Because the River Marne can sometimes be extraordinarily unpredictable, in 1970 it was decided to build a reservoir dam called Lac du Der. This gigantic building project gave rise, on the border between Marne and Haute-Marne, to one of the largest artificial lakes in Europe, with 4800 ha of water and 77 km of shoreline.

The Lac du Der is now, in the heart of Champagne-Ardenne, an outdoor leisure resort and a major site for migrating birds.
In the area in and around the lake, there are over 270 species of bird (some of which are rare and endangered), 40 species of mammals, 45 varieties of dragonflies, 20 kinds of amphibians and more than 200 different plants. Lac du Der has been listed as a national hunting and wildlife reserve since 1978, and is one of the most important sites in France for bird watching, attracting specialists from all over the world.