The lake, lying in a zungenbecken or hollow, was created by ice age glaciers from the Alps, and extends 21 km (13 mi) from north to south and has a width of 3-5 km (2-3.5 miles) from east to west. It has a single, small island, the Roseninsel, and a single outlet, the W�rm river (because of this river the lake was called the W�rmsee until 1962). Its major inflow comes from a chain of small lakes in the south, Osterseen. This small river is called Ach or Ostersee-Ach. The lake's water is of excellent quality due to a circular sewerage system being introduced in the 1960s, collecting all wastewater from around the lake and transporting it to a treatment plant below the lake's outlet at Starnberg.
It is possible to circumnavigate the lake on a bicycle. Passenger services have operated on the lake since 1851. Today they are operated by the Bayerische Seenschifffahrt company, using modern motor ships
