The Isar, which flows as a whitewater river from the Austrian part of the Karwendel mountains, is dammed between Mittenwald and Krün by a weir to form the Krüner Isar reservoir () and is then diverted to the Walchensee. This water flows past the Krün hydroelectric plant in an open channel, through a culvert, under the B 11 road at Wallgau and then via a tunnel to the Sachensee lake (). Here a 3.9 kilometer long penstock begins. At the end the water enters the hydro-electric power plant at (), propels the turbines, and finally flows into the lake.