Griessee, Reservoir in Valais, Switzerland

Griessee is a small reservoir in Switzerland's Valais region, situated at over 2,380 meters elevation among mountains. The water comes from the Gries Glacier and is held back by a dam built in 1965 to store water and manage flow in the area.

The Gries gravity dam was completed in 1966 after three years of construction, creating a reservoir of 18 million m³ on the territory of Ulrichen, now part of Obergoms. Its water feeds the cavern power plant Altstafel, which has a capacity of 9.2 MW and produces 21.6 million kWh per year, almost 80% of it in the winter half-year. The water then passes under the cantonal border into Ticino and is used again in the Lago di Robièi, Bavona, Cavergno and Verbano plants.

The name Gries refers to the Gries Glacier that feeds the lake and to the Gries Pass, the old mule track running south from the shore into Italy. Since 2011 the first turbine of the Gries wind farm has stood beside the reservoir, so hydropower and wind power are produced at the same spot.

The lake sits at over 2,380 meters elevation and is normally reached by car via the nearby Nufenen Pass, with the drive passing through rocky mountain terrain. Bring your own food and water since there are no facilities at the site.

The drop from the reservoir at 2387.5 m to Lake Maggiore at 194 m gives the water an unusually high energy value: one cubic metre yields about 4.3 kWh on its way down, among the highest figures in Switzerland. The water passes through the Altstafel cavern power station (9.2 MW) and then three plants in Ticino.