The Grimselsee or Lake Grimsel is an artificial lake near the Grimsel Pass in the Canton of Bern , Switzerland . With a volume of 95 mio m3 (20.9 billion imperial gallons, 20.1 billion US gallons), it is larger than other hydroelectric reservoirs in the Aare headwaters region: Oberaarsee , Räterichsbodensee and Gelmersee . The dam was completed in 1932 and is operated by Kraftwerke Oberhasli AG (KWO). It is located in the municipality of Guttannen .