Beavers: Worth a Dam

October 2016 

 by Linda Martinson 

We live in the Blue Ridge geographic province of the Southern Appalachian Region, an eco-region that contains one of the world’s richest and most biodiverse temperate deciduous forests. There are plants and other species in our area that now occur in only one other location in the world: the temperate deciduous forests of central China. What is left here now, in the Southern Appalachians, are the relic species and habitats of an ancient forest that once spread over almost the entire Northern hemisphere. What is not here now is what has been removed by intensive logging and cleared for agriculture, urban, and industrial development, which is most of what was here originally. There are only small sections of the original extensive forests of the Appalachian-Blue Ridge mountains, with the largest in the Great Smoky Mountains National Park.

Most of our regional forests are regrown and even though they still provide vital habitat for wildlife, they now contain large areas of heath vegetation such as rhododendron and mountain laurel, indicative of poor soil, instead of the original hardwood trees. And because there aren’t any wolves and cougars or other large predators left, our largely regrown woodlands, forests, and remaining undeveloped river valleys are overgrazed by deer. There are also significant threats from introduced species such as the hemlock woolly adelgid and from invasive vegetation such as multiflora rose. Still, we live in an area of the world which, although small, is extremely valuable for many reasons, especially for its amazing biodiversity.

What does this description of the Blue Ridge forests eco-region have to do with beavers?  

Research indicates that the physical and ecological landscapes of most of the Northern Hemisphere were shaped primarily by beavers and that what is left has also been shaped by the near extinction of beavers over most of the Northern Hemisphere, including the Southern Appalachians. Fossil records from as far back as 60 million years ago indicate that 30 genera of the beaver family (Castoridae), most smaller than today but at least one giant one, about the size of a black bear, co-evolved with the great forests and wetlands of Eurasia and North America.  Wherever they lived, beavers were the main shaper of the physical and ecological landscape. If there was water (i.e., streams, ponds, marshes, wetlands), there were beavers. And it is probable that, after several millennia of ice ages and catastrophic climate changes, without beavers there would have been little or no water by the Late Pleistocene epoch in the Northern hemisphere except for major rivers and streams. That situation would have been regrettable for us humans. 

How did our part of the world look about 15 – 20,000 years ago – near the end of the Late Pleistocene? 

That would have been just about when humans, the ultimate invasive species, arrived on the North American continent, but beavers were already here. In some ways, the landscape would be fairly familiar to us now, because most of our major mountains and rivers were already in place and many of the trees and other plant species now associated with northern North America were common. The mega fauna were nearly extinct by the end of the Late Pleistocene, and the glacial sheets that had nearly covered the continent had significantly retreated. The climate was colder and drier than now, with only about half the amount of rainfall but just as in Eurasia, the entire continent was teeming with beavers (estimates are from as many as 65 to 400 million) and the wetlands they had created. When humans arrived across the Bering Land Bridge about 15,000 to 12,000 years ago, they spread fairly rapidly across a familiar and hospitable landscape, one shaped by beavers, by then down to only basically two species: one species in Europe and one species in North America genetically similar to a singular species in Eurasia. 

Beavers had a significant head start on humans; by the time the Neanderthals appeared in Eurasia around 250,000 years ago, beavers had been there for several millennia gathering food and building their homes by significantly changing and shaping the landscape. All our ancestors in Eurasia and eventually North America only knew a world that had been shaped by beavers, and one hypothesis is that humans were able to evolve and advance so relatively quickly across Eurasia into North America because the landscape there was so familiar and hospitable.

Before we pick up the tale of what happened with beavers and humans after the end of the Pleistocene, let’s consider the beaver—a Water Superhero, Continental Change Agent, and a Keystone Species—and why they are so successful at changing the landscape to suit themselves. 

There are only two species of mammals that have been successful at changing their environment to suit themselves: beavers using water and humans using fire. Beavers are a member of the order Rodentia, the largest mammal order. About 40% of all mammal species are rodents, and they are gnawing animals — they have to keep gnawing to live. Beavers have been called the perfect animal, and they are an ecological marvel: a semi-aquatic animal at home on the land and in the water with an incredibly persistent drive to sustain water on their landscape. Beavers need both land and water to live, and they function well in both worlds. On land, they use their dexterous front paws and strong teeth to cut down and move trees, bushes and branches; they are excellent swimmers and hydrologic engineers, building dams and lodges and storing food underwater.

