Lac de Malpasset, reservoir in France

Lac de Malpasset was the artificial reservoir held back by the Malpasset dam in the Reyran valley, from its completion in 1954 until the dam's collapse on 2 December 1959 at 21:13. At maximum level it ran about 4 km north-south, was some 300 m wide east-west, covered 2 km² and held close to 50 million m³ of water, with depths averaging up to 50 m in its middle.

The dam was built in the early 1950s to support irrigation and water supply, but its construction was based on incomplete geological studies. On December 2, 1959, heavy rains caused water to rise rapidly and the dam suddenly failed, causing one of France's worst civil disasters of the 20th century.

The lake was stocked with fish once the water rose high enough, and its banks drew anglers and onlookers who came to see the arch dam holding back the Reyran. Since the 1959 rupture the valley has been taken over by Var vegetation, with trees growing again where the water once stood.

The site is open to visitors today with walking paths along the riverbed and around the ruins. The landscape is mostly rocky with gneiss hills, so wear sturdy shoes and bring water.

The dam was never rebuilt, and part of its arch still stands in the Reyran valley. The reservoir it held filled only slowly because the Reyran runs dry nine months a year, yet it reached its maximum level in the hours before the collapse of 2 December 1959, which killed more than four hundred people.