Lac du Ballon

The Lac du Ballon owes its name to the summit at the foot of which it is located, the Grand Ballon, the highest point in the Vosges.
Embedded in a glacial cirque, it offers a setting of all splendor, the reflections of the sun making its surface shimmer in an alternation of shadow and light created by the rich surrounding vegetation.
Although natural, it is forbidden to bathe there because it is used to supply water to the valley.

The Lac du Ballon (German Belchensee) is a reservoir in a side valley of the Florival in the Vosges, at the foot of the Grand Ballon. Its outflow flows into the Lauch after 3.9 km.

The small cirque lake was enlarged as early as the 16th century by raising it on the moraine.

In 1699 Vauban had the dam built, and the lake now supplied water for canals during the construction of the Neuf-Brisach Fortress. The remains of the dam were washed away by floods in 1740 and 1778.

In 1869, manufacturers from the Lauchtal built a tunnel under the lake bed as a drain in order to increase the usable volume.

By 1955 the bedrock had leaked.

1966 to 1968 the lake was sealed, the dam raised again, with a flood drain on the left bank. The outlet tunnel was filled, and a new bottom outlet with a service tower was built. 1999-2000 the dam and the lake shore were bricked up and the dam crest was concreted.

The lake, together with the smaller Lauchsee, serves as a drinking water reservoir and flood retention basin for the Lauchtal and the town of Guebwiller.
Source: Wikipedia

Beautiful mountain lake surrounded by forest.. a break in the cool water is worth it