They have disappeared from nine states and one Canadian province. The primary cause of decline was habitat pollution and destruction. Hunting, currently legal in some US states and all Canadian provinces, also reduces otter numbers. River otters use a variety of habitats. They frequent lakes and ponds, but usually live in marshes and along wooded rivers and streams.
Otters live in dens, usually borrowed from beavers, muskrats or woodchucks.
Brush piles and root areas under large trees are used as temporary homes. Beavers are important to otters, for the dens they build, and because beaver ponds make ideal otter habitat. Dens have openings above water in summer, but in winter these are closed and the only entrance is below water. The entrance opens up to a large nest chamber, which may have a bare floor or a slight accumulation of leaves or grass. River otters require a large amount of space.