the environments and their consequences on the biocoenoses that develop there? This project assesses the impact of dams on aquatic animal and plant communities in relation to water quality (zooplankton, macroinvertebrates, phytoplankton, periphyton, macrophytes). Methods : IBMR ; IBGN ; IBD ; suivi des biofilms ; caractérisation physico-chimique Data : - Monitoring of physicochemical conditions (water temperature, conductivity, pH, etc.) - Concentrations of nitrates, silicates, nitrogen, and total phosphorus - Monitoring of invertebrate communities - Monitoring of phytoplankton and zooplankton communities, and measurement of phytoplankton biomass - Monitoring of biofilm (2015–2017) - Monitoring of aquatic plants (1996) - Floristic surveys of aquatic plants (2015, 2017) - Toxicological characterization of fish in the Vezins Reservoir - Analysis of fish stomach contents (GCA)

The re-establishment of the river continuum on the Sélune after the dams have been levelled will enable colonization of the upper reaches of the basin, which were previously inaccessible to eels. Before the dams were levelled, two main tributaries were used by amphihaline fish for reproduction, in addition to the main river: the Beuvron and the Oir. The eel population of the Sélune was monitored in the pre-flushing phase by abundance index campaigns carried out in 2013, 2015, 2019 on a network of stations covering the entire Sélune hydrographic network. Since 2021, this network of stations has evolved to monitor changes in the part of the watershed accessible following the dismantling of the 2 dams. These data enable us to estimate changes in eel abundance along the Sélune and its tributaries, as well as their size structure before and after the removal of the dams. The inventory is carried out in early September, every other year between 2013 and 2019, then every year since, using an electric fishing device and dip nets. Thirty thirty-second fishing stations are set up. A fishing station comprises approximately 100 meters of river. All eels caught are anaesthetized and biometric measurements are taken (weight, length, horizontal and vertical eye diameter, determination of yellow or silver stage). All eels are released directly at the fishing site. Two different protocols were implemented: the Eel Abundance Index (Pottier and Chapon, 2022) at 23 stations and exhaustion fishing at eight other stations (IAA01 to IAA08) on the Oir River and some of its tributaries, according to an internal protocol. This layer provides station abundances by fishing session.

In a nutshell: This research project focuses on the social and geographical dimensions of the ecological restoration of the Sélune River. Its objective is to assess the current state of the landscapes, land uses, and practices in the Sélune Valley (and more specifically in the gorge section) prior to the removal of the dams, and then to monitor the changes brought about by the removal: the state of both material transformations (land use, practices, visitor traffic) and conceptual transformations (perceptions, discourse) of the territory in order to assess the impacts of this project and thereby measure its success from the users’ perspective (quality of life, regional development, practices, etc.) to understand, using the example of the Sélune, the difficulties in gaining acceptance and ownership of such a project by various local stakeholders, as well as the challenges related to its definition and implementation. Methods used: surveys, questionnaires, analysis of the local press, photography Data: - A collection of photographs (~40) covering the entire Sélune Valley to track changes in the landscape - Historical photographs and postcards. Photographs will be annotated and geotagged - Inventory of land uses (fishing, recreation, lodging, etc.), analysis of visitor traffic in the gorges, and mapping - Semi-structured interviews will be conducted with representatives of user groups (to gather their opinions) - Text analysis (using Alceste or WordMapper software) of transcribed interviews - Collection of residents’ opinions and assessment of their satisfaction with the project - Corpus of articles published in the regional daily press to track the history of reactions - Collection of information disseminated via other media platforms such as blogs, forums, and websites by various stakeholder groups, particularly fishermen, environmentalists, and opponents of the clearing - Codification of the corpus followed by a lexical and semantic analysis using the TXM platform

The opening of the dams should impact the functioning of Atlantic salmon populations migrating up the Sélune River and its tributaries. Restoring the river's ecological continuity will alter population flows by allowing amphihaline sea trout to migrate further upstream and native trout to move downstream and upstream of the dams. The trout population is monitored at a network of stations covering the entire Sélune basin using an abundance index, which is carried out every two years before 2021 and every year since then. Field campaigns comply with the fishing protocol developed by INRAE and OFB. The aim of this method is to estimate the abundance of juvenile Atlantic salmon (Salmo salar) at a station (or sector). This protocol targets juveniles of the year (aged 0+) whose abundance reflects the renewal of generations within the population (or recruitment) and survival after the embryonic development phases under gravel and the first months of life in the open environment. Field campaigns are carried out by INRAE and the Manche d'Ille-et-Vilaine Federation for Fishing and the Protection of the Aquatic Environment (FDAPPMA50 and FDAAPPMA35). This dataset provides individual biometric data on Atlantic salmon caught during PAS fishing sessions in the Sélune watershed, by station and by fishing session.

This project compares the implementation, acceptance, and progress of dam removal projects in France, using the example of the Sélune, and in the United States, where several similar removal projects have taken place.