A beaver may work alone or with family members to build a dam, using piled logs and trees secured with mud, masses of plants, rocks, and sticks. Very large trees are not moved but the bark is stripped off and eaten. Smaller trees are cut into moveable pieces, dragged into the water for repairing dams and lodges. This work is done mainly in autumn. The den is used as a food cache, rearing area, and general home.
Dens are of two types depending on water level fluctuations.
Bank dens are simply dug into the stream or river bank with a mass of sticks, mud, and rocks constructed over the top of the den. Lodges are constructed of the same materials as bank dens, but are located where the water level is more stable and slower moving, like in a pond or lake. Where streams are too large or swift to dam but do provide ample water throughout the year, the beavers may use bank dens. These may have several tunnel exits with at least one above the high water mark and another below the low water mark. The exit provides quick and easy access for food gathering and emergency escape from predators.