Lake Joux

Lake Joux lies about 1,004 metres above sea level between Mont Risoux and Mont Tendre. During the last glaciation, clay deposits sealed the valley floor above the permeable Jura limestone, allowing the lake to form. Its 209-square-kilometre catchment extends into France. Neighbouring Lac Brenet belongs to this lake system; the similarly named Lac des Brenets on the Doubs is a different lake.

The Orbe arrives from Lake des Rousses and enters the lake near Le Sentier. Originally, water left the lake system through infiltration and reappeared at the Orbe spring about 200 metres lower down. Since 1903, Lakes Joux and Brenet have supplied storage for the La Dernier power station near Vallorbe. Infiltration points were partly sealed and artificial waterways built: this naturally formed lake is now actively managed.

Average levels for 1991–2025 are lower in late winter and higher in late spring: 1,003.51 metres above sea level in February and 1,004.37 in June. This is the curve of a regulated lake, not simply a rainfall calendar. To interpret a low- or high-water period, the season and changes over several days are therefore more useful than comparison with a single supposedly normal elevation.

FOEN's Lac de Joux–Le Pont station 2007 records water level in metres above sea level. The value gives surface elevation, not remaining depth or usable energy reserves. Location matters too: a measurement on neighbouring Lac Brenet does not automatically represent Lake Joux. Before comparing readings, identify the full station name as well as the date and unit.

Vaud's 2025 assessment describes the lake as moderately nutrient-rich following earlier eutrophication. Deep water remains a concern: oxygen can be almost absent there by the end of summer stratification. The reported average summer surface temperature for 1984–2025 is 18.2 °C. This is a historical reference, not a current bathing temperature or a description of the entire water column.