Beaver Dam Control

Learn why beavers build dams and how to outsmart them using water control structures.

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by Adobestock/Ronnie Howard

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Beavers have been integrated into our lives for centuries. Sayings such as “busy as a beaver,” “eager beaver,” and “leave it to beaver” (OK, I might be stretching here) all come from the apparent industriousness of beavers. The Greek word kastor, which means “one who excels,” is probably the root for the beaver’s genus Castor. The provinces of Saskatchewan and Alberta use the beaver in their official seals. Oregon is the Beaver State. Beaver lore abounds in Native American cultures. Even Aesop had a story about how a beaver saved itself, although the story is not for gentle ears.

The beaver has played many roles on the North American continent. Its meat, fur, and castoreum were used by Native Americans for thousands of years before the beaver became one of the principal financial sources that funded the exploration and colonization of the continent by the Europeans. Today, after near extinction by the fur trade, beavers have made a dramatic comeback. They have repopulated much of their historic range, to the point that now, in many places, the beaver has become a pest with a propensity for chewing trees, building dams in unwanted places, and digging into man-made ponds.

Beaver History

Castor canadensis is a rodent. It is a member of a larger order that includes gophers, kangaroo rats, and pocket mice, and it’s the second largest in body size behind capybaras. Fossil evidence indicates that modern beavers evolved from the family Castoridae starting about 24 million years ago, during the Miocene Epoch. Some members of this family reached 6.5 feet in length and 220 pounds in weight. Even at the end of the Pleistocene (11,000 years ago), there were bear-sized beavers in Canada. As our modern beaver evolved into the bark-eating animal of today, it also reduced in size. Beavers of today have a body length of roughly 40 inches, plus a tail in the range of 15 inches. Adults weigh about 30 to 65 pounds.

The beaver is superbly adapted to its watery lifestyle. Beavers have evolved streamlined bodies, webbed feet for swimming (they don’t swim with their tails), and muscular bodies for pulling heavy loads. The beaver’s iron-imbedded, ever-growing teeth and the skull that supports those teeth are designed for one thing: gnawing wood. Beavers feed on many types of plants, but their extra-long gut is designed to extract nutrition from the cambium of woody plants. They even have the habit of eating their own feces as a method of extracting the maximum amount of nutrients from their lignin-laden diet. The coat of a beaver has 75,000 to 150,000 hairs per square inch. That’s more hair in the area of a postage stamp than humans have on their entire head. This dense fur keeps beavers warm, protects them from predators, and helps them float. Daily grooming using a modified toenail on their hind feet spreads oil collected from a gland under their tails to make their fur water-repellent. The coat of oil also traps a layer of air next to the skin that helps insulate the body, much as a neoprene wetsuit does for divers. The beaver has adaptations to its throat and a second set of fur-lined lips that keep water from entering its lungs so it can carry limbs underwater. These are only a few of the many ways beavers have evolved into the geoengineers that changed the face of North America.

It is, of course, the beaver’s ability to cut and use trees for the building of dams and lodges that has so intrigued humans. Birds can build elaborate nests, prairie dogs can tunnel into acres of prairie, termites can build huge mounds – but none of these animals change the basic structure of their environment. Like humans, beavers can alter the ecology of an area. Beavers can change dry land into wetlands and open water. Their dams and channel-digging can divert the direction of streams. Their ponds capture and retain sediments and act as nutrient and carbon traps. Their constant need to gnaw on saplings and trees can turn a dense forest into an open forest, which in turn impacts the plants and animals living there.

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European Beaver Exploitation

It was principally its fur that made the beaver such an economic driving force in the European colonization of North America. In the mid-1800s, beaver felt hats were the height of fashion. Basically, the underfur was put through a series of processes to produce felt. The felt could then be shaped into hats and other clothing. Castoreum, an oily fluid from the beavers’ castor glands, was used in perfume, food flavoring (artificial vanilla), and medicine. The Hudson’s Bay Company, which still exists today in the guise of several different retail stores, was incorporated in 1670 to control, politically and economically, the trade of beaver pelts. This driving force literally changed the face of North America culturally, politically, and ecologically.

Why Do Beavers Build Dams?

Although beavers can be beneficial, they can also become at best a pest and at worst a danger, and can cause tens of thousands of dollars’ worth of damages, when they do their thing in unwanted places. Their dam building on free-flowing streams and at culverts and bridges can cause water to overflow and undermine roads and bridge abutments. The resulting ponds, while beneficial to many types of wildlife, are destructive to other types, flooding dry habitat, woodlands, and farmland.

My beavers seem to have an appetite for hundreds of saplings that have grown from the thousands of seedlings I laboriously planted 10 years ago. High-value trees (in both economic and environmental terms), such as oak, tamarack, and maple, must taste much better than low-value prickly ash and invasive buckthorn and honeysuckle. So while I appreciate and value the benefits of beavers, I often wish they would take their industriousness somewhere else.

In the late ’50s, my uncle built a dike across a spring-fed marsh that was once used to harvest marsh hay. Some modifications over the years have produced a nice 6-acre pond that is used by ducks, geese, mink, muskrats, and now beavers.

The normal course through the year was that the pond would rise in spring with the snowmelt and rains. Water would run out of the culvert for a couple of months, until the water table dropped and many springs dried up for the rest of the year. The water level of the pond would fall below the culvert, and whatever inflow from springs that came into the pond was taken care of by evaporation and seepage through the dike. Over the past five or six years, however, the water table has remained high – high enough that my well has become artisanal, and the springs flow all year. This means water flows out the culvert all year. This is not an issue in and of itself, but when you add beavers to the mix, some dire problems arise.

