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Beaver Dam Construction
Charles L. Dobbins

(This article was published in the June/July 1996 issue of The Trapper and Predator Caller magazine)


    I have seen temporary dams built of green corn stalks, soybean plants,and gravel. I have even encountered dams built within city limits that contained wooden lawn furniture, hobbyhorses and parts of picket fences as part of the construction materials. My observation of hundreds of permanent beaver dams tells me that most are made up of mud with some woody material holding them together.
    Beavers prefer to build their permanent dams on a clay-type soil. Clay can be packed with the woody material, leaves and grasses to hold a dam together. Gravely or sandy soils can't be packed to hold water.
    Beavers have good reason for building their permanent dams on a clay bottom. When the beaver uses sticks, poles, limbs or other woody material, the heavy end of this wood is placed on the dam and the smaller ends are on the lower side of the dam. On the heavy end there is usually some heavy mud packed on it to help hold it in place. When the water is high and there is force against the upper side of the dam, the smaller ends of the poles and limbs dig into the bottom on the lower side of the dam and help hold the dam in place:  If the bottom is solid stone or shale, the water pressure behind the dam during periods of high water will push the dam off its foundation.
    If the bottom under the dam is gravely or mostly sand, the turbulent high water will erode the foundation from under the dam construction. By selecting a clay bottom, beavers can be sure of a strong dam.
    It is on the upper side or deep water side of the dam where the animal digs the clay for the dam construction or repair. This removal of mud and clay also deepens the water close to the dam.
    Beavers seem to have a preference for waterlogged wood gathered from the bottom of streams or other bodies of water for dam construction. This material doesn't float because of its waterlogged condition and the heavy clay from the bottom used by the beaver helps hold this wood in position on the dam.

Heavy material
   Notice that both of these materials are heavy. The waterlogged wood is heavier than ordinary wood and the clay is heavier than ordinary soil. The heaviness of these materials helps to keep the dam in place during periods of high water. Of course any floating sticks, fallen branches or other wood will be used in dam construction or repair.
   Another thing I have observed about beaver, and don't fully understand, is that all material used in dam construction or repair is brought downstream. I have torn holes in their dams, and deposited the material I removed from the dam only a few feet away on the downstream side. The beavers repaired the dam, but they did not use any of the material that was only a few feet away.  They sometimes travel quite a distance to gather repair material upstream from the ruptured dam. Why they don’t use the material that was removed from the dam and is only a few feet away remains a mystery to me. It seems the longer I am in this trapping business, the more questions arise that I have no answers to.  

Oddball dams
    I have seen some oddball beaver dams during my trapper career. At one place there were about a half dozen apple trees, loaded with fruit, beside a small tributary. They were about a 1/4 mile up a small tributary on a sizeable river. In September some beavers built a gravel dam in the small stream.  The apple trees were in open pasture land with no woody material available to use in dam building. The beaver piled the gravel across the narrow stream, but the dam would not hold water. It flowed right through the piled gravel. The beaver traveled over a mile upstream to where they cut tall weeds of several varieties and used them to weave against the breast work of the dam to hold the water. It worked.
    The water at its deepest was only a couple of feet. At the root system of a tree they dug their den, its entrance barely underwater. The beaver stayed there until the apples were gone, then went back to the river.
    There was another beaver dam that was appropriately named "The Shoe Dam." A second-hand store had gone out of business, and they disposed of their stock of used shoes in a nearby stream. About a mile downstream from the dump site the beavers had just started to build a dam. Many of the shoes floated to the dam and the beavers used them as building material. This happened several years ago, and the 'Shoe Dam" is still in place.
    I have also learned that beavers will move and bury an animal carcass if it is close to a dam. It was in the early 1960s when I first observed this. I was using a beaver dam to cross a deep stream, and had been using it for several days. Deer season was in progress, and I noticed that hunters were also using the dam.
    The day after deer season, I saw a deer carcass floating about 20 yards upstream from the dam in deep water. It was a small deer, and I assumed it had been wounded and took to the water to escape the dogs. (Dogs were legal to use for deer hunting.) The deer carcass was there for a couple of days, then it was missing. I couldn't imagine what had happened to it. It would have been almost impossible for a human to retrieve the carcass because of the deep water and floating debris. The only sign on top of the dam were my own boot tracks from the previous few days.
    I hadn’t gone more than 25 steps when I came to a fresh pile of mud and decaying vegetation on top of the narrow dam. It was easy to see that it had been made by the beavers. I had never before seen where beavers had made such a large pile of debris on top of their dam. I studied it for a few seconds, then stepped on it to continue across the dam. When I stepped on the pile of debris, some of it slid off into the water, and left part of a deer carcass exposed.    With my shovel I scraped most of the mud and debris from the carcass. Beavers had moved the carcass from deep water to the top of the dam and covered it. The next day the carcass was covered over again.
    The following day the sign showed that several dogs had found the carcass and pulled it to higher ground and devoured it. All that was left was some hair scattered around and tracks left by several different size dogs.
    At another place and time there was an established beaver colony in a manmade ditch. They had a dam about 100 yards downstream from where a county road crossed the ditch. I had mink, coon and muskrat traps set, and used the beaver dam to cross the deep water below the bridge when checking traps. 
    One day I noticed a dead collie that someone had tossed off the bridge into the water. I said a few words under my breath regarding anyone who would defile a clean waterway. The dead dog had floated close to the bank below the bridge, and stayed there for a few days. One day the collie was missing. I thought it strange that a dead dog would disappear. When I crossed the beaver dam, there was the dead dog under a pile of debris. The beavers had buried the dog.
    I have several unanswered questions about carcasses that beaver bury on their dams. Why couldn't the beaver had have placed the carcasses on shore anywhere and covered them? What do beaver do with carcasses in streams or natural lakes with no dams? It seems the more I learn about any wild animals the more questions arise that I don’t have the answers to.


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