Lake Sihl is a reservoir located entirely within the municipal area of Einsiedeln in the canton of Schwyz. The lake is dammed on the north shore, it was flooded in 1937 and lies at an altitude of 889 metres above sea level. The tributaries of Lake Sihl are the Sihl, which gives the lake its name, and other smaller streams. With a length of about 8.5 km and a width of between one and 2.5 km, the reservoir is the largest of its kind in Switzerland.