The Forest Preserves' Best Dam Builders

At 4 feet long and 40 to 60 pounds, the North American beaver (Castor canadensis) is the continent's largest rodent. It has a stocky body with glossy brown fur, short legs with webbed hind feet and a paddle-shaped tail.

Beavers are aquatic mammals and can spend up to 15 minutes underwater. Valves in their ears and noses close to keep out water, and clear membranes cover and protect their eyes. The animals can seal their lips behind their incisors, too, so they can forage underwater.

As herbivores, they eat the roots of cattails, sedges and other wetland or aquatic plants and the tender twigs and bark of trees such as cottonwood, willow, aspen, birch and poplar. When a beaver cuts down a tree, it gnaws it into portable pieces, eats what it can and moves the branches to a nearby waterway. (Its two lower and two ever-growing upper incisors make smooth, distinctive cuts in wood.)

Beavers mate for life and have one litter of three to four kits each spring. Beaver colonies are strictly family units that consist of the parents, this year’s kits and last year’s offspring. Populations spread when young leave their colonies to seek their own homes along waterways, often recolonizing vacant habitat.

Most beavers in this area live in burrows they dig in riverbanks or along lakeshores, but some use branches and mud to build lodges, dome-shaped shelters that can rise 5 feet above the waterline. Burrows and lodges are warm and dry inside and accessible through underwater passages. Beavers create winter food supplies by anchoring large groups of branches and twigs into muddy pond bottoms around their lodges.

Beavers also construct dams out of branches stabilized with rocks and mud. Dams are important because a dammed stream becomes a pond that provides beavers with drinking water, travel routes, easy access to underwater food supplies and protection from predators.

The ponds also create irreplaceable habitat for other wild animals. Because of the significant, positive effect beavers have on other animals, ecologists consider them a “keystone species,” one that plays a critical role in maintaining the structure of an ecosystem and influencing the variety and abundance of animals that live in that system.