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Beaver Dams
Beavers can make impressive dams. The tallest I've seen was at least 12 feet high,
and I've seen them maintain a dam 275 feet long
But we should be careful when we use the same word to describe what people make and what animals make. We put too much of our emotions, not to mention grit and determination, into what we make, especially when the finished product is something so massive and landscape changing as a dam.
We have no way of knowing how beavers regard a dam and the art of dam building. Most people who write about beavers and dams credit the animal with a large measure of forethought, engineering skill, and an obsession for stilling flowing water. I disagree. We humans think of stream gradients, rates of flow, force vectors, and what might cause the dam to fail. By making a dam we make a grab for power. Beavers make a dam to make it easier for them to get food. Beavers can move faster in the water then they can on land so naturally they feel more comfortable in water.
By making a dam, they flood the valley behind the dam, making it easy to swim to trees they want to cut down for food,
and when they cut the trees down on the shores of the growing pond, they can drag branches and logs they cut into the pond and then more easily pull them through the water to build up their lodge or dam (double click the Google Video box below the photo to see a 10 MB video of the beaver working on the dam)
or take logs to a more comfortable place to eat the inner bark that is their principal food.
They can also eat the plants that grow in the water behind the dam.
So I think a dam is more a line of continuity than it is an interruption of a fundamental force of nature. A beaver dam is that arc of a circle
that enables a beaver pond to be the home of a colony of beavers.
I've seen beavers build a pond though for the first three months I thought it was a joke, that a few baby beavers had climbed over the old dam and were imitating the big beavers by pushing some leaves and mud together to back up a little rivulet
But even a little puddle of water takes on a logic of its own. The beavers kept building small dams a few yards farther downstream and then water would back up flooding the dams behind. The beavers would then make a hole in those dams so they could more easily swim down stream and then in a matter of six months or so, they found the logical spot to complete the circle of the largest possible pond. This crucial spot does require a bit of engineering to stop the force of the flow
but this conglomeration of logs, mud, grasses and rocks is not the concrete of the human dam builder, it is more akin to scar tissue holding in the life blood of an organism.
Yes, beaver dams do break catastrophically and the resulting flood can damage roads, but more commonly beaver dams gently sweat. The flooding water can lap over the dam in many places
and within a few days the beaver will patch the leaks with mud, grasses and branches.
Well, that sounds gentle, but they might also move in some big lumber to shore up a weak wall.
When beavers neglect a dam, it usually dies a slow death -- unless there is an otter around. In the dead of winter these fish foraging, long-tailed tubes of energy commonly dig holes deep through beaver dams
so the fish behind the dam have less water to hide in and the fish downstream swim up so the otters can eat them. It took the beavers about a month to patch that holes which they did by building a little dam just below to still and back up the escaping water and then positioning logs, branches and mud to make the dam whole again.
I think it took the beavers that long to patch the dam because of the cold weather making it harder to work with mud. Humans busted another dam I watch in the late spring
and the beavers had it patched in two days.
The photos, I hope, are detailed enough to give you some idea of how the beavers go about dam building. In essence they back the mud with logs and compress the mud with rocks. The arrangement of logs strikes us as quite intricate,
but from my observations these logs are tossed over and brought onto the dam quite haphazardly. In the interior of beaver lodge I have seen some lattice work with sticks woven together, but never on dams. The strength comes from the weight and random entanglements.
Beavers usually operate on both sides of a dam. I've frequently seen them drag freshly cut saplings and branches over the dam, and they always take them into the pond to be stored for winter food or stripped of their bark and leaves right away. Eventually the remaining log might be brought to the dam and dragged up and pushed over. I think the beavers prefer waiting for a log to get rather waterlogged. I've seen them dive and bring a log up from the bottom. They want weight to brace the dam.
While I don't want to diminish the poetry of an animal using what it doesn't eat, the logs, to build dams and lodges, the beavers use of mud truly marks the rhythm and rhyme of its life.
because what it dredges up and carries and pushes with its arms and body creates the channels in the pond that will allow the beavers to survive when their seemingly shallow pond freezes in the winter. The pond ice will often break behind the dam sometimes creating a gaping hole, as if the pond were gasping for air. And sometimes a beaver lurks in the exposed pool of water with an eye out for spring.
That colony of beavers, six of them, I think, a typical size for a colony, made their lodge right in the dam that winter. Far from having an innate urge to stop flowing water, as some suggest, in the winter especially beavers manipulate the dam so water flows through to keep ice open and create space for living inside the dam and for eating in the pool of open water forming behind and in front of the leak in the dam
Recently, vandals made a gap in one of the dams I watch.
The beavers immediately began to patch it by pushing logs down from the pond into the gap. When I took this photo the dam still leaked, and a week later was still leaking. We've had a great deal of rain and snow lately so, I think, the beavers simply decided to keep the leak. Better to have one gap that can be monitored rather than high water lapping over the dam and making leaks all over. The flood from the dam also made a gap in a smaller dam just below. Here the beavers made a better patch.
But I can see how a little leaking served them well. It kept the water behind that part of the dam from freezing and made a comfortable place for the beavers to sit and nibble sticks, and the stripped sticks could be filed into the mass of sticks holding back the water of the pond. The ponds and dams are always changing. I'll keep you posted. In the spring of 2009 I watched in amazement as the dam a beaver family built failed twice. I wrote about it in one of my blog postings. Death and Destruction in Shangri-la Pond and soon I will update that because in the winter of 2009-2010, those same beavers dug a hole in their new dam and then repaired it in an ingenious way.
by Bob Arnebeck, November 2005, revised March 2010
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