page_title: Obersee (Zürichsee)
section_title: Geography
hierarchical_section_title: Obersee (Zürichsee) / Geography
caption_attribution_description: Obersee (upper Lake Zürich), as seen from Stampf in Jona (Switzerland)
context_page_description: The Obersee is the smaller of the two parts of Zürichsee in the cantons of St. Gallen and Schwyz in Switzerland.
context_section_description: Zürichsee is the common name for the lower (Untersee) northwestern section of 68.2 square kilometres (26.3 sq mi), while the smaller southeastern upper (Obersee) lake area measures 20.8 square kilometres (8.0 sq mi), separated by the Seedamm causeway, a Molasse formation connecting Rapperswil with the Hurden peninsula. Before 1951 the annual water level fluctuated more than 1 metre (3.3 ft), but since then the water level is strictly regulated and therefore between summer and winter differs an average of 40 centimetres (15.7 in). The average lake level is now at 406 metres above sea level, while Obersee and Untersee differ by only 1 centimetre (0.39 in).
  The Seedamm between Rapperswil and Hurden was used since about 5,000 years as a historical lake crossing. Since the 1870s a partially artificial road causeway and two bridges were added, to cross the most narrow and flatbedded area of the lake, carrying a railway line and road from Rapperswil to Pfäffikon. Seedamm divides the southeastern section of lower Zürichsee and Obersee, connected by the Hurden canal at Hurden, where also the Frauenwinkel protected area is situated. That bigger part of the Zürichsee to the west of the Seedamm, is largely situated in the canton of Zürich, smaller parts in the cantons of Schwyz and St. Gallen (Kempraten and Rapperswil), whilst the upper lake is shared just between the cantons of St. Gallen and Schwyz.
  Zürichsee was formed by the Linth river, which rises in the glaciers of the Glarus Alps. In 1811 the Escher canal was completed, which diverted the Linth river into Walensee (Lake Walen) from where its waters are carried to the east end of Obersee by the Linth canal (completed in 1816), and separated by the Buechberg hill. No 
  other streams of importance flow into the Obersee besides the Linth and the much smaller Jona river.
caption: Obersee towards Buechberg
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