Dam building is instinctual in beavers. They build dams to create open water. They need water at least 3 feet deep or more to keep the entrances to their lodges submerged. The question is, how do they know where to build their dams? The answer appears to be: by the sound of flowing water. And not just any flowing water – they don’t try to dam a fast-flowing river. But they will try to dam a low-gradient small flow, such as that through a culvert.

How to Beaver Proof Water Control Structures

The typical beaver lodge starts as a pile of sticks with an underwater entrance. Beavers enlarge the lodge by adding sticks, mud, and plant material to the outside and chewing away the inside. A bank lodge is started as an underwater entrance that comes out onto the surface of land. The beavers add sticks and mud-plant cement on top of the hole. As the hole is widened, they add more sticks and mud, and the process continues, leaving a relatively flat roof. Again, in and of itself, this might not be a problem. My dike that contains the water is mostly sand. When I dismantled the roof of the bank lodge, I found a hole about 4 feet deep and 6 feet in diameter. It was more than a third of the way into the dike. My options were ignoring it and letting nature take its course, trapping, or destruction and subterfuge.

Letting nature run its course was not a viable option. On my pond, there is an 18-inch outflow culvert, which is enough to handle any outflow. The outflow culvert on my pond was plugged by beavers. Before the damming, the water was 4 feet deep right in front of the culvert. After the beavers dammed up the outlet, the water level raised by at least 18 inches, and I knew that without daily removal, it would probably overflow the dike. If I let the beavers continue to dig their lodge into the dike, there would eventually be a blowout somewhere along the dike, and the pond would drain. The beavers, being who they are, would probably quickly plug the breach, but this boom-and-bust cycle wouldn’t be conducive to the health of all the other creatures that use the pond.

I’ve tried trapping in the past, having trapped as a teen, but it’s a time- and labor-intensive operation. Traps have to be checked daily, and the pelt has to be processed or it will go to waste. Removing beavers is like putting up a vacancy sign – new beavers will come and fill the empty niche.

Destruction and subterfuge was the course I chose. I figured if the triggers for occupancy (food and shelter) were taken away, maybe the beavers would leave. Taking food away wasn’t realistic, as I wanted the saplings to grow into trees. I used some plastic snow fencing to protect some of the young trees, but there are a lot of trees. Destruction of the lodge proved effective at getting the beavers out of the pond. There were three bank lodges being constructed. I removed the roof and collapsed the entrance of each lodge. The following evening, the beavers tried to repair the roof. The hole was more than 3.5 feet deep and 5 feet across. By the third day, they gave up and left for parts unknown.

If this was all I had done, the beavers would’ve just kept coming back and digging more lodges and continually plugging the culvert, so I launched the subterfuge step. It had two parts: stopping the beavers from plugging the culvert, and removing the sound of flowing water while still letting the water flow – it’s a “Castor con” (“beaver deceiver” is already taken). See the two illustrations for the basic principle behind this flow device. The inlet is below the surface, so there’s no sound. Even if the culvert is plugged or the dam repaired, the water will still flow through the black pipe. It doesn’t stop any flooding, but you can control the depth and breadth.

To achieve this, I started by building a barrier around the culvert that will let water flow but can’t be plugged by the beavers. I used cattle panels (approximately $25 each); 3-foot, 16-gauge Econo fencing with 3-by-2-inch spacing; and 6-foot T-posts to support everything. The cattle panels are 16-by-4-1/2-foot panels made of 4- or 5-gauge wire. They are heavy-duty and easy to manipulate, especially when I’m standing waist-deep in water, and they form the basic structure. If the beaver tries to plug the culvert, it will build on the outside of the fence. Beavers seem to get confused as they build farther and farther from the source of the sound and it doesn’t disappear. Hopefully, they will not be able or willing to build along the entire barrier, leaving some open space through which water can flow. The actual shape of the enclosure can vary depending on your specific situation. I thought the cattle fencing would be enough, but the large spacing allowed the beavers to slip through, so I added the Econo fencing on the outside. That has kept the beavers from getting to the culvert.

This may be enough in some situations, but it doesn’t remove the sound of flowing water, so the beavers will keep trying to plug the “leak.” So my next step was to eliminate the sound by inserting a pipe into the culvert with the inlet resting on the bottom of the pond. I inserted two 30-foot sections of flexible 8-inch black pipes ($100 per 100 feet) into the culvert. The outflow end is about 6 to 8 feet into the culvert, and the inflow end extends about 20 feet into the pond and rests on the bottom. I covered the inlet with some fencing, and I drilled a series of 1-inch holes along the pipes to allow air to escape so they would sink to the bottom with the help of some bricks and water in the pipe. Because the inlet is on the bottom, there is no sound to attract the beaver. With the water level kept lower, the entrance to the lodges is too shallow for the beavers to feel secure, and eventually, they’ll give up and leave. Or at least, that’s the idea.

If a beaver is making a new pond by building a dam that’s causing problems, this same effect can be achieved by burying a drainpipe in a beaver dam. After checking with your game warden, tear out part of the dam, lay down a drainage pipe, and when the beavers repair the dam, the sound will disappear, but the water can still flow out.

I made my improvements in early winter, and then the pond froze, which stopped any dam-building activities. It’s now spring, and the pond is open again. There hasn’t been any repair activity at the culvert … yet. The water is flowing well, and it’s coming through the black pipes. I don’t expect the beavers to stay away, but now the water level will stay within acceptable levels. I might still have to deal with the bank dens, but I know beavers won’t like me as a landlord and will soon move on. Beavers are amazing and in the right place can do good things – just not on my pond.


Doug Thalacker grew up on a Wisconsin dairy farm. He has degrees in agriculture, biology, and environmental science. He has taught agriculture, earth science, environmental science, and horticulture for 30 years.

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