Oberaarsee, reservoir in the Grimsel area, Guttannen, Bern, Switzerland

Oberaarsee is a 2.8 km long and 500 m wide reservoir in the municipality of Guttannen, canton of Bern, impounded by a dam completed in 1953. It lies below the Oberaar glacier, its main inflow, and holds 65 million m³ of water. The Oberaarbach drains it into the Grimselsee.

The Oberaar dam was completed in 1953, creating the highest reservoir in the Aare headwaters and feeding the Kraftwerke Oberhasli hydroelectric plants. The dam is operated by Kraftwerke Oberhasli. The Oberaar glacier, west of the lake, supplies the main inflow; the lake is drained by the Oberaarbach, which flows into the Grimselsee.

The name Oberaarsee combines Aar, the river rising from the Oberaar glacier, with Ober (upper), distinguishing it from the lower Aar reservoirs Grimselsee and Räterichsbodensee. The lake belongs to the municipality of Guttannen, and its dam is run by Kraftwerke Oberhasli AG, the power company whose plants the reservoir feeds.

Oberaarsee is reached from the Grimsel Pass by a one-way road whose direction changes every half hour, and a cable car owned by the power company runs to the dam year-round. The Oberaar dam stands at the lake's outflow, where the Oberaarbach leaves for the Grimselsee.

The Oberaar dam is reached by a single-lane road from the Grimsel Pass whose direction of travel reverses every half hour, and a cable car owned by the power company runs to the dam wall all year. The lake is about 2.8 km long and 500 m wide, holding 65 million m³ of water.