From this story in The Ottawa Globe and Mail:
Experts say beavers have been busy since mid-70s, building what's believed to be largest known dam on earth. Ottawa ecologist Jean Thie located the dam using NASA and Google satellite technology in October 2007, posting it online, but only recently has it grabbed the world's attention. The dam is situated in northern Alberta's Wood Buffalo National Park and stretches about 850 metres, more than eight football fields long. Mr. Thie has told reporters the beavers who laid the foundation for the dam likely got busy building in the mid-1970s. Alerted to global media's beaver fever, a park spokesman had rangers fly over the area to investigate because the area isn't accessible by any roads or streams. The dam is now being pegged as the largest known habitat of its kind on earth, beating a 652-metre structure in Three Forks, Montana.