Why do beavers build dams?

Beavers are clumsy on land but excellent swimmers, so deep water is their safest habitat. By damming streams with branches, stones and mud, they raise the water level and create a pond around their lodge. The lodge entrance is underwater, keeping wolves and other predators out, and the pond stays deep enough not to freeze solid in winter. Beavers store branches underwater near the lodge as a winter food supply. The longest known beaver dam, in Wood Buffalo National Park, Canada, is about 850 metres long and was spotted on satellite images in 2007.