Nature’s Hydrologists
From chapter 2, “Nature’s Hydrologists” in Water: a natural history by Alice Outwater.
Copyright 1996 by Alice Outwater.
The beaver is utterly familiar. Forty inches long and over a foot upright, a beaver seems like a little person with a fondness for engineering. Good-natured, gentle, and clean, it makes a friendly pet that follows its owner around much like a dog, scrambling up onto a lap to be rubbed on the belly whenever it’s invited. Beavers were commonly kept as pets around Indian encampments, but they do have a fatal flaw in a modern household: they never stop building. When kept indoors, they will cut down the legs of tables and chairs and build little dams between pieces of furniture. Left on their own, they will rearrange waterways.
Beavers do more to shape their landscape than any other mammal except for human beings , and their ancestors were building dams ten million years ago. These Miocene beavers were 7 feet long, felling trees ages before the mammoths roamed. Their underground spiral burrows can be found from western Europe to central Asia and North America; after their extinction, some of these burrows filled with debris that fossilized. Creating twisted masses of stone that geologists call devil’s corkscrews. Legends of these prehistoric giants were once widespread. The Indians of Nova Scotia claimed to know of an ancient beaver dam so cast that it flooded the Annapolis valley; farther west, tales circulated of tribal ancestors using immense beaver teeth to hollow out their canoes.
In tribes across North America, legend had it that the beaver helped the Great Spirit build the land, make the seas, and fill both well with animals and people: Long, long ago when the Great Waters surged in a blind and shoreless world, the gigantic beaver swam and dove and spoke with the Great Spirit. The two of them brought up all the mud they could carry, digging out the caves and canyons and shaping the mud into hills and dales, making mountains where cataracts plunged and sang. Some tribes believed that thunder was caused by the great beaver slapping his tail.
Until European colonization of the New World, Castor canadensis, the North American beaver, was one of the most successful animals on the continent, living almost everywhere there was water, from the Arctic tundra to the deserts of northern Mexico. It was scarce only in the swamps of Florida and Louisiana, where its dams and lodges were no match for voracious alligators. Everywhere else, along thousands of streams, lived colony after colony, as many as three hundred dams per square mile, each with its own ring of wetlands.
It is estimated that as many as two hundred million beavers once lived in the continental United States, their dams making meadows out of forests, their wetlands slowly capturing silt. The result of the beaver’s engineering was a remarkably uniform buildup of organic material in valleys, a checkerboard of meadows through the woodlands, and a great deal of edge, that fruitful zone where natural communities meet. Beavers are a keystone species, for where beavers build dams the wetlands spread behind them, providing home and food for dozens of species, from migrating ducks to moose, from fish to frogs to great blue herons.
A total vegetarian, the beaver eats roots, tubers, and the inner bark of trees. As a consequence, its own meat is sweet and tender, and to avoid becoming everyone’s favorite prey it took to the water long ago and is well adapted to aquatic life. Its dense coat conserves heat, and its multipurpose scaly tail functions as a rudder, as a place to store fat for lean times, as an internal temperature regulator, and as an early warning system to other beavers thanks to the noise it makes when slapped against the water.
Served at medieval banquets as “bear’s paws,” the tail of the beaver covers an even more singular feature. Beavers have neither external testicles nor penis, hence their name – castor, from the Latin castratum. (Some Victorian references claim that castor is derived from gaster – belly, in Greek – but that was a shyer era.) The beaver’s sexual organs are modestly tucked up inside its body, while a pare of glands in the anal area of both sexes secrete castoreum, the musky oil the beaver uses to grease its coat and mark scent mounds to delineate its territory. Castoreum was a popular medicine in the Middle Ages, said to cure ailments ranging from headaches to impotence; it is high in salicylic acid – the basic ingredient of aspirin – which the beaver ingests by dining on willow bark. Long used as a base for perfume, its scent is described as a pungent, waxy, burnt-orange odor, with smoky notes of Irish peat fires and good pipe tobacco and undertones of cardamom and tea.
