The River otter is adapted for both terrestrial and aquatic environments. The heavily muscled, somewhat cylindrical body is thickest at the thorax and tapers posteriorly to a thick, flattened tail. The body tapers to a blunt and slightly flattened head. The legs are short and powerful; and the toes are webbed. The eyes are small, forwardly directed and set high on the head. The underfur is grayish, short, and dense, and overlain by longer, stiff and shiny guard hairs. The dorsum is brown and the venter a lighter brown or tan; the lower jaw and throat are whitish.
In Oregon, River otters are mostly found west of the Cascade Range but have been found in eastern Klamath County and in Deschutes, Wallowa, and Malheur counties. The River otter is associated with river, lake, pond or marsh habitats, but may make extensive overland excursions from one such habitat to another.
River otters are considered among the more social members of the mustelidae family. In addition to the adult male-female association during the breeding season and the maternal female to young association, a variety of groupings of otters in different sex and age-classes have been observed.