The Sihlsee is a reservoir, and it is honest about that, which is part of its charm. The Sihl river once wandered through marshy farmland south of Einsiedeln. In the 1930s the Swiss Federal Railways dammed it to store water for hydroelectric power for the railway network, and by 1937 the valley floor had become a long blue lake, with farms moved and a stretch of moorland drowned. The lake still has the job of feeding the Etzel power station, so the water level changes with the seasons, and in late winter you can see the tide-line of bare shore.
Today the setting is as pleasant as any natural lake. The water is surrounded by meadows, small farms and low forested hills, with the Mythen peaks above Schwyz showing sharp on the southern skyline on a clear day. A causeway crosses the lake at Willerzell, where you can pause and look out in both directions. Swimming is possible from the shore in warm months, and the water is cooler and clearer than you might guess, though it never gets properly warm until August.
Riders reach it from the Ibergeregg side or from the lake of Zurich, and the road around the water is open, easy and scenic, with moderate bends and a good surface. After the climb and descent of the pass, the lake feels like a pause. It is also close to Einsiedeln, a short ride away, where the great Benedictine abbey has been a pilgrimage destination since the Middle Ages, famous for its Black Madonna and the baroque church on a broad square. The abbey is believed to have been founded in the tenth century, and the pilgrim route to Santiago de Compostela passes through town.
Fuel and food are in Einsiedeln, about three kilometres away, and the town has plenty of cafés near the abbey. At the lakeside the facilities are simple, so bring water and a snack if you plan to linger. Parking is easy near the shore, but check signs because some areas are for locals.
Wind picks up in the afternoons, ruffling the surface into small waves, and the lake shows a different colour in each kind of light. Morning is calm and glassy, and evening is soft and golden.