The Lac du Der owes its name to the region of the same name. In the Middle Ages, the Der region was covered by a large swampy forest dotted with a few villages. An Aquitanian monk named Berchaire founded a monastery there around the year 673. The monks of the monastery drained the marshes and partly cleared the forest. At the end of the 1960s, the construction of the Lac du Der-Chantecoq reduced the forest by a further 1,000 hectares. Three villages were also swallowed up by the waters of the lake, after having been burnt and levelled. In periods of low water, the ruins of certain walls become visible.