Lac de Cap-de-Long, Artificial reservoir in Hautes-Pyrénées, France.

Lac de Cap-de-Long is an artificial reservoir in the French Pyrenees at roughly 2,160 meters elevation. It holds water for hydroelectric power generation and helps manage water flow and flooding in the region.

The reservoir was built during the 20th century as part of France's hydroelectric development in the mountains. Construction required specialized techniques to create a stable structure at this extreme altitude.

The name "Cap de Long" means "head of the valley at the foot of Pic Long", the glacial threshold below that peak. The 1950s dam submerged the old natural Lac de Cap de Long and the Lac de l'Oustallat into one reservoir. The dam is listed as Remarkable Contemporary Architecture, and the road along its crest ends at the structure itself.

The road to the lake leaves the D929 at Fabian, a hamlet of Aragnouet, climbs past Lac d'Orédon and ends at the dam; it is generally closed in winter, with signs at the Fabian junction. The lake sits at 2,160 m, so bring equipment for cold, changeable mountain weather. Trails from the dam lead to Lacs d'Aubert and d'Orédon and are the start of climbs to Pic Long and Pic de Campbieil.

The lake's catchment covers only 11 km², yet a system of 40 km of galleries and 38 intakes drains the equivalent of 95 km² into it. Water from the Ossoue dam below the Vignemale is pumped back up after a 900 m siphon crossing the gave de Pau valley.