The Seedamm, a partially artificial causeway and bridge, crosses a narrow point of the lake carrying a railway line and road from Rapperswil to Pfäffikon. The eastern section of the lake is known as the Obersee, German for "upper lake". West of this dam lie the small islands of Lützelau and Ufenau, where in 1523 Ulrich von Hutten took refuge and died.