The common muskrat is the largest arvicoline rodent on Oregon. The body is heavy and rounded, and except for the tail and feet, heavily furred. The eyes are beady; the ears are rounded and almost covered with fur; the tail is flattened laterally, scaly, keeled, and naked except for a few hairs on the keel. The forefeet are relatively small, but the hind feet are large and partly webbed. The pelage consists of an exceedingly dense underfur usually overlain by glossy dark-brown guard hairs; the underfur is waterproof. Reddish, blackish, silvery, and white individuals have been recorded.
In Oregon, muskrats originally were distributed throughout the Willamette Valley and coastal regions as far as Coquille, Coos County, and east of the Cascade Range east of a line from The Dalles, Hood River County, to Shirk, Harney County. High mountain regions and the Klamath, Summer, Albert, and Warner valleys were unoccupied. However, introductions to unpopulated lakes and marshes in Lake, Klamath, and Curry counties have been chronicled.
Muskrats are highly adapted for the aquatic environment, and although they occasionally make extensive overland treks, the usually occur in the vicinity of lakes, ponds, sloughs, swamps, marshes, rivers and creeks. Muskrats are powerful swimmers and can stay submerged as long as 20 minutes.