the beaver

The Dam's Primary Role: The Beaver's Home

Beaver dams are created both as a protection against predators, such as coyotes, wolves and bears, and to provide easy access to food during winter. Beavers always work at night and are prolific builders, carrying mud and stones with their fore-paws and timber between their teeth. Because of this, destroying a beaver dam without removing the beavers is difficult, especially if the dam is downstream of an active lodge. Beavers can rebuild such primary dams overnight, though they may not defend secondary dams as vigorously.


Shape of dam

Lassen Volcanic National Park.A dam's shape depends on the strength of the stream's current. Relatively still water encourages dams that are almost straight; while dams in stronger currents are curved, with the convexity pointing upstream. The beavers use driftwood, green willows, birch and poplars; and they mix in mud and stones in ways that contribute to the dam's strength.

Beavers have been known to build very large dams.[6] The largest known was discovered near Three Forks, Montana, and was 2,140 feet long, 14 feet high, and 23 feet thick at the base.[7] When objectionable beaver flooding occurs, modern water level control devices can be installed for a cost-effective and environmentally sound solution. Unwanted damage to trees can be prevented by wrapping chicken wire or sheet metal around the base of trees.[8]


Canoeists try unsuccessfully to run a beaver dam in Algonquin Park. The dam is about 1 m high."In places," writes Hearne, "which have been long frequented by beavers undisturbed, their dams, by frequent repairing, become a solid bank, capable of resisting a great force both of ice and water; and as the willow, poplar and birch generally take root and shoot up, they by degrees form a kind of regular planted hedge, which I have seen in some places so tall that birds have built their nests among the branches."[citation needed]

Stimulus for dam-building
It is primarily prolonged exposure to the sound of water in motion that stimulates the beavers to build. However, studies involving beaver habitual activities have indicated that beavers may respond to an array of stimuli, not just the sound of running water. In two experiments Wilson (1971) and Richard (1967, 1980) demonstrate that, although beavers will pile material close to a loudspeaker emitting sounds of water running,[9] they only do so after a considerable period of time. Additionally the beavers, when faced with a pipe allowing water to pass through their dam, eventually stopped the flow of water by plugging the pipe with mud and sticks. The beavers were observed to do this even when the pipe extended several meters upstream and near the bottom of the stream and thus produced no sound of running water. Beavers normally repair damage to the dam and build it higher as long as the sound continues. However, in times of high water, they often allow spillways in the dam to flow freely.