How Beavers Build Dams Beaver Dams Beaver Aquatic Animals Wetlands Wetland Animals

The Beaver (Castor canadensis) is a large aquatic rodent that builds intricate dams, canals and lodges. They range from 35 1/2″ to 46″ in length, are dark brown and have a hairless, black scaly tail that is horizontally flat. They are excellent swimmers, with large, webbed back feet that can be up to 5″ wide. They inhabit lakes, ponds, rivers, streams and marshes.

Beavers are very industrious workers, hence the phrase “busy as a beaver” is in common usage. During frontier days, beavers were almost trapped into extinction for their luxurious fur. They have been restocked in many areas and are now widespread throughout the U.S. and Canada.

Beavers and humans are considered to be the two species that alter their habitat most to meet their needs and make themselves more comfortable and successful in their environment. Beavers on rivers and streams burrow into the banks underwater and then slope the tunnels upward to form a den area for shelter and protection. In lakes and ponds, they will build lodges from piles of limbs. These lodges will have several underwater entrances and there will be a well ventilated chamber above the high water level.  Wood chips and the loose construction of the lodge material allows excess water to drain from the chamber.

Beavers build dams on streams from small size to create small ponds to huge affairs that create large ponds behind them. These dams can be destroyed by major floods, but most are quite strong and withstand strong water flows.

Beavers have large incisors and can cut huge chips easily from standing trees. They feed on bark and cambium layers of the wood and use the stripped limbs to build dams and lodges. The dams they build back water into timbered areas and allow them to access more wood for food, lodge building and dam building. As the supply of standing timber is consumed, they will dig canals from the pond into wooded areas so that the limbs and trunks of small trees can be gnawed down and pulled into the canals. The water makes the limbs buoyant and more easy to transport than dragging them over the ground.  In addition, the beaver are more protected in the canals and not as vulnerable to predators. Beaver are sometimes killed when they fell large trees. They can’t determine the direction in which the tree will fall and may be crushed when it falls.

Beaver can reach weights of 60 to 70 pounds and the large incisors make them a formidable foe, but they can be brought down by coyotes or feral dogs. When alarmed, a beaver will slap its large, flat tail on the water with a resounding splash to warn others in the area. Humans traveling by canoe or kayak in the pre-dawn and dusk hours often pass close to beaver and are startled by this alarm signal.

The dams are quite interesting. They are built with short sections of wood that the beaver have gnawed. The ends are usually pointed, and they are placed with the one point in the downstream direction and pushed into the muddy bottom.  Water pressure pushes against the upstream end to keep it embedded. Others are placed strategically and then mud and grass are packed into the crevices. After the dam reaches a significant height and begins to back water into a pond, silt collects on the front side of the pond as water runs over the top. This adds strength and parts of the dam above the water may even have vegetation growing on them.

These industrious animals are still trapped in some areas, but the reduced interest in the fur trade in the U.S. and the anti-hunting groups have contributed to a signigicant population growth. Beaver can cause a lot of damage to commercial timber operations and can create dams that flood road or valuable agricultural property. They are sometimes trapped to minimize this kind of damage. When dams are destroyed to discourage them, humans that dynamite the dams are astounded to return a few days later and find the dam restored. Special drain mechanisms have been designed that will drain the water even when the beaver rebuild the wooden structure. Even with these measures, it is unlikely that the beaver will be threatened by extinction any time in the immediate future.