| Sihl | |
|---|---|
 The Sihl nearSihlbrugg | |
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| Location | |
| Country | Switzerland |
| Physical characteristics | |
| Source | |
| • location | Canton of Schwyz,Switzerland |
| Mouth | |
• location | Limmat,City of Zürich |
• coordinates | 47°22′57″N8°32′18″E / 47.3826°N 8.5382°E /47.3826; 8.5382 |
| Length | 73 km (45 mi) |
| Basin features | |
| Progression | Limmat→Aare→Rhine→North Sea |
TheSihl is aSwissriver that rises near theDruesberg mountain in thecanton of Schwyz, and eventually flows into theLimmat in the centre of thecity of Zürich, after crossing theZürich–Winterthur railway atZürich HB. It has a length of 73 km (45 mi), including theSihlsee reservoir, through which the river flows. Water is abstracted from the river at the Sihlsee, leading to decreased downstream water flows and a consequent reduction in water quality.[1][2]
The river flows through, or along the border of, the cantons ofSchwyz,Zürich andZug. The main settlements of theSihl Valley are all in the canton of Zürich, and include the towns ofLangnau am Albis andAdliswil, along with a south-western segment of the city of Zürich. Above Langnau am Albis, some 13 km (8.1 mi) from the confluence with the Limmat, there are no major settlements alongside the river, and only a few small villages. Whilst thetown of Einsiedeln is situated close to the Sihlsee, it is actually in the valley of a tributary river, theAlp.[1]
The first written reference to the name dates to 1018, in the formSylaha.[3] The name may be ofOld European orCeltic origin:*Sîla ("quiet watercourse", from a root*sîl = "to trickle, wet") >RomanceSila with the addition of the Old High German elementaha "flowing water".[4]
The river rises to the north-eastern side of theDruesberg mountain, in themunicipality of Unteriberg of thecanton of Schwyz, and flows north to thevillage of Studen. Some 2 km (1.2 mi) downstream of Studen, the river enters the artificialSihlsee reservoir at its southern end. Nearby, theMinster also flows into the Sihlsee, and was, prior to construction of the reservoir, a direct tributary of the Sihl. TheEubach,Rickentalbach andGrossbach tributary streams also flow into the Sihl via the Sihlsee.[1][2]
The Sihlsee is some 8.5 km (5.3 mi) long, and lies close to thetown of Einsiedeln. It is Switzerland's largest artificial lake, and is impounded by a 33-metre (108 ft) high dam. Electricity is generated by diverting water down a tunnel from the Sihlsee, which has a water level elevation of 889 m (2,917 ft), through theEtzelwerkhydroelectricpower station atAltendorf, intoLake Zürich, which has a water level elevation of 406 m (1,332 ft). This has led to decreased water flows downstream of the dam, with negative impact on water quality and a tendency for the river to dry out in places during low flow conditions in winter.[1][2][5]
Below the dam, the Sihl enters a narrow glacial valley and continues in an easterly direction, receiving the water of its tributary theAlp, before briefly entering thecanton of Zürich near thevillage of Hütten. The river then continues as the border between the cantons ofZug and Zürich, passing through theSihlsprungrapids along the way. At thevillage of Sihlbrugg, the river enters the canton of Zürich for a second and final time. North of Sihlbrugg, the river flows through a lightly populated and heavily wooded valley, accompanied by a main road and railway line. TheSihlwald forest, a rare example of a large-scale and original forest, lies on the slopes of theAlbis that rise up from the river to the west. At this point, Lake Zürich lies only 2 km (1.2 mi) to the east, but is separated from the river by theZimmerberg mountain.[1][2][6]
Reaching thetown of Langnau am Albis, on the west bank of the river, and thevillage of Gattikon, on the east bank, the valley becomes more populous and industrial, with industry originally attracted by the available water power. From here the valley bottom is continuously built-up through thetown of Adliswil to the boundary of thecity of Zürich. Once in the city, the river flows through and between theLeimbach,Wollishofen,Friesenberg,Alt-Wiedikon andEnge quarters before reaching the city centre. In this stretch the river is crossed by many bridges, and for one 1.2 km (0.75 mi) stretch theA3 motorway runs along and over the river on a viaduct.[1][2]
Nearer the city centre, theSZU railway line runs in tunnel along and under the river. One access to that line'sSelnau station is constructed in the form of an artificial island within the river. Just downstream of here, theSchanzengraben moat, which is fed from Lake Zürich and formerly protected the western perimeter of theAltstadt, flows into the Sihl, just before the latter flows underZürich Hauptbahnhof railway station. The station has platforms on two levels, and the river actually tunnels under the upper ones, but above the lower ones, in 5 culverts each with a length of 190 metres (623 ft) and a clear opening of 12 metres (39 ft) by 3 metres (10 ft). Shortly after passing under the station, the Sihl joins theLimmat at the apex of thePlatzspitz park.[1][7]
The river flows through theSihlsee some 50 kilometres (31 mi) upstream of Zürich city centre, and studies have shown that a failure of the dam could lead to an 8-metre (26 ft) high flood wave reaching the city within 2 hours. This threat is exacerbated by the fact that the river passes through Zürich Hauptbahnhof station in a tunnel that limits the river’s flow capacity, raising concerns about the capacity of the tunnel to deal with extreme flood events. This threat has led the City of Zürich to develop, publish and test evacuation plans for the affected areas of the city.[7][8][9]
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