Lake Superior, largest of the Great Lakes and the world’s largest freshwater reservoir, has fallen to its lowest level in 81 years. Its decline is being seen as evidence of the effect that climate change is having on the North American continent.

The lake is a foot and a half below its long-term average. The last time it was this low was in 1926. Falling water levels mean that once-floating boat docks are high and dry, cargo vessels are severely restricted, hydroelectric power is curtailed and lakeshore ecology is changing fast.

Researchers at the University of Minnesota have measured an average temperature rise of 4.5F since 1979. The warmer water is evaporating faster and holding less ice in winter. For the first time in living memory, the ice and snow that usually covers the lake by early December arrived late last year.