Beavers keep growing their whole lives and can weigh up to 60 pounds and live to be 20+ years old. They have thick, luxurious fur and special valves for their nose and ears, so that they can swim easily under water. They also have a special membrane that closes over their eyes like swim goggles and specialized eye muscles that correct their vision underwater, so that what they see is not distorted. They can carry branches and twigs and even eat efficiently underwater, because they have an extra set of fur-lined lips that close behind their teeth while they carry and chew. With their webbed back feet and paddle-like tail, they are excellent swimmers and can stay underwater for 15 minutes, or more, without breathing.    

Beavers have survived and thrived for millennia, and shaped the physical and ecological landscapes of entire continents. How do they change the landscape so effectively?

The short answer is that they incessantly cut down trees and move them, swim, dig and build dams. Primarily, beavers build dams on smaller water sources, not on large rivers or streams where they excavate the banks instead. They cut down trees and bushes and drag them into the water. Then using their teeth and front paws, they push them into the bottom of a small stream area and then dig up mud and rocks and pile them onto the branches. They continue working diligently until they have a strong dam, up to 1/2 mile wide, sturdy enough to walk on. If their dam is damaged, they work incessantly again until it is repaired or replaced. And beavers work cooperatively, steadily, and quickly without tools.

What happens after a dam is built?

Water backs up and a small pond is created. When the water is deep enough, beavers add to the dam or build more dams, and then build lodges and dig tunnels for their living chambers and to store food. Beavers are generally monogamous and cooperative, raising their young and working together. They eat the bark or cambium layers of the branches of deciduous and other tender shoots and plants. They create food stores underwater, if they live where the ice freezes over in the winter.

When beavers build a dam, they start a form of aquatic succession that creates a burst of biodiversity.

First, the standing water created from their dams gradually floods the nearby land, and the trees in the area are soon cut or killed by the standing water. More sunlight on the water triggers an explosion of biological activity — algae and aquatic plants flourish, and their organic material supports microscopic organisms, which are then eaten by invertebrates, which in turn provide food for fish, birds, and mammals. Soon, an entire food chain is created in the beaver pond, and the growing wetland area becomes a protected spawning area and magnet for a rich variety of wildlife, including plants and wildflowers, butterflies and other insects, frogs, and game species of mammals, birds, and fish.

The wetlands resulting from beaver ponds have been described as the “Earth’s kidneys.”  Why?

Beaver dams and ponds significantly improve water quality and flow. They function as natural sponges to reduce downstream flooding and erosion, and the algae and plants in the ponds they create process organic wastes and detoxify runoff toxins such as pesticides, fertilizers, and heavy metals. Beaver ponds can stabilize the water table, recharge drinking water aquifers, and maintain stream flow during droughts. No other single species creates and benefits watershed areas more effectively than beavers.

And further succession of beaver ponds/wetlands creates meadows and rich bottomland and eventually a mature forest.

After many years, sometimes even centuries, the ponds and watershed area formed from beaver dams become shallow and fill in with silt and plant debris. Grasses, sedges & shrubs begin to choke the water and more silt is trapped, creating meadows and rich bottomland. As it becomes harder for the beavers to move around the area and to find trees and bushes nearby, they move on and build their dams in other locations. Eventually there is a healthy, mature, and diverse forest growing where a beaver pond once was, and the beavers have moved on to start over in their own tireless, tenacious way!

Picking up the tale of North American beavers after the end of the Late Pleistocene…let’s consider the modern history of humans and beavers in the Blue Ridge ecoregion of North America.

There is evidence that indigenous people lived in the Blue Ridge as early as 11,000 years ago, with the Cherokees being the largest tribe here. They used beavers for fur, food, and medicine, and celebrated them in their stories and rituals. Certainly they benefited hugely from the significant physical and ecological changes made by the beavers that had been here for millennia, and for thousands of years they used the streams, ponds, and watershed areas for their own lodges and villages and to hunt and raise their food.

