Initially created to prevent flooding of the Seine in Paris, the Lac du Der-Chantecoq also limits flooding in the Marne valley. In summer and autumn, the water stored in the reservoir lake is released to reinforce the low water level of the Marne, which flows into the Seine. In November, the lake is almost empty, only the “dead slice” remains, the minimum volume vital for fish, and mudflats that attract birds are formed. The water level of the Lac du Der therefore varies according to the needs of the Marne and the Blaise, the two rivers regulated by the lake.