Lac des Brenets

Lac des Brenets occupies the Jura valley of the Doubs on the French–Swiss border; on the French side it is also called Lac de Chaillexon. Rockslides and a natural sill at the Saut du Doubs shaped the basin. The sill lies around 751 metres above sea level, but the water can fall well below it. This lake belongs to the Doubs and is not Lac Brenet beside Lake Joux.

The Doubs flows through the lake, joined by the Rançonnière and karst springs such as Arvoux. In the lower part of the lake, some water infiltrates fractured limestone and returns to the river below the Saut du Doubs. The visible outlet therefore does not tell the whole water-balance story. Farther downstream, Lake Moron is held back by the Châtelot dam; it is a separate artificial reservoir, not the natural Lac des Brenets.

Monthly averages for 1991–2025 are higher in winter and early spring than in summer and early autumn: 751.02 metres above sea level in March and 748.84 in August. During prolonged drought, karst losses can deepen the fall until the lake partly dries out. This is not a controlled reservoir drawdown at Châtelot, but a feature of the natural hydrology of Lac des Brenets.

Lac des Brenets–Les Brenets station 2057 measures lake level, whereas discharge station 2247 describes surface outflow. The hydrogeological study explains that this discharge does not fully include underground losses. Particularly at low water, timestamps and the source's quality notices also matter. A repeated number demonstrates a stable lake level only if the underlying measurement is valid.

The lake belongs to the federally inventoried Doubs landscape, with its aquatic, rocky and shoreline habitats. Its ecological condition cannot be read from one level value. Station 2057's water-level record is not a temperature record either: a dry or wet period does not imply a particular bathing temperature. Check whether a temperature was actually measured in the lake, in a river section or calculated by a model.