Europeans began arriving in North America starting in the mid-1500s, and early explorers reported beavers in every single small stream and river – any place that had suitable habitat – millions and millions of them. By that time, beavers were almost extinct in Europe and Asia from over-harvesting them for food and especially for their pelts. Beginning seriously in the mid-1700s, by 1900 beavers were almost extinct in North America, too, harvested by invading Europeans with the help of Native Americans.

How did this happen? If the population of beavers in North America in the 1500s was up to as high as 400 million, how could it drop to almost 0 by 1900? How could an animal that had survived multiple ice ages, global climate changes, and prolonged droughts for several millennia be almost eliminated in most of the temperate regions of the Northern Hemisphere in just a few hundred years? 

It was the intense global fur trade, a huge and profitable business stretching back centuries, even millennia. Probably since the beginning of human history, furs have been used for clothing, shoes, housing, and decoration. Global trade in furs and felts has been traced from modern history back to the Classic Greek and Roman times. Through the 15th century, Russia, Northern Scandinavia, and Central Asia were the major suppliers to Northern and central Europe and even throughout the Mediterranean and Middle East. Beaver pelts were particularly prized because of their luxurious fur and how easily the pelts could be felted into a tough hide or cloth, but by the beginning of the 17th century, the beavers in Europe and Eurasia were almost eliminated and the fur trade of Eurasian beaver pelts stopped. 

Fortunately for fur merchants, but unfortunately for North American beavers, the establishment of European colonies in North America coincided with the nearly total extinction of Eurasian beavers. European merchants, mainly from France, Holland, and England, rapidly established extremely profitable trade deals with North American colonists, beginning in what is now Canada but eventually all across North America, mostly by enlisting the help of Native Americans with their well-developed hunting and trapping skills. The European merchants traded knives, tools, cooking pots, textiles, and guns for what must have seemed like endless sources of beaver pelts, turning profits of 1000 to 2000 percent.  

This extremely profitable North American fur trade became an international phenomenon, with beaver pelts even used as currency. The Hudson Bay trading company alone shipped 5 million prime beaver pelts in 100 years between 1769 to 1868, not including those pelts discarded or sold for less because they were not prime. The Cherokees and other Native Americans fully cooperated in this fur trade economic bubble, which changed their lifestyles forever, not the least because the spread of European diseases such as small pox, chicken pox, the bubonic plague, and influenza decimated populations of indigenous people across North American and eventually South America. 

In the years following, after almost every single beaver on the North American continent had been killed, an unprecedented draining of the extensive wetlands of the continent for agricultural land creation began. By 1990, 6 states had lost at least 85% of their wetlands, and 22 others had lost over 50% of their wetlands; also, 65 – 80% of the coastal marshes were drained.

For many years, these changes in North America were generally interpreted as progress, and it took a while before researchers (at first primarily in Canada) began to ask the question: Wouldn’t the loss of at least 70% of the wetlands on the third largest continent in the world AND the near extinction of the one animal capable of engineering wetlands creation have some negative effect?

Indeed, severe droughts throughout the continent followed the near extinction of beavers, and the Civil War drought, the Dust Bowl phenomenon, and the Dirty Thirties followed by the Great Depression are now thought to be correlated with the severe loss of beavers in the United States. Gradually, an understanding developed, and has been generally accepted that beavers account for 85% of open water area even in times of drought, and there have been significant efforts since the 1990s to restore beavers in many areas of the United States and Canada.

In the Blue Ridge Mountains, the effects of over-harvesting beavers with some draining of wetlands were confounded by the severe industrial logging that took place in Western North Carolina and Tennessee in the late 1800s/early decades of the 1900s and by the loss of the American Chestnut trees to the blight beginning at about the same time. 

By the early 1900s, not only the beaver, but also wild turkeys and trout and many other less noticeable species were almost eliminated in this area. Severe logging on the mountainous areas was followed by fire and floods, and it has taken decades to restore land, forests, and watershed areas in Western North Carolina — and it’s not done yet.

