River otters have long, streamlined bodies, short legs, webbed toes, and long, tapered tails—all adaptations for their mostly aquatic lives. Their short thick fur is a rich brown above, and lighter, with a silvery sheen, below. Adult male river otters average 4 feet in length, including the tail, and weigh 20 to 28 pounds. Female adults are somewhat smaller than males. The North American river otter has long whiskers that are used to detect prey in dark waters. It differs from the Eurasian otter by its longer neck, narrower visage, the smaller space between the ears and its shorter tail.

Their aquatic habitats can be both marine and fresh water: streams, rivers, lakes, ponds and marshes. They prefer unpolluted water with a minimal human disturbance. An extremely adaptable animal, otters tolerate hot and cold climates, as well as high elevations and lowland coastal waters. River otters are common throughout the northeast in ponds, lakes, rivers, sloughs, estuaries, bays, and in open waters along the coast. In colder locations, otters frequent areas that remain ice-free in winter—rapids, the outflows of lakes, and waterfalls. River otters avoid polluted waterways, but will seek out a concentrated food source upstream in urban areas.