Mauvoisin Dam

Mauvoisin Dam is a concrete variable radius arch dam across the Val de Bagnes on the Dranse de Bagnes stream, in the canton of Valais, Switzerland. Initial construction on the dam commenced in 1951 and was completed in 1957, with the reservoir filling by 1958. In 1991, the dam was raised to increase the capacity of the reservoir for winter storage. The dam's primary purpose is hydroelectric power generation.

The eighth highest dam in the world, Mauvoisin stands 250 m high and 520 m long, with a structural volume of 2030000 m3. The impounded water behind the dam forms the 4.9 km Lac de Mauvoisin, which has a capacity of 211.5 million m3 (171,500 acre feet) and a full surface area of 208 ha. The dam and reservoir control runoff from a catchment of 167 km2. Flood waters are released by a gated spillway with a capacity of 107 m3/s.

Water from the dam is fed to two hydroelectric power stations with a combined capacity of 363 megawatts (MW). Mauvoisin Dam provides a hydraulic head of 482 m to the Fionnay generating station, which can produce 138 MW from three Francis turbines. The water then drops another 1014 m to the Riddes generating station, where it drives five Pelton turbines with a combined capacity of 225 MW. The two plants produce about 943 million kilowatt hours (KWh) each year, with Fionnay generating 278 million KWh (29.5%) and Riddes generating 665 million KWh (70.5%).

Mauvoisin Dam also serves for flood prevention and sediment control. The dam helps protect the Bagnes and Rhône river valleys from glacial lake outburst floods such as ones that occurred in 1595 and 1818. During the 1960s and 1970s, Giétro Glacier adjacent to Lac de Mauvoisin threatened to produce icefalls, which could have overtopped the dam. Giétro has retreated since 1980, eliminating the threat of such an event. The dam also traps about 300000 m3 of sediment each year, helping to extend the life of downstream hydroelectric plants. However, sediment accumulation also poses a threat to the dam's useful life; a project to remove sediment has been proposed but has not been implemented because of safety concerns.

In 2016, the dam was the location of the world-record highest successful basketball shot. 28-year-old Australian Derek Herron (from the YouTube channel How Ridiculous) launched a basketball from the top of the dam, where it fell 180 m directly into a net placed on the ground below.