Dams cause significant changes to the hydrological regime, water temperature, and biological and biogeochemical flows. This is the case for the Vezins and La Roche qui Boit dams, located on the main course of the Sélune River, which resulted in the existence of two reservoirs between 1919 and 2022 for La Roche qui Boit and between 1932 and 2019 for Vezins. Since the end of the last emptying of the La Roche qui Boit dam at the end of 2022, these two lakes no longer exist in the valley landscape. This layer represents the two reservoirs as they were referenced in 2017, i.e. before the start of the dismantling work on the two hydroelectric dams on the Sélune. Among the attribute data associated with these two entities, the length is based on a calculation made by the Sélune project team using the reference points established across the entire watershed (and therefore across the entire course of the Sélune) and their knowledge of the areas of influence of the lakes/natural watercourse.

The removal of the Sélune dams is the subject of a scientific program initiated in 2012 in which environmental parameters are monitored to provide real feedback on this restoration operation. In 2019, a data observatory was set up to ensure the collection and processing of these parameters (biotic and abiotic). With the leveling of the La Roche Qui Boit dam (LRQB) in 2022, it is likely that the sands and sedimentary deposits, previously stopped by the dam, will continue to transit downstream. In order to quantify the transit mode of these sands and sedimentary deposits, the granulometry and progression of the sands downstream of LRQB as well as the evolution of the sedimentary banks are monitored annually. Regarding the sedimentary deposits benches: The complete opening of the La-Roche-Qui-Boit dam, in May 2022, caused the displacement of a significant volume of fine sediments that were contained in the reservoir. These sediments were deposited massively between the RQB and the Quincampoix mill, mainly in the form of sediment beds along the banks of the river. In order to monitor the evolution of this stock, some deposits were mapped in 2022, 2023, 2024 and 2025. Bench shapes and sediment volumes were estimated from measurements of the width and thickness of sedimentary deposits located at DGPS. The measurements were carried out at 4 sites downstream of the reservoir in order to observe whether there was an upstream-downstream logic in the distribution or size of these accumulations. These are the 4 sites that are represented in this dataset. At these sites, all visible and accessible benches were systematically recorded except those that seemed to be due to the presence of natural obstacles (blocks, stumps). On each bench, the measurements of width and thickness were carried out along transect (2 to 4 points per transect according to the width of the bench) systematically upstream, at the center and downstream of each shape (Figure 9). Intermediate transects were added when the bench was particularly long and variable in shape. The sediment thicknesses were measured by means of a reefiron inserted into the sediments until it reached the roof of the coarse sediments constituting the bottom of the bed. The widths were measured using a test pattern placed on the benches. Each transect and each thickness measurement point along these transects was georeferenced to DGPS (Trimble GPS R8). Concerning fine sediments (sands): Sediment traps are positioned to analyze the evolution of sand sizes according to the distance from the LRQB dam. The objective of these traps is to monitor the evolution of the grain size carried by the watercourse, and to account for a possible change in sediment size, with the appearance of sands transported downstream from dams. These systems do not consist of quantifying sedimentary flows with greater precision. This device does not allow for more precise quantification of sand flows. The traps were tested in August 2021 and then officially implemented in May 2022. This monitoring is only carried out in summer when flows are low and allow the traps to be collected. Their manufacture and installation were described in the 2021 report (Stubbe et al., 2022). There are four traps in all that have been installed. The first is located 200m downstream of the upstream station of Virey as a witness of the conditions upstream. The other three traps were placed downstream of the RQB: immediately after the dam, 100m upstream from the boat deck and 200m downstream from the Signy station (Figure 5). These traps were placed between the deep pools in the idea that they could accommodate sediment deposits once the dam is destroyed. NB: the data recorded by these traps are not made available in the IS but can be obtained by contacting the referees (contact point in the metadata sheet). Regarding the transport of coarse sediments: The study is carried out using pebbles marked by passive transponders (pit tags). These pit tags were introduced into several hundred pebbles of different sizes in the river. Their movements during the hydrological cycle are monitored using an antenna that is manually moved by navigating the river, at a rate of one campaign per year. This dataset therefore includes the displacement monitoring zones for coarse sediment, otherwise known as «Station traceur galets». This monitoring was carried out from 2022 to 2027. This layer gives the location of the various sedimentary monitoring systems installed downstream of the former La-Roche-Qui-Boit dam, which is part of the Sélune Observatory.

The removal of the Sélune dams is the subject of a scientific program initiated in 2012 in which environmental parameters are monitored to provide real feedback on this restoration operation. In 2019, a data observatory was set up to ensure the collection and processing of these parameters (biotic and abiotic). With the leveling of the La Roche Qui Boit dam (LRQB), it is likely that the sands, which are currently stopped by the dam, will continue their transit downstream. In order to quantify the transit mode of sands, we will follow the grain size and the progression of sands downstream from LRQB at different locations. The complete opening of the La-Roche-Qui-Boit dam, in May 2022, caused the displacement of a significant volume of fine sediments that were contained in the reservoir. These sediments were deposited massively between the RQB and the Quincampoix mill, mainly in the form of sediment beds along the banks of the river. In order to monitor the evolution of this stock, some deposits were mapped in 2022, 2023, 2024 and 2025. Bench shapes and sediment volumes were estimated from measurements of the width and thickness of sedimentary deposits located at DGPS. The measurements were carried out at 4 sites downstream of the reservoir in order to observe whether there was an upstream-downstream logic in the distribution or size of these accumulations. These are the 4 sites that are represented in this dataset. This layer gives the location of the measurement points, by sedimentary bench and by session, and proposes all the measurements made at these different points. It was carried out as part of the Sélune Observatory.

