Once the most widely distributed mammals in the United States and Canada, early European settlers trapped otters for their fur. By the early 1900s, river otters had disappeared from much of their historic range due to over trapping. In the 20th century, loss of habitat from wetland drainage and water pollution have added to the pressures on the species. Its life expectancy is 10-15 years. Along with weasels, minks and several other species, the river otter belongs to the family Mustelidae.
Males are usually about seventeen percent larger than females.
Otters have a sleek body with a short, blunt snout, a thick neck, and a thick tail that is flattened on the top and tapers to a point. The waterproof fur is short and dense. It is generally dark brown with light brown coloring under the neck, chest, and stomach. Otter feet have five toes with non-retractable claws and webbing between each toe. The heel pads on the hind feet are adapted to provide better traction on slippery surfaces.