Beavers (Castor canadensis) are thickset animals with paddle-shaped tails and hind legs that are webbed for swimming. They use their tail for swimming and as a rudder to steer. They are quite dexterous with their forepaws and use them like hands.
Their front teeth or incisors are chisel-shaped enabling beavers to gnaw away at trees and use their trunks and limbs to build dams and lodges. Beavers can cut down a 12 cm (5") diameter tree in about 30 minutes.
Beavers construct dams to slow down water to create ponds in order to maintain a steady depth of water. Water depth is important in order to (1) provide a place to store branches that supply bark for food, especially through the winter, and (2) provide for predator-resistant and ice-free underwater entrances to their homes. Beavers build lodges out of sticks, stones and mud that can be 2m (6 ft) and as much as 12m (40 ft) wide at their base under the water.
Beavers cut trees and drag them to the water to form dams and lodges. Using any rocks or branches lodged underwater or on the bank to build upon, beavers begin adding branches hauled to the water. Lodges are mounds built in the middle of the water, usually not far from the dam. Each lodge has one nesting chamber located above the water's surface and several entrances through underwater channels. Wonderful diggers, beavers may also burrow underground tunnels from the banks up to favorite feeding grounds and excavate channels to other parts of the stream or river. Beavers eagerly maintain their structures by patching them with branches and mud scooped from the floor of the waterway or pool. At times, flooding or storms may destroy lodges and dams, but many withstand the weather.
Beavers are active at dawn and dusk. Beavers often work on their dams and lodges during the night. They are seldom seen during the day. Beavers eat all parts of a tree: leaves, buds, bark, roots, and the living layer beneath the bark called the cambium. Beavers also eat aquatic plants to include the roots.
During the fall, beavers make a deep hole in the water near their lodge and fill it with branches that they munch on through the winter. The cold water helps to keep the food fresh.
Beavers are one of the few animals that significantly modify the environment in which they live. Beavers destroy habitat by flooding an area, but compensate for that by creating habitat for other animals and plants.
Beaver dams prevent the normal flow of water, flooding the banks of waterways and creating pond environments. These pools attract many insects, birds, and other creatures that thrive in still water. Certain animals—such as fish that must migrate long distances up or down a river—may not benefit from the beaver's work, and may perish. Flooding caused by the dam may also kill engulfed trees, plants, and insects. But beavers also encourage new life, as they create a marshy area. New insects are attracted to dead wood, and the dampened soil encourages new trees to sprout and grow. As beavers cut trees, branches then grow from the remaining stumps, providing beavers with fresh food.
