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North American beavers have also been introduced into South America (Patagonia) and Europe (primarily Finland and Karelia).
The North American Beaver is often referred to simply as "beaver," although this can cause some confusion because another distantly related rodent, Aplodontia rufa, is often called the "mountain beaver." Other vernacular names, including American beaver and Canadian beaver, distinguish this species from the other extant beaver species, Castor fiber, which is native to Eurasia.
The beaver is the largest rodent in North America and competes with its Eurasian counterpart, the European beaver, for being the third-largest in the world, both following the South American capybara and lesser capybara. The European species is slightly larger on average but the American has a larger known maximum size. Adults usually weigh from 11 to 32 kg (24 to 71 lb), with 20 kg (44 lb) being typical.
The first fossil records of beavers are 10 to 12 million years old in Germany, and they are thought to have migrated to North America across the Bering Strait.The oldest fossil record of beavers in North America are of two beaver teeth near Dayville, Oregon, and are 7 million years old
At one time, 25 subspecies of beavers were identified in North America, with distinctions based primarily on slight morphological differences and geographical isolation at the time of discovery.
However, modern techniques generally use genetics rather than morphology to distinguish between subspecies, and currently the Integrated Taxonomic Information System (which provides authoritative taxonomic information on plants, animals, fungi, and microbes of North America and the world) does not recognize any subspecies of C. canadensis, though a definitive genetic analysis has not been performed.
Such an analysis would be complicated by the fact that substantial genetic mixing of populations has occurred because of the numerous reintroduction efforts intended to help the species recover following extirpation from many regions.
Information and Image taken directly from Wikipedia