Castor’s teeth never stop growing: the pair on the upper and lower jaws form curved blades that chisel through wood as hard as rock maple and are perfectly designed for felling trees. Beavers are the largest rodent in North America: at close to sixty pounds, the adult females slightly outweigh the males. The beaver’s rotund belly is filled with an enormous gut packed with vegetative matter and the bacteria that convert vegetation into calories. To extract the most calories from its high-fiber diet, the beaver eats everything twice when food supplies are low, a practice called coecotrophy. Ruminants manage this by burping up their ingested food and chewing the cud., but the beaver actually passes food through its entire digestive tract twice, eating the gelatinous, porridge-like substance that comes out of its anus the first time through. Double-digested beaver stool looks almost like pure sawdust.
When they are about three years old, beaver kits leave home to find a companion, with whom they mate for life. During this quest for new territory, they are at their most vulnerable to predators (and today to cars), but they are remarkably safe once they’ve built their living quarters. When the new couple finds a suitable stream, they mark the area with scent mounds and dig out a den in the stream bank. Beavers are burrowers, and they have powerful, curved claws on all four feet. They’re also equipped with a number of features that aid underwater construction: valves close off their nose and ears; thin membranes over their eyes act as goggles; and skin flaps behind their front teeth allow them to tow tree branches in their teeth without swallowing half the pond. With these adaptations, beavers are able to dig their burrow’s entrance well below the surface of the water. Slanting the tunnel upward to above the high-water line, they clear out a room three feet wide and line it well with shredded wood and grass. The underwater entrance keeps them safe from lynx and wolverines, and to ensure that the water stays high enough to hide their burrow’s mouth during the low summer flows, they build a dam.
Choosing a dam site where the stream is not too deep and the bottom muck is firm, they fell saplings first and then larger trees. Working by night – sometimes on separate trees, sometimes together on a single trunk – they sit with their paws around the tree, their tails either folded beneath them like a seat or extending behind as a prop. Tilting their heads from side to side, they make deep bites in the tree, driving their long yellow teeth into the wood to wedge, pry, or pull out a chip, chiseling the trunk until the tree topples. After cutting the tree into manageable lengths, they push and pull the logs into position on the dam, pointing the butt ends upstream, and hold them fast with piled mud and stones. As the dam grows higher, the water slows, and the beavers weave in more branches and pat on more mortar until a substantial barrier is completed.
Dams must be continuously maintained, and beavers do so every night, replacing shifted sticks and poles and patting on more mud. They build dams throughout their territory; some for water control; some, it seems, just for fun. A family of beavers can build a 35-foot long dam in a week.
Where the streams have clearly cut banks and a channel with a uniform current, beavers build a solid bank dam with the poles underneath and earth on top; water discharges through an opening in the dam’s crest. If the stream is wide, they bow the dam into the flow of the water, increasing the structure’s stability. When the young trees nearby are all consumed and the edge of the forest is too far away for the beavers to drag their branches easily, they dig canals about 2 feet wide and 1 foot deep in which to float the branches back to home base, sometimes extending these canals for hundreds of feet to reach new trees. If a riverbank is steep, they build slides down to the water. Dams more than 4,000 feet long have been found, built by generations of beavers, and nineteenth-century reports describe dams encrusted with lime and half petrified, attesting to hundreds of years of continuous repair.
The beaver is a clever engineer, but its brain is embarrassingly small – smooth and unconvoluted except for the well-developed olfactory lobe. The beaver’s ratio of brain size to body weight is the lowest found among mammals: like that of the primitive marsupials, the beaver’s brain is about a third the size of the average mammal’s; a beaver-size human would have a brain fifteen times as large as a beaver’s. Beavers don’t have much gray matter, and they don’t see well. Nevertheless, there is abundant evidence – noted in Enos Mill’s 1913 study In Beaver World and confirmed by the contemporary naturalist Hope Ryden, who studied a colony of beavers in New York State – that much of their building technique appears to be learned during their long childhood. Oddly enough, although the European beaver (Castor fiber) is nearly identical in appearance to the American beaver, it has no interest in dam construction; ion most regions, European beavers confine their efforts to digging burrows in the stream bank. It seems likely that the fine points of dam construction were lost to Castor fiber during the centuries when it only survived in parks.