Fortunately, many individuals in WNC took it upon themselves to reintroduce beavers, turkey and trout to restored areas in the 1960s and ‘70s,

including the managers and rangers of the Great Smoky Mountains National park and the Pisgah and Nantahala National Forests. Now, we have fairly healthy populations of all these species throughout the Blue Ridge area, and many efforts and organizations have been established to sustain them. 

The population of beavers is now up to 6 to 12 million in the United States, and there are now several concerted efforts to restore beavers in more arid locations in the Midwest and West coast.  But now that beavers have been restored in settled, human-dominated areas, we are finding that beavers don’t always play by our rules. They don’t always respect human boundaries, and build dams in culverts, for example. Also, many people don’t understand the environmental value of beavers and consider them pests, so there is often an organized effort, sometimes at community, county, and even state levels, to destroy dams, which in the long run doesn’t work (unless you remove or kill the beavers, too) and is environmentally counter-productive.

Because beavers have been restored to largely settled areas, not wilderness, and there has been resulting conflict between humans and beavers – the question is, can they peacefully co-exist? 

The answer is, “Yes, with Adaptive Management Approaches.”

Here is one example: the story of the Martinez, California beavers.

Martinez County lies just outside San Francisco, and they had been suffering from water shortages for decades. Their main water source, Alhambro Creek, slowed to a trickle during the summer of 1998, but was still prone to severe flooding in the winter and spring.

To address the problem, in 1999, the county earmarked $9.7 million dollars for a flood improvement plan along the river – mostly they planted thousands of willows and other landscaping plants. And they had some moderate success with this effort after a few years.

Then, in 2006, a pair of beavers moved into Alhambra Creek, probably from the Sacramento/San Joaquin river delta, which at one time had one of the largest concentration of beavers in North America and where some beavers had recently been introduced. In no time, the two beavers had built a dam 30 ft wide and 6 ft high and had chewed through half of the willows and landscaping plants that had been planted. Then, with some deliberation, the Martinez City Council decided to destroy the dam and remove the beavers because they thought that, with the dam and the loss of the trees and bushes they had just bought and planted, there would be excessive flooding again.

Almost immediately, a grassroots citizen group called Beavers: Worth a Dam (I borrowed this name for this newsletter) started up to save the beavers and the dam. To appease the City Council quickly, they put small pipes through the dam in several places. With some experimentation, the group found that with small pipes, the beavers couldn’t hear the running water through the dam so they didn’t try to fix the dam and the water continued to drain. And with this small fix, the danger of flooding from the beaver dams was eliminated . The City Council was appeased, the water levels stayed manageable, and there has been no flooding in Martinez County since the beavers moved in.

By 2008, the trickle that was Alhambra Creek had become a series of multiple beaver dams and beaver ponds. There is an ample water supply, and both the flow and the water quality of the creek has improved significantly. Toxic run-off from agriculture has been mitigated, and steelhead and other fish, water birds, song birds, river otters, etc. have returned. The technique of using pipes to keep water levels manageable behind the dams has been refined and other methods of using the work of beavers, while controlling and protecting them at the same time, are now being developed all over the country. Enough easy and relatively inexpensive techniques have now been developed to address most beaver control issues with some patience and ingenuity.

Consider our corner of the Blue Ridge Mountains, Richland Ridge, and specifically the old farm site along the West Fork of the French Broad river. 

Suppose when we acquired the farm site many years ago, we had decided to restore this site to a vibrant montane riparian site with ponds and streams that stay full even during dry spells and with native vegetation, fish, wildflowers, rare butterflies, legions of frogs, and non-toxic run-off into the French Broad River? Could we have done it? Maybe, but it would have taken a lot of planning, cooperation, time, and money. However, the beavers that moved back here, probably during the 1980s, have done it for free without any effort on our part except for leaving them alone. Clearly, our beavers are worth a dam.

Native American lore was that beavers hummed when they were happy and satisfied with their work and one afternoon when I watched my daughter fish in our restored riparian area, I thought I heard the beavers humming.

References: Hood, Glynnis. The Beaver Manifesto. Rocky Mountain Books, 2011.  

    Hardisky, Tom. Beaver Management in Pennsylvania (2010-2019)

      The Fur Trade cwh.ucsc.edu