Obersee (Lake Zurich)
The Obersee is the smaller of the two parts of Lake Zurich, located upstream in the cantons of St. Gallen and Schwyz in Switzerland. It is almost completely separated from the lower part of Lake Zurich by the Hurden Peninsula, an Ice Age glacial moraine.
The lake is home to several species of birds, amphibians, fish and invertebrates. There are several protected areas along the lake's shores. Located in the Prealps, the lake is also a popular recreational area.Geography
"Zürichsee", in certain contexts, refers to the lower northwestern section of of Lake Zurich, or the lake as a whole. While the smaller southeastern upper lake area measures, separated by a peninsula, a glacial moraine that almost reaches the opposite coast. Before 1951 the annual water level fluctuated more than, but since then the water level is strictly regulated and therefore between summer and winter differs an average of. The average lake level is now at 406 metres above sea level, while Obersee and Untersee differ by only.
The narrow strait between Rapperswil and Hurden was used already 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 Zurich, smaller parts in the cantons of Schwyz and St. Gallen, whilst the upper lake is shared just between the cantons of St. Gallen and Schwyz.
Zürichsee was formed by a glacier and is supplied 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, from where its waters are carried to the east end of Obersee by the Linth canal, south of Schmerikon. Besides the Linth, other streams that flow into the Obersee are the Jona river, the Wägitaler Aa and the Schmerikoner Aa.
To the south-east, Obersee borders the Buechberg, a Molasse hill. Further to the east are the Appenzell Alps, with the Speer, Chüemettler and Federispitz overlooking the lake. Further south are the Schwyz Alps.Hydrology
Obersee is characterized by its shallow depth and, compared to its volume, by the very large inflow from the Linth canal. The theoretical residence time of water in the upper lake at medium Linth inflow is thus only around 10 weeks. In the near-surface layers of water, a slight upward trend in average temperature has been observed, resulting in the temperature stratification of the lake being more stable much longer into late summer. The increasingly warm winters result in incomplete mixing of lake water to the depths.
The Walensee and Obersee are the main water suppliers for Zürichsee which is used in turn for more than a million inhabitants as drinking water storage. To ensure the quality of the drinking water supply, the water supply authorities of the city of Zurich therefore regularly examine the quality of these two lakes. The neighboring cantons of Zurich, Schwyz, Glarus and St. Gallen participate in the monitoring costs. Since the early 1970s the water quality has greatly improved thanks to extensive restructuring measures. Almost all residents in the large catchment area are connected to sewage plants. Although every spring a circulation of the water occurs, often a visible lack of oxygen in July below a depth of is observable, which lasts for several months.Nature
Flora
Following a 1978 inventory, in 1988 the University of Geneva took a second inventory of aquatic plants in the riparian zone of the entire lake area. The lake shore in the canton of St. Gallen was divided into 50 portions of approximately in length and described by 12 different parameters. The most important are the plants environmental information on aquatic plants, divided into reeds, floating plants and submerged vegetation. The dense vegetation zones are generally between and up to depth. Instead of nutrients, there is a high load with floating mineral particles, which prevents colonization of the sea floor in greater depths. The grade of colonization of the shallow water zone has risen again since the last observation in 1978, which is directly related to the increase in the maximum colonization zone. The vegetation consists four-fifths of submerged plants and only about one-fifth of emerging plants. The latter includes the reedbed: In 1850, of reed beds were alone in the lower lake area. A comparison of aerial photographs from 1954 and 1987 shows in most shore sections of the machining area a significant decrease in the reedbeds. The reed area has increased from 1978 to 1987 in the upper lake from 15 to 16 ha. It can be concluded that the reed banks are mostly declined in the 1960s and 1970s, but that since then at a low level takes place a slight recovery. Lily, as floating leaf plants, covered an area of on the lower lake, with a declining trend; in contrast, a doubling to can be detected on Obersee. Of the 61 species in the entire Zürichsee, 56 colonize the upper lake and 34 the lower lake. Most frequently represented are the pondweed ; 24 species are considered rare, more than 15 very rarely.Fens, open fields and parks
There are no more original riverside forests on the lake shore, except on steep slopes like the Fuchsberger Horn where a species-rich forest with oak and pine trees breeds. Also worth mentioning is the small wooded area at the far end of the upper Bollingen peninsula. Outside of urban areas, the bank is often planted with different density and broad trees stocks. Extensive shore sections are occupied by large, private gardens and parks. Although often non-native and exotic shrubs and tree species occur and the lawns usually are intensively cultivated, the upstream water habitats often form relatively low-noise buffer areas to the underlying recreation, transport and settlement areas. At several banks sections agricultural meadows and pastures directly to the water. However, the upper Bollingen peninsula is the only major agricultural area with lake access. All remaining grasslands have less than lake access and are usually relatively little interference. A disadvantage often is the missing, extensively used transition to riparian vegetation.Geotopes
s comprise geological outcrops, boulders and river delta formations. Geological rock outcrops, i.e. scenically striking rocky ridges are found in Fuchsberg and Stafflen in Jona, Heilig Hüsli in Rapperswil and the Hörnli glacial erratic in Oberbollingen. Extensive river deltas included that of the Jona and Schmerikoner Aa, to a lesser degree of Sägenbächlein, Wagner and Lehholz in Jona and the Linth with side channels in Schmerikon.Fauna
Birds
Perennial systematic birdwatching and studies illustrate the importance of reed beds as an outstanding habitat for many bird species. Among the 15 most valuable ornithological reeds on Obersee include Rapperswil Strandweg, Bollingen Unter Stafflen, Wurmsbach bay, Wurmsbach nunnery, and Schmerikoner Allmeind in the canton St. Gallen. The extensive area of shallow water with its diverse flora and the still widespread shore trees play an extremely important role for many bird species, both as food, breeding and resting place. A critical phase in the life of birds is the time of moulting, during which they are more or less incapable of flying and depending on quiet water areas. The Reed warblers inhabit the seaward edges of the reed belts and need significantly more space than other reed birds. Due to the decline of the reed banks and the increase in interference from recovery operations and water sports on the lake shore, the number of breeding pairs is constantly decreasing. Today the Great reed warbler is acutely threatened. According to the Red List of endangered species in Switzerland, it is listed in the top risk category of disappearing birds whose survival in Switzerland is unlikely if the threats continue. In the Obersee area, there are three important resting places of waders: Joner Allmeind annually depending on the water level in spring and autumn, Rapperswil in winter. and Schmerikoner Allmeind in spring and autumn. For some northern European birds, such as the common eider, the lake is a wintering area.Amphibians
Many amphibian species are dependent on both natural waters for spawning, as well as on adjacent food and wintering areas. Because the connection between these two partial habitats is interrupted by settlement patterns and transport systems, the amphibians have declined in these landscapes. With regard to a cross-linking of priority areas by the lake with the backlands, the most important amphibian spawning areas have been identified in 1997. Amphibian spawning areas of national importance are situated at the Allmeind areas in Jona and Schmerikon. At the Joner Allmeind two endangered species on the Red List can be found in large to very large populations. Schmerikoner Allmeind is home to the same two endangered species. For both areas the preservation and promotion of natural corridors are required, and in Schmerikon along the Linth and the side channels a large-scale networking with Kaltenbrunner Riet and other protected areas in the Linthebene region.Fish stocks
The few existing inventory data are compiled by the competent fisheries overseer respectively a report in 1993. This is a rough, spatially highly generalized statement to the spawning grounds of the Northern pike and Coregonus. In addition to these specific fishing information, ecological data and studies on the other species of fish and their habitat requirements, but also to invertebrates, such as crabs and mussels, is missing.