Virtually every Castor canadensis builds dams, however, and behind each dam the water slowly backs up and covers the land. A rush of insects, animals, and plants transforms that thin sheet of water into a place where every level, every nook and cranny, is teeming with life. Ecologically, wetlands are an example of an ecotone – a transition between two diverse communities. Uniquely, an ecotone contains organisms native to each overlapping community as well as organisms characteristic solely of the ecotone itself. The so-called edge effect – the increased variety and density at community junctions – is what makes wetlands so productive of life, and the beaver’s role in this system is to build the dams that make wetlands, increasing the edge between waterways and dry land.
[Lyrical discussion of the biologic wonder of wetlands elided.]
The summer after they build their first dam (having wintered in their streamside burrow while the pond grew), a beaver pair constructs a lodge in the lily pads near the shore. They build a platform of interlaced branches mortared with mud and dried leaves, and when the platform is a few inches above the water, they construct a domed roof of mud and branches above it. Some lodges have one room, some more than one.
The entrances to the lodges are narrow and steep, and all lodges have at least two (some have up to five). The most modest homes have one opening for entry and exit and another for food transport. The entire lodge, which may enclose a room more than 5 feet high, is plastered with mud before winter.
The kits are born in April, in litters from one to six. Their eyes open into the dark as soon as they are born, and within two weeks they begin to swim, accompanied by their mother. Beavers have a happy childhood. Posted around the edge of the lodge, they nose and push each other about, tumbling into the water. They race, wrestle, and dive in the pond, slapping tails with abandon.
After a leisurely summer, the adults and the two-year-olds spend fall harvesting food for the winter, while the younger kits get in the way. Gnawing together, the beaver fell trees, slice them up, and drag them over their shoulders and under their arms to the pond, where the branches are cached in piles. Beavers store tons of wood in their pond, thinning the forest and removing saplings that were unlikely to survive in the over-story shade. Most animals grow thin in the winter, but beavers fare well and even grow fat when it’s cold, feasting on inner bark and the roots and tubers of water plants, scrupulously digesting every bite twice and living amiably in their close quarters.
Since the kits stay at home through two winters, an established beaver colony includes the parents, this year’s adolescents and this year’s newborn kits – six to twelve beavers living together, cutting down trees, digging canals, and building dams up and down their valley. A beaver lives a dozen years or so, and works with its fellows for most of its life to build more and more dams, ponds , and lodges, until an entire valley becomes a mosaic of beaver handiwork. The original dam matures, and the old streambed runs like a ribbon through the marsh’s center, almost filled with shifting silt and the billowing growth of bright marsh marigolds and water hyacinths. The trees have long since drowned, and old stumps break the water’s surface while willow sprouts grow thick along the wetland’s borders. Over time, meadows emerge from the wetlands and thinned woodlands.
The beaver’s dam is a telltale flag, however, and hunting beaver requires little more than persistence, which the Indians had plenty of. Trapping took place in winter, when beaver fur was prime. Since Indians did most of the trapping and traders wrote the journals, the stories may not be accurate in all respects, but according to these early accounts the Indians would block the stream above the dam with stakes, so that the beavers could not escape upstream, and tap the ice along the edge of the pond with chisels to sound out where the entrances to the burrows lay. At every entrance along the bank, they would make an opening in the ice, and then the lodge would be torn open. Those beavers that ran out of the lodge were clubbed, and those that ran into the mouth of a burrow in the bank were trapped there; the burrow was then broken into, or the beavers were pulled out with grappling hooks.
Beavers had also long been hunted with bow and arrow, and trapped by deadfalls and snares. The snare was made from rope, with a loop large enough to encircle an animal’s head or leg. Sometimes a snare would be attached to a young tree, which would be bent over and held by a triggering device: when a beaver put its head or leg through the lop, the prop was dislodged and the tree sprang up, hauling the animal with it. The deadfall consisted of some large, heavy object – a boulder or a log – delicately positioned above the bait. The bait and trigger were smeared with castoreum, which beavers invariably investigate, and when a beaver went for the bait the deadfall came crashing down, pinning the beaver beneath it.
