A concrete structure built to protect residents from flooding in Brittany has had significant environmental consequences on the Vilaine estuary. The Arzal dam, constructed between the communes of Arzal and Camoël in the Morbihan and put into service in 1970, was designed to protect districts of the city of Redon from recurring floods and to supply fresh water to Rennes, Saint-Nazaire, and Auray. The dam has been technically successful in its primary functions but has also altered sediment and water circulation in the estuary. Before the dam's construction, sediments resuspended by currents could deposit up to 50 kilometers upstream of the mouth. The movement of tides ensured a natural dispersion of sediments over a large area. Since 1970, this mechanism has been disrupted, with the dam significantly reducing the speed of currents and concentrating silt deposits on a much smaller area, limited to about 8 kilometers. Estimates of siltation vary, with a technical document mentioning about 22 million cubic meters of sediments and a more recent assessment by EPTB Eaux et Vilaine estimating 30 million cubic meters. The accumulation of silt has led to ecological consequences, including anoxia in deep waters, which caused a massive fish mortality observed in 1982. This event triggered an in-depth study by Ifremer, which linked the phenomenon directly to the presence of the dam. The eel fishing industry has experienced a significant decline, with catches dropping from more than 200 tons in 1979 to only 18 tons in 1998. The entire river ecosystem has shifted, with eels practically disappearing from the area and freshwater species gradually replacing those that characterized the river with a maritime influence. To address the continuous siltation, site managers have organized dredging operations. In 2006, Eaux et Vilaine put into service a dredger called Rochevilaine, designed to carry out dredging campaigns mainly in winter. These interventions help contain the phenomenon but do not solve the problem at its root, as the siltation continues to rebuild year after year as long as the dam remains in place. The Arzal dam has rendered immense services in terms of freshwater management but has also created a permanent environmental challenge that Brittany must manage in the long term.