The artificial lake of Saint-Agnan is located in the Yonne department southeast of Paris and Montereau-Fault-Yonne and northwest of Sens, on the border of the Seine-et-Marne department. The territory of the municipality extends over 3,266 acres (1,322 ha) on the Gâtinais plateau and rises from 288 to 551 ft (88 to 168 m). The village, which has 953 inhabitants, clings to the western flank of a steep limestone outcrop whereas the lake has invaded the Cousin valley below.