The removal of the Sélune dams is the subject of a scientific program initiated in 2012 in which environmental parameters are monitored to provide real feedback on this restoration operation. In 2019, a data observatory was set up to ensure the collection and processing of these parameters (biotic and abiotic). With the leveling of the La Roche Qui Boit dam (LRQB), it is likely that the sands, which are currently stopped by the dam, will continue their transit downstream. In order to quantify the transit mode of sands, we will follow the grain size and the progression of sands downstream from LRQB at different locations. The complete opening of the La-Roche-Qui-Boit dam, in May 2022, caused the displacement of a significant volume of fine sediments that were contained in the reservoir. These sediments were deposited massively between the RQB and the Quincampoix mill, mainly in the form of sediment beds along the banks of the river. In order to monitor the evolution of this stock, some deposits were mapped in 2022, 2023, 2024 and 2025. Bench shapes and sediment volumes were estimated from measurements of the width and thickness of sedimentary deposits located at DGPS. The measurements were carried out at 4 sites downstream of the reservoir in order to observe whether there was an upstream-downstream logic in the distribution or size of these accumulations. These are the 4 sites that are represented in this dataset. At these sites, all visible and accessible benches were systematically recorded except those that seemed to be due to the presence of natural obstacles (blocks, stumps). On each bench, the measurements of width and thickness were carried out along transect (2 to 4 points per transect according to the width of the bench) systematically upstream, at the center and downstream of each shape (Figure 9). Intermediate transects were added when the bench was particularly long and variable in shape. The sediment thicknesses were measured by means of a reefiron inserted into the sediments until it reached the roof of the coarse sediments constituting the bottom of the bed. The widths were measured using a test pattern placed on the benches. Each transect and each thickness measurement point along these transects was georeferenced to DGPS (Trimble GPS R8). This layer gives the location and shape (polygon) of sedimentary banks and by session and proposes some general measures to assess their evolution. It was carried out as part of the Sélune Observatory.

The restoration of the Sélune river continuum after the dams were demolished will allow colonization of the upper reaches of the basin, which were previously inaccessible to eels. Before the demolition, 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-demolition phase by abundance index campaigns carried out in 2013, 2015, 2019, 2021, and 2023 on a network of stations covering the entire Sélune river system. These data make it possible to estimate changes in eel abundance along the Sélune and its tributaries and their size structure before and after the destruction of the dams. The inventory is carried out in early September, every other year, using electric fishing equipment and landing nets. Thirty fishing points are carried out for a minimum of 30 seconds each. A fishing station covers approximately 100 meters of river. All eels caught are anesthetized and undergo biometric measurements (weight, length, horizontal and vertical eye diameter, determination of yellow or silver stage). All eels are released directly at the fishing site. This layer contains individual data on the eels caught.

The restoration of the Sélune river continuum after the dams were demolished will allow colonization of the upper reaches of the basin, which were previously inaccessible to eels. Before the demolition, 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-demolition phase by abundance index campaigns carried out in 2013, 2015, 2019, and then every year since 2021, on a network of stations covering the entire Sélune river system. These data make it possible to estimate changes in eel abundance along the Sélune and its tributaries and their size structure before and after the destruction of the dams. The inventory is carried out in early September, every other year, then every year since 2021, using electric fishing equipment and landing nets. Thirty fishing points are carried out for a minimum of 30 seconds. A fishing station covers approximately 100 meters of river. All eels caught are anesthetized and undergo biometric measurements (weight, length, horizontal and vertical eye diameter, determination of yellow or silver stage). All eels are released directly at the fishing site. This sheet refers to two layers, one showing only the study areas and their location, and the other specifying the concept of capture sessions.

The hydrographic sections, which make up the waterways, were extracted from the Topage database covering the study area.

In 2020, scientific plots were set up, marked out on the ground with colored stakes. These plots are intended to facilitate scientific monitoring, particularly of renaturation. These plots represent privileged areas for scientists, where no sediment or vegetation disturbance is planned until the end of the Sélune program.

The removal of the dams on the Sélune River is the subject of a scientific program launched in 2012, in which environmental parameters are monitored in order to provide real feedback on this restoration operation. In 2019, a data observatory was set up to collect and process these parameters (biotic and abiotic). The data acquired from 2012 and at least until 2027 will be centralized through an information system (IS) called SISelune. SISelune aims to assist scientists associated with the Sélune program and make the data accessible to all. This layer shows the study area, based on hydrographic sub-basins for the continental part and the transitional water body for the Bay of Mont St-Michel.