Initially constructed to prevent flooding of the Seine in Paris, the Lac du Der-Chantecoq also limits flooding in the valley of Marne. In summer and autumn, the water stored in the reservoir-lake is released to reinforce the low water level of the Marne and the Seine. In November, the lake is almost empty, leaving only the “dead slice”, the minimum volume vital for fish, and forming mudflats that attract birds. The ornithologist guide Antoine Cubaixo observes them and also takes part in counting the species for the League for the Protection of Birds (LPO).