Lake Nantua, 2.5km long and 650m wide, was formed in a cluse, behind a deposit left by a former glacier. Numerous sources contribute to it, including the source of Neyrolles. By the Bras du Lac, the waters flow into the Oignin, a tributary of the Ain.

The esplanade by the lake, shaded by handsome plane trees, and the avenue du Lac afford beautiful views over the water framed by the peaks of Haut-Bugey. The watersports centre offers numerous sporting and leisure activities.