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North Words - The American beaver performs amazing engineering feats

Last fall once again in the backcountry of a state wildlife management area north of Karlstad, Minn., I walked upon a single beaver dam for nearly its full quarter-mile length. I began my hike as I have in the past within the forested uplands where I first discovered the interesting beginnings of a very small beaver dam over ten years ago.

And, as it has always been, much of the area remained void of any sizable aspen trees. The location where the beaver dam begins has all the look and feel of a timber harvest site -- as if human loggers had just harvested all the merchantable timber. As I walked on top of the dam, it began increasing in height the further I walked until the dam itself was five-plus feet high.

The dam's size and length never ceases to astonish me -- the mud, sticks, and other debris, and even the age of the dam. Obviously the dam is a structure that has taken decades and decades to build, because, all along its sinewy length, through the great cattail and shrub-choked marsh, the dam also supports the growth of many fair-sized trees and shrubs. Grasses and other herbaceous plants cover the dry side of the dam from bottom to top.

Indeed, many, many generations of beavers, perhaps all of which were descendents of the original pair of beavers that arrived in the marsh long, long ago, all working to maintain and expand the dam to contain the pond and make the pond larger, were responsible for the dam's never ending construction. The dam, like the animal, is a true marvel of nature.

American beavers are the largest, certainly the most industrious, rodents in North America. Adults average 35 pounds with some individuals reaching as much as 65 pounds. Specimens of over 90 pounds have also been recorded. Beavers can reach four to five feet in length from nose to the tip of their tail.

Long-lived for rodents, beavers can reach twenty-plus years of age. They tend to form pair bonds for life. Kits (newborn beavers) remain with their parents for two years until they disperse to locate their own mates and territories. One litter per year of two to four kits is born to a mated pair.

So many marvelous adaptations help to make the beaver perfectly suited to an aquatic lifestyle: powerful hind legs with flipper-like webbed feet propel the beaver through the water; nostrils that close to keep water out; clawed toes to help assist digging and gripping sticks; dense, oily fur that repels water and keeps them warm in frigid conditions; massive chisel-like incisors to cut through wood; and an unusual tail that's used as a rudder for swimming, as support when standing, and as a warning signal when alarmed. A loud slap on the surface of the water warns other beavers in the pond that danger lurks nearby.

Aside from the impressive dams that beavers construct to retain water that they require in order to survive, the lodges, too, are unique and elaborate in design. The inside chamber may be as wide as eight feet, as high as three feet, and is lined with soft vegetative materials like grass and woodchips. And depending also in the structure's design, the entrances may differ as well. Some are straight up and through the floor of the lodge, whereas other lodges are entered through gradually sloped entrance holes. Still, too, other lodges are "bank dens", in that the lodge is built along the embankment of a river or lake and is entered from beaver "runs" underneath the lodge from the water.

Beaver are especially active in the fall as they prepare for winter. Branches from felled trees and shrubs are carried near to their lodge where they are stored underwater. Called "food rafts", beavers cache these sub-surface food items for consuming throughout the cold winter months. When a meal is needed the beaver simply exits the lodge, swims below the ice to the food raft, and carries back with them something to eat inside the warm and cozy lodge. Once there, the animal holds the branch in its "hands" like you and I would a cob-of-corn and nibbles off the nutritious bark and cambium layers.

Though sometimes viewed as pests because of the problems they can create for people from their activities (flooding roads and fields, felling desirable trees), beaver ponds provide many benefits for other species of wildlife. Flooded trees often die and subsequently become excellent nesting trees for wood ducks and other cavity using wildlife.

River otters, mink, turtles, frogs and toads and salamanders, aquatic invertebrates, fishes, and many species of birds such as belted kingfishers, great blue herons, and scores of songbirds, shorebirds, and waterfowl use beaver ponds and the associated wetland habitat, too. But over time, because of sedimentation, diminishing timber, or hydrologic and environmental changes, most beavers eventually abandon their pond. These old beaver ponds often become lush meadows afterwards.

To illustrate how environmental conditions can affect wildlife such as beavers, I recently had the pleasure of talking with Minnesota Public Radio's Tom Robertson about the subject. He and I visited a wetland northeast of Detroit Lakes that only a couple of years ago was brimming with water and was alive with beavers and other wildlife. In the case of this particular three-acre wetland, we were able to walk across the frozen mud-bottom of the pond directly to a recently abandoned beaver lodge. But one day the water will return -- and soon after, a pair of beavers, too.

How and why this amazing animal does what it does is one of the many mysteries in the animal kingdom. With all its astounding engineering feats and beneficial contributions to the landscape and other species of wildlife, the American beaver provides us with a multitude of things to appreciate as we get out and enjoy the great outdoors.

(Klemek is the DNR Area Wildlife Supervisor in Detroit Lakes. You can contact him at