Lac du Vieux Émosson is a reservoir in Valais, Switzerland, in the municipality of Finhaut at an elevation of 2205 m. The lake drains into the reservoir Lac d'Émosson, 300 m below.
In September 2014, an increase of the dam by 21.5 meters was completed. The capacity of the lake was thus doubled to 25 million m³. The larger capacity is used for the Nant de Drance pumped storage plant, for which Lac du Vieux Emosson forms the upper basin and Emosson reservoir the lower basin.[1] The storage power plant went into operation on July 1, 2022, after 14 years of construction. The construction of the plant cost the equivalent of 2 billion euros. The installed capacity is 900 MW, and 20 GWh can be stored.[2]
The arch dam Vieux Émosson was completed in 1955. Efforts to raise the dam 20 m (66 ft) began in 2013. The lake forms the upper reservoir for the 900 MW Nant de Drance Hydropower Plant which began operations in 2022.[3]