Researcher Jean Thie said Wednesday he used satellite imagery and Google Earth software to locate the dam, which is about 850 metres (2,800 feet) long on the southern edge of Wood Buffalo National Park.
Average beaver dams in Canada are 10 to 100 metres long, and only rarely do they reach 500 metres.
First discovered in October 2007, the gigantic dam is located in a virtually inaccessible part of the park south of Lac Claire, about 190 kilometres (120 miles) northeast of Fort McMurray.
Construction of the dam likely started in the mid-1970s, said Thie, who made his discovery quite by accident while tracking melting permafrost in Canada’s far north.
“Several generations of beavers worked on it and it’s still growing,” he told AFP in Ottawa.
Mike Keizer, spokesman for the park, said rangers flew over the heavily forested marshlands last year to try to “have a look.” They found significant vegetation growing on the dam itself, suggesting it’s very old, he said.
“A new dam would have a lot of fresh sticks,” Keizer explained. “This one has grasses growing on it and it’s very green.” …