Yellowstone’s unique geology makes the perfect environment for river otters. Active hydro-thermals keep lakes and rivers from freezing over and ensure a steady source of fish throughout the winter.  The river otter is one of six species of otters in the world but definitely one step up from its relative, the sea otter. Sea otters can barely walk and almost never leave the water. 

River otters share the same characteristic thick fur as weasels, perfect insulation against Yellowstone's cold winters. This thick coat of dark, oily fur is lined with a lofty undercoat of shorter hair and an insulting layer of air is effectively trapped between them. Otters must constantly groom their coats to prevent the fur from becoming flat and matted and losing its protective air pocket.

Like weasels, river otters are highly active, constant predators. Otters eat an average two pounds per day of insects, frogs, crustaceans and small mammals. They can swim in circles, creating a whirlpool-like motion that brings fish from the bottom of the water up to the top. Occasionally, larger river otters will attack water birds such as ducks, geese, and even herons.

If an otter is not sleeping, it's moving. They are generally more active at night, dusk and dawn, but are active during the day where undisturbed by human activity. They are very playful, chasing, sliding, swimming, jumping, and wrestling. They can be highly curious animals and have been known to follow trout fisherman along the opposite bank. They also like to slide down river or snow banks into the water.

River otters often use dens built by other animals, and will sometimes kill beavers or muskrats to take over their lodges. Otters will usually mate once in late winter or early spring although the male often mate with more than one female. After two months, 2-3 pups are born and males are kicked out and do not return until the pups are half-grown.

River otters are usually between 26 to 43” long, with their tail making up 30% of their total body length. Last winter, a large river otter was easily spotted underneath the bridge going to the Grand Canyon of the Yellowstone.

Watch for otters along the waterways of Yellowstone, especially where creeks empty into larger rivers or lakes. Some places where otters have been spotted include along the Madison River between Seven Mile Bridge and Nine Mile Hole, near Mt. Haynes, the West Thumb, along the Yellowstone River near the 5-mile marker in Hayden Valley, Trout Creek, on the banks of Soda Butte Creek, and where DeLacy Creek empties into Shoshone Lake. If you miss a river otter in the wild, just stop into the Fishing Bridge Visitor’s Center where one exhibit features a family of otters.

Along with river otters, you can also enjoy watching beavers in Yellowstone. Last winter, beavers were spotted along the Yellowstone River near the Otter Creek inlet. Although not as playful (no snow bank sliding) as otters, you'll enjoy taking some time to watch them emerging from under the ice and gnawing on a few branches.

Beaver are often harder to spot than otters. They are primarily nocturnal and are concentrated in certain areas of the Park where the habitat (waterways and meadows with willow, aspen and cottonwoods.) As of March 2006, it is estimated that there are approximately 500 beavers living in 85 colonies in Yellowstone.

Beaver colonies are located on major rivers (Lamar, Gardiner, and Madison) or where water levels fluctuate in the park. One lodge can support from 6 to 13 related beavers. Beaver dams, on the other hand, are built on small, more placid streams. You can spot one old beaver dam at Beaver Lake between Norris and Mammoth.

Your best chance to spot beavers in Yellowstone is early morning or at dusk. To combine hiking and beaver spotting, take the 4.5 mile (round trip) Beaver Pond Loop Trail from the Mammoth Hot Springs area. The trail begins between Liberty Cap and the stone house, and follows a small, shaded creek up Clematis Gulch.

The trail splits after ascending 350 feet through spruce and fir trees. Take the west trail head toward Sepulcher Mountain and continue north (right). Approximately two miles from the trailhead are a series of step-like beaver-created ponds along the stream.

Other places to watch for beaver include Willow Park and Grizzly Hike (if you hike in) between Mammoth and Norris, Harlequin Lake in the Madison area, and along the Gallatin River (U.S. 191 between West Yellowstone and Big Sky).  

Beaver colonies are also concentrated in the back country hiking areas including the Bechler area (Boundary Creek, Mountain Ash, Proposition Creek and Falls River) and the Yellowstone River delta area in the southeast (from the Southeast Arm of Yellowstone Lake to Thorofare).  This year, you could also spot beaver dams and beaver along the Lamar River and Slough Creek. Watch around new willow growth where the branches are low enough for beaver to reach.