The Ourcq (Urc in 855) is an 87-kilometre-long (54 mi) river in France, a tributary of the River Marne. Its source is near the village Ronchères, and its course crosses the departments of Aisne, Oise, and Seine-et-Marne. It flows southwest through the towns of Fère-en-Tardenois, La Ferté-Milon, Mareuil-sur-Ourcq, and Crouy-sur-Ourcq, finally flowing into the Marne near Lizy-sur-Ourcq. Napoleon I made use of the river as a water source, and it supplied the city of Paris until Baron Haussmann's rebuilding of Paris.