The Grande Dixence dam, in the canton of Valais in Switzerland, is at 285 metres high one of the worlds tallest dams. It holds back a lake, the Lac de Dix , around 16km long.

The top of the dam is at an altitude of 2,365 metres in a steep mountain valley. Tunnels bring the water to three power stations in the Rhone valley over 1,800 metres below.

The dam is situated on the relatively small Dixence river, but collects a large amount of water thanks to a system of water supply tunnels over 100km long bringing water from other rivers and valleys. Most of the water comes from glaciers which melt during the summer. The lake is usually full of water by late September, and is allowed to empty during the winter, reaching its lowest point around April.

There is another dam situated a little further upstream, constructed several decades earlier, which was flooded by the later dam. It can still be seen when the water level is particularly low.