Lac d'Enghien, Lake in Enghien-les-Bains, France

Lac d'Enghien is a 43-hectare freshwater lake in Enghien-les-Bains, Val-d'Oise, formed by a dam built in the Middle Ages at the outlet of a marsh in the Montmorency valley. It measures 930 m long and 500 m wide, with a 3 km perimeter, and is only 1.10 to 1.85 m deep, reaching 2.55 m at the spillway under the jetty. It has one artificial island, Île des Cygnes, created in 1860 from mud dredged from the lake bed.

The lake originated as a marsh at the foot of the Montmorency valley, dammed in the Middle Ages to form a pond. Enghien-les-Bains grew on its shores in the 19th century after the discovery of France's most sulfurous thermal springs, which rise here at 11.8 to 13 °C. The Île des Cygnes was formed in 1860 from mud dredged during cleaning.

The lake gave its name to Enghien-les-Bains, a commune created on its shores in the 19th century after the discovery of France's most sulphurous thermal waters. The water is cold, between 11.8 and 13 °C, unlike the warm Pyrenean spas. The lake lies at 43 m altitude, 22 m above the Seine at Épinay-sur-Seine.

The lake lies at the outlet of the Montmorency valley, 11 km north of Paris and 43 m above sea level, with the Seine at Épinay 22 m below. Its water is shallow, between 1.10 and 1.85 m deep, reaching 2.55 m at the spillway under the jetty. It drains through the covered ru d'Ormesson toward the Seine at Épinay-sur-Seine.

The lake's only island, Île des Cygnes, was formed in 1860 from mud dredged during a clean-up of the lake bed, replacing an older island of the same name slightly to the north. The lake bottom holds a layer of mud about 80 cm thick that thickens over the years and requires regular dredging.