The Bassin de Saint-Ferréol was and remains the main water reservoir for the canal with a surface of 67 hectares. It receives the waters of the Montagne Noire via the Channel of the Mountain which is itself supplied by the basin of Lampy. The lake waters are retained by a dam 786m long and 149m wide consisting of three parallel walls. The channel surrounds the basin and can evacuate any excess water. A museum is located near the outlets and has displays on the history of the construction of the Lake.