The first reference to a modern trap is in Leonard Mascall’s 1590 British classic, A Book of Fishing with Hook and Line . Another of Sundrie Engines and Trappes to take Polecats, Buzzards, Rates. Widely deployed in Europe in the 1600s, the trap now known as a steel trap was pictured, and described as “a griping trappe made all of yrne, the lowest barre, and the ring or hoope with two clickets.” The first steel traps used in North America were based on the Mascall trap, and had a round or oval baseplate. By the 1800s, the design had changed to a flat baseplate, with jaw pillars mounted at either end. The steel trap made simple work of harvesting beavers: instead of staking the stream and destroying the lodge, the trappers could drown the animals one by one. The traps cost $12 to $16 in the early 1800s, weighed 5 pounds, and were secured by a 5-foot chain with a swivel to prevent kinking. The trapper would wade up the stream to cover his tracks, and set the trap near the bank under 3 or 4 inches of water. To secure the trap, the chain was stretched to its full length and anchored to the streambed with a strong stake. A castoreum-coated twig was fixed above the trap, waving a few inches above the surface of the water. Any beaver that happened along would swim over to sniff the castoreum, place its foot on the trigger, and spring the trap’s semi-circular jaws. Diving down to conceal itself underwater, the beaver would find its movement restricted by the chain. If the trap caught only its paw, the beaver could gnaw the paw off and would try to gnaw through the chain – most often, it would be unsuccessful and drown. Even if a beaver succeeded in wrestling the stake out of the streambed, the combined weight of the trap, chain, and stake would eventually exhaust and drown the animal.
The beavers disappeared trap by trap, and hat by hat. But across the country, they disappeared by the tens of millions. When the beavers were removed, their old dams slowly collapsed, and the streams were released from the series of ponds and impoundments that had been built throughout the watershed. Each watershed lost wetlands, and the water that had once seeped quietly down to the aquifer now flowed to the sea, and flowed much more rapidly. Some of the spring and freshets that had bubbled throughout each watershed began to dwindle, while other disappeared entirely; in the undammed land, the water table soon dropped. Wetlands disappeared by the acre as the frontier rolled West.
Not only was there less water in the land but the water quality changed for the worse. In a land full of beaver, the stillness of ponds and wetland had allowed sediment to settle, clearing the water and providing a large reserve of nutrients that stabilized the ecosystem. Over time, this collected sediment had formed rich bottomland valleys, building layers of topsoil. With the dams and wetland to slow the flow and allow the sediment to settle, the rivers became laden with silt.
Without wetlands, the runoff from the high flow of storms and snowmelt was unimpeded, and storm or spring flooding could be two and three times higher than it was before. The swift-flowing water swept more soil into the stream – soil that was more like to stay suspended – and muddy water blocked the sunlight from the algae. Without still warm ponds, the plankton were no longer as populous, and fewer minnows and insects found food. Without these tasty tidbits, the birds and animals that used to feast in the wetlands went hungry.
With fewer beaver ponds, there were fewer places for black ducks, ring-necked ducks, goldeneyes, and hooded mergansers to drop down to breed. Without the dams that maintained constant pond levels, muskrats and otters were either flooded or frozen out. Mink and raccoons, fond of eating the frogs, snakes, and suckers near ponds, found less food when the beavers were gone. The rabbits that had once nibbled safely among the brush and hidden among the felled logs were no longer so plentiful, and the red foxes found fewer to stalk. Moose and deer, which had browsed on the plants and waded in the cool water of the beaver colonies, lost their habitats. The beaver’s wetlands had been home to rich diversity of creatures of the air, land, and water, and without the beavers the fertility of vast areas was subtly reduced.
Today, the beaver has returned in part, but its numbers are nothing like what they once were, and we have forgotten that beaver wetlands once enlivened the now arid rangelands of the West. The total land area of the contiguous United States is 2.96 million square miles. Since the arrival of the Europeans, the beaver population of the United States has dropped from perhaps two hundred million to ten million. This decline in beaver population, and in beaver dams, caused the first major shift in the country’s water cycle. If each of those pre-Columbian beaver had built only a single acre of wetlands, then an area of more than 300,000 square miles – a tenth of total land area of the country – was once a beaver-built wetland. Now these wetlands are gone. The river of life receded when the water receded, and the primeval splendor of the land disappeared with the beaver’s demise.
