Lac Brenet, Reservoir in the Vallée de Joux, canton of Vaud, Switzerland

Lac Brenet is a reservoir in the Vallée de Joux in the canton of Vaud, Switzerland, separated from the larger Lac de Joux by a 200-metre-wide land bridge. It measures 1.5 km long, about 500 m wide and 18 m deep, lying at 1,002 m above sea level. Its north and east shores are steep, while a small marshland borders the southwest edge.

The lake was created as a reservoir to manage water flow for the region and now serves a hydroelectric plant in Vallorbe. From 1879 to the early 1900s, ice harvesting was a major economic activity, with blocks of ice transported by railway to cities including Bern, Geneva, and Paris.

The lake lies in the French-speaking Vallée de Joux, and its name is shared with the nearby village of Les Charbonnières, the only settlement on its shore. The narrow-gauge railway from Vallorbe to Le Brassus runs along the eastern bank, and the Dent de Vaulion rises in the background.

The lake lies at the northeastern edge of the Vallée de Joux, near Les Charbonnières in the municipality of Le Lieu. A branch railway line linking Vallorbe and Le Brassus runs along the eastern shore. The lake freezes only partially in winter because of currents, so it is not open for skating.

From 1879 to the early 1900s, the lake's ice was harvested and transported by horse-drawn carts and train to major cities. Informational panels along the shore tell the story of this forgotten ice industry and how local commerce once depended on this seasonal resource.