SI Vault
Bil Gilbert
June 27, 1966
Those cute little furry animals with the sharp teeth are industrious workers, so if it is your woodland retreat they are working on, brace yourself. When you wake up tomorrow morning a trout stream may be flowing through your cellar
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June 27, 1966

Beavers Dam And Be Dammed

Those cute little furry animals with the sharp teeth are industrious workers, so if it is your woodland retreat they are working on, brace yourself. When you wake up tomorrow morning a trout stream may be flowing through your cellar

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For more years than they would perhaps like to count, if they could count, beavers were the Patsies of the World. As the object of a continent-wide skin game, they had more to do with the founding of Montreal, the Mackinac Bridge and the Astor fortune than the faithful hound, the dumb ox and spirited horse put together. Beavers are so much part of the very early history of the United States and Canada that a good many people assume these big rodents are now pretty much confined to zoos, the novels of James Fenimore Cooper and the films of Walt Disney. The truth is quite the opposite. Some time ago (SI, Feb. 9, 1959) we noted that all over North America, even in supposedly tame and urbanized areas, the beaver was making a comeback. We can now report that comeback is far too mild a word.

The insurgency of the beavers strongly suggests the tactics of the Viet Cong. Beavers are constantly turning up in little enclaves supposedly well behind the front lines of civilization and then running everyone else out. The native population often invites in or at least welcomes the first penetration by beavers, believing that the creatures will be an adornment to the countryside. Later, innocent nature lovers discover that, given a figurative inch of babbling brook, beavers will make a literal half mile of muddy bog. Once they have got a big webbed foot in the door, beavers are harder to dislodge than commissars.

The experiences of a Baltimorean, whom I shall call Burt (to protect him in case the Senate Internal Security Subcommittee should begin investigating beaver sympathizers), are typical of what happens to humans who start fellow-traveling with beavers. In the early 1950s Burt bought some land in West Virginia for a vacation retreat. It was a pretty place in a shallow, heavily timbered valley. For Burt, who is an ardent fly-fisherman, the prize feature of the property was a fast, cold trout stream which rose from springs above his place and flowed into a very large, tangled swamp below. Burt's first improvement was to dig two small trout-rearing ponds below his cabin. He stocked the ponds with rainbows and browns and enjoyed the whole project immensely. His next move was less successful and in fact marked the beginning of the end of his Shangri-La. Being an all-round nature lover, Burt was infected with the idea that it would be a lovely, woodsy thing if beavers could be established in the swamp below his cabin. After some discussion the game commission agreed to the plan, and three pairs of beavers were dropped, by parachute, into the swamp. To say that the beavers established themselves is again to understate the facts. In the past 14 years these six beavers and their offspring have simply taken over the joint.

To appreciate what beavers can do and have done to people like Burt, a little understanding of beaver psychology and physiology is necessary. Except perhaps for the Army Corps of Engineers (creatures that they closely resemble in spirit, if not appearance), no other animals have such ability and compulsion to rearrange the landscape. Beavers have strong notions as to what constitutes a good environment, and they are not the timid compromising sort who, if they don't find what they want in one place, will look elsewhere. Beavers will start with the most unpromising terrain and soon whip it into a shape that suits them.

The first thing a pair of beavers wants is a deep water hole across which they can float logs, under which they can store food and which will give them protection from land-based enemies. If deep water is naturally available, in a lake or big river, beavers make use of what is there. They dig their dens back under the bank and go about their business without having to spend much time on intricate engineering projects. However, if there is no body of water of sufficient size they make one by damming up the nearest stream. In the middle of their artificial pond, dam beavers will build a lodge, a dry platform roofed over with a conical structure of sticks and mud. The lodge has an underwater entrance. Around the lodge in the deep water beavers store cut lengths of unpeeled logs, the butt ends anchored in the mud. During the winter, beavers mostly laze around the lodge. When they are hungry they swim out, grab a log, carry it back inside and sit around nibbling off the nutritional bark. When a log is cleaned they shove it outside as a man would throw an empty beer can over his shoulder. This is a comfortable and sensible way to behave in bad weather.

Beavers also insist on having a system of waterways for bringing their construction materials and groceries back to the main pond. Obviously, it is more efficient to float a fallen tree than it is to drag it through the underbrush. Also, from a beaver's standpoint, it is a lot safer. Though an excellent swimmer, he is a clumsy waddler on land, and so he does not like to work far from water. Each season, as they go farther and farther from the lodge in search of food, beavers extend their waterways. They excavate canals back into the woods. They throw up small dams above the main one, creating a system of secondary ponds. Most spectacularly, they continue to increase the length and height of their principal dam, thus enlarging the perimeter of the pond and their effective working area. A well-established colony may have a dam six feet high and a quarter of a mile long, creating a pond of up to 75 acres. Only when they have absolutely exhausted the food supplies in an area (an active colony cuts between one and two thousand trees a year) will beavers, of their own volition, move elsewhere.

Not only does a pair of beavers constantly extend its watery holdings, but it also regularly produces two to six beavers each spring. In time each of these kits will want a little pond of its own. Young beavers hang around the parent colony for a year or two, helping out as apprentice lumberjacks and construction workers. Eventually the youngsters wander away or are driven off by the patriarchs. Sooner or later a dislodged youngster will meet up with a beaver of the opposite sex, and the whole cycle—stream, dam, pond, lodge, mud flat—will start again.

The beavers that were parachuted into Burt's swamp had all the instincts and ambitions of their clan. They apparently decided that the place had possibilities (there was a lot of alder, a favorite beaver food) but would need considerable improvement. The first season one pair came out of the swamp and threw up a small dam across the trout stream below Burt's cabin. Burt was overjoyed, figuring this was just the touch his retreat needed, a real true beaver pond. But by the next season his enthusiasm for the wilderness bit was tempered by growing alarm. Surely and not so slowly the beavers began to drown the trout stream. In five seasons the original pair and their progeny built a 300-yard main dam and a series of six smaller impoundments that reached all the way back to the source of the short stream. The brook was converted into a brown slough, a vast improvement if you happen to be a beaver. Because it had been heavily shaded, the spring-fed stream had remained cold throughout its length. The beavers felled many of the shade trees, and the water from their ponds killed most of the remainder. Rushes and water-tolerant scrub replaced the trees. The sun beating down on the water raised its temperature above the point that trout can tolerate.

While Burt's trout stream was sacrificed, he still had his trout ponds—but not for long. Arriving one summer, Burt found that the beavers, with their insatiable craving for Lebensraum, had incorporated his ponds into theirs by raising the height of their dam a foot or so. Where the dikes around the pond had stood in their way, the beavers had flattened them. The trout had, of course, dispersed into the bog. Just to add insult to injury the beavers had rummaged through Burt's firewood pile and nipped off the emerging shoots of a row of red raspberry canes he had planted the year before.

Burt's cabin, situated on a knoll, is presumably safe for the time being, but the watery waste that now surrounds the building is the kind of place only a beaver could love. Since he no longer can spend his days trout fishing, Burt spends his time politicking. He is agitating for the removal of the beavers, but his campaign is not as easy as was the one to get the beavers into the swamp. A biological survey shows that there are now approximately 100 beavers, working a dozen locations in the vicinity. Each year there are more beavers—and consequently more swamp. It is quite certain that no one is going to parachute these beavers out of the place. They are dug in good. The jungled morass they have created protects them from virtually everything but tactical nuclear weapons.

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