The North American Beaver (Castor Canadensis) is the only species of beaver in the Americas, native to North America and introduced to South America. In the United States and Canada, where no other species of beaver occurs, it is usually simply referred to as beaver. Its other vernacular names, including American beaver and Canadian beaver, distinguish this species from the one other extant beaver, Castor fiber, native to Eurasia.
This beaver is the largest rodent in North America and the third largest rodent in the world, after the South American capybara and the Eurasian beaver. Adults usually weigh 15 to 35 kg (33 to 77 lb), with 20 kg (44 lb) a typical mass, and measure around 1 m (3.3 ft) in total body length. Very old individuals can weigh as much as 45 kg (99 lb).
Beavers are mainly active at night. They are excellent swimmers but are more vulnerable on land and tend to remain in the water as much as possible. They are able to remain submerged for up to 15 minutes. They use their flat, scaly tail both to signal danger by slapping the surface of the water and as a location for fat storage.
They construct their homes, or "lodges," out of sticks, twigs, and mud in lakes, streams, and tidal river deltas. These lodges may be surrounded by water, or touching land, including burrows dug into river banks. They are well known for building dams across streams and constructing their lodge in the artificial pond which forms.
When building in a pond, the beavers first make a pile of sticks and then eat out one or more underwater entrances and two platforms above the water surface inside the pile. The first is used for drying off. Towards winter, the lodge is often plastered with mud which when it freezes has the consistency of concrete. A small air hole is left in the top of the lodge.