The English language is replete with idioms about beavers, like “beaver away” or “busy as a beaver,” all signifying hard work and industry. In his new book, Eager, Ben Goldfarb takes us inside the amazing world of nature’s premier construction engineer—which can create dams as long as half a mile—and shows us why the restoration of an animal almost driven to extinction is producing wide-ranging, positive effects on our landscapes, ecology, and even our economy.

When National Geographic caught up with Goldfarb by phone in New York, he explained how beavers are playing a crucial role in the American West, how a beaver named Jose set up home on the previously poisonous Bronx River, and why the only way to tell a beaver’s sex is to sniff its butt.

You call beavers, “ecological and hydrological Swiss army knives” and “one of our most triumphant wildlife success stories.” Elaborate on those two statements, and showcase some of the economic and even medical benefits of beaver restoration.

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Classic beaver behavior, which every third grader can identify, is building dams. By doing this, they create ponds and wetlands that turn out to be important for many reasons. The first is biodiversity habitat, providing places to live for fish and wildlife. In the American West, where things are pretty dry, wetlands cover just 2 percent of the total land area, but support about 80 percent of the biodiversity. Any creature capable of creating wetlands becomes immensely important. Imagine being a frog that breeds in a pond, a juvenile salmon that grows up in one, or a duck that nests near one. The number of species that depend on these beaver habitats is virtually limitless.

Beavers provide all kinds of great services for us humans, too. Beaver ponds filter out pollution, store water for use by farms and ranches, slow down floods, and act as firebreaks or reduce erosion. One study in Utah found that restoring beavers to a single river basin produced tens of millions of dollars in economic benefits each year.

In North America, when the first white traders and trappers arrived, there were as many as 400 million beavers. By 1900, there were perhaps 100,000. For three centuries they were trapped for their pelts; their furs made great hats. Then, in the early 1900s, we woke up and realized that incessantly trapping these animals was not sustainable and that these were important creatures, which we needed back in our landscapes. The recovery