Lac de Montbel, Reservoir lake west of Chalabre, France

Lac de Montbel is a 550-hectare reservoir created in 1985 by a 270-metre earth dam on the Trière, west of Chalabre on the Ariège–Aude border. Fed by a diversion from the Hers-Vif, its open water is backed by low hills; when the level drops, flooded hillsides, tree stumps and sticky marl appear.

The reservoir was built between 1982 and 1985 as a dam designed to support irrigation for farming areas in the surrounding region. Before the flooding, the valley was covered with forest and crossed by smaller streams.

The lake's name comes from the commune of Montbel, where the dam stands on the Trière. The Léran-Montbel water sports base hosts a sailing club, and the shores hold a picnic area, campsite, playground and the Maison du Lac cultural centre. Waterfowl such as ducks, coots and grebes winter here, counted regularly under the Wetlands International programme.

The lake has two swimming areas and facilities for water sports such as canoeing and pedal boats. The shores are mostly open and exposed, so arriving early on hot days or sticking to the wooded paths nearby helps make the visit more comfortable.

When water levels drop, old tree stumps and pale clay layers emerge along the exposed shores. These remnants mark the former forest floor of the valley before it was flooded.