The lac Noir (Black lake) is downstream from the lac Blanc (White lake), more extensive and to which it is connected by a stream and pipes. They were fitted out between the two world wars to preserve a level of water necessary for the functioning of a power station built on the north shore of the lake. Its outlet is the Black Lake Brook, which joins the Weiss river below Orbey. The White lake extends approximately 0,6 mi (1 km) upstream of its counterpart. The two cirques are separated by the granite ridge of the Reisberg, which rises to 4,173 ft (1,272 m).