IV.

The Dam

The dams used by beavers to widen the area and increase the depth of water around their homes are constructed either of sticks and logs or more firmly and solidly of mud, brushwood, and stones. As time goes by the beaver repairs and adds to the dam. Floating material that becomes caught in the dam, and the roots of vegetation that grows on the top both strengthen the whole structure. Frequently the beaver builds a smaller dam downstream in order to back up some water against the original dam and thus decrease the pressure of water on it from the other side. The dams are about 1.5 m (5 ft) high, usually more than 3 m (10 ft) wide at the base, and narrow at the top. A beaver dam more than 300 m (1,000 ft) long was found in Rocky Mountain National Park, Colorado. Beaver ponds attract fish, ducks, and other aquatic animals. Although the dams cause local flooding, they also help to control run-off and reduce flooding downstream. The ponds eventually fill with sediment, and the animals move to a new location. The abandoned area becomes good meadowland.

Although the beaver is a powerful swimmer, it is awkward on land and has difficulty dragging the logs and branches it needs for building and for food. Colonies of beavers therefore often dig canals from the pond to a grove of trees. Such canals are up to 1 m (3 ft) wide and deep and often a few hundred metres long. The timber is then readily floated down the canal towards the pond. Some observations and experiments suggest that dam construction is the beaver's response to the sound of running water.