The construction of the Guerlédan dam lasted seven years (from 1923 to 1930). Placed on the waters of Blavet, it is 45 meters high and retains a volume of 55 million m3 of water over an area of 400 hectares, thus forming the lake of Guerlédan, the largest of the Breton lakes. Its impoundment engulfed more than 12 km of the Haut Blavet valley. During the emptying of the lake, one can discover, like the legendary town of Ys in Cornouaille, a few walls of houses (in particular lock keepers) engulfed in the old valley as well as its towpath, its gardens, its locks and its weirs. The opening of the site took place in June 1923 and the official commissioning of the hydroelectric plant in January 1931.
