Adult North American river otters weigh from about 5 to 15 kg (Melquist et al. 2003). Size varies among geographic areas, with males typically larger than females. River otters display delayed implantation, resulting in a period of almost one year from time of breeding until giving birth. Young generally are born in February and March, and breeding occurs shortly thereafter, well before the young are independent (Liers 1951, Hamilton and Eadie 1964). Males and females are sexually mature at two years of age, but variation in the reproductive age has been reported (Hamilton and Eadie 1964, Docktor et al. 1987). Litters typically are comprised of 1 to 3 young (Hamilton and Eadie 1964, Tabor and Wight 1977, Docktor et al. 1987). Maximum life expectancy is typically about 10 years of age in the wild and up to 20 years of age in captivity (Stephenson 1977, Melquist et al. 2003).

The North American river otter is an aquatic-habitat generalist, capable of exploiting virtually all freshwater systems, estuaries, and some coastal and marine areas depending on the availability of adequate prey and riparian cover (Melquist et al. 2003). The diet of the river otter is comprised mostly of fish, but amphibians (mostly frogs), crustaceans (mainly crayfish), and birds may also be eaten depending on the region and season (Sheldon and Toll 1964, Knudsen and Hale 1968, Stenson et al. 1984, Serfass et al. 1990, Reid et al. 1994, Stearns et al. 2011). Typically, the slowest moving and most abundant fish (e.g., members of the sucker and minnow families) species are preyed upon most (Serfass et al. 1990, Stearns et al. 2011). Depending on availability, crayfish in some areas exceed the importance of fish in the river otter diet (Serfass et al. 1990). River otters prefer undisturbed riparian with adequate cover to serve as denning and resting sites (Swimley et al. 1998, Stevens et al. 2011). Both diet and riparian habitat use vary based on regional differences in aquatic systems and associated conditions.

Ultimately, river otter populations are limited by the distribution of suitable aquatic and riparian habitats. Consequently, any factors that reduce the quantity or degrade the quality of aquatic environments will adversely affect populations. River otters are often associated with aquatic habitats modified by the construction of dams and lodges by the American beaver (Castor canadensis) (Reid et al. 1994; Swimley et al. 1998, 1999). River otters are otherwise most active during nocturnal and crepuscular periods (Melquist and Hornocker 1983, Stevens and Serfass 2008). The extent of travel is influenced by sex, age class, region, season, and habitat conditions (Spinola et al. 2008). Males occupy larger home ranges than females (Melquist and Hornocker 1983, Reid et al. 1994, Melquist et al. 2003, Spinola et al.2008). Home ranges for river otters occupying riverine habitats in Idaho ranged from 8 to 78 km² (Melquist and Hornocker 1983). River otters generally maintain low population densities (e.g., about 1 otter per 3.58 km of riparian habitat in Idaho; Melquist and Hornocker 1983), apparently without the need for overt (aggressive) displays of territoriality. Although considered to be territorial, home ranges of adjacent individuals may overlap to varying degrees based on gender and season (Melquist and Hornocker 1983, Spinola et al. 2008). River otters likely avoid aggressive interactions by practicing mutual avoidance through olfactory communication facilitated by scent marking at latrines. Predation on river otters has been reported, but is generally considered rare. River otters are presumably most vulnerable to being attacked and killed by larger predators when travelling overland or visiting latrine sites (e.g., gray wolves [Canis lupus], bobcats [Lynx rufus], coyotes [Canis latrans], domestic dogs [Canis familiaris] (Gable et al. (2017), and mountain lions [Puma concolor]; see Melquist et al. [2003] for a review of these and other reported examples of predation). However, there are isolated reports of river otters being preyed on in water (e.g., by the American alligator (Alligator mississippiensis); (Crocodylus acutus) (see Lariviere and Walton 1998). Trapping river otters for fur is an important human-induced mortality factor in many conservation jurisdictions in the USA and Canada (Bricker et al. 2016).