Glattalpsee, body of water

Glattalpsee is a 27.6-hectare reservoir on the Glattalp plateau at 1,853 m above Bisisthal in the municipality of Muotathal, Canton of Schwyz. Although in Schwyz, it lies in the Glarus Alps. It has no surface outflow: water seeps through the karst bed or feeds the Sahli power plant, and the lake dries out in winter.

Glattalpsee is a natural lake with no surface outflow; its water seeps through the karst bed, and since 1991 part of it has fed the Sahli power plant in Bisisthal through a steep pressure pipe, producing electricity for about 4,000 households. Because the bed leaks, the lake dries out in winter and refills when the snow melts in spring. Up to 700 litres per second are lost to seepage, and a partial sealing of roughly 12% of the surface is planned.

The name pairs "Glatt", from the smooth alpine pasture, with "See", lake; the surrounding Glattalp is summer pasture for Muotathal cattle. Walkers today pass the Glattalphütte, a hut of the Swiss Alpine Club, and the Berggasthaus restaurant on the loop around the shore.

A gondola lift runs from Sahli in Bisisthal up to the Glattalp; from its top station the lake is an easy walk, passing the Glattalphütte and the Berggasthaus restaurant. The roughly 7 km loop around the shore has little elevation gain and takes about two hours. Climbing on foot from Bisisthal is demanding because of the height difference and needs sure footing.

On 7 February 1991 a temperature of −52.5 °C was recorded at Glattalpsee, the lowest value ever measured in Switzerland. Because the lake bed is karst and leaks, the lake dries out in winter and refills only when the snow melts in spring; up to 700 litres per second seep away, and roughly 12% of the surface is to be sealed to raise the Sahli power plant's output.