Hydroelectric power accounts for about 58% of Switzerland's electricity production. Stations using reservoirs - constructed by building huge dams in the mountains - supply over a third of the total power produced. These stations include the Grande Dixence, in canton Valais, whose dam wall is the third highest in the world, at 285 meters (935 feet).

Another quarter comes from run-of-river stations, which exploit the water's natural flow to drive their turbines.

Switzerland also has a number of pumped-storage stations, which have a system of paired reservoirs.

Hydroelectric energy is a renewable resource, which produces no carbon dioxide emissions, and is hence regarded as relatively environmentally friendly. However, it can have serious negative consequences on the surroundings.

The problem is not only that large areas of land are lost under new reservoirs, but - even more importantly - that the dams have a serious impact on the whole river system. As river levels have fallen, the temperature of the water in them has risen, killing many of their fish - which are also threatened by a lack of fish ladders to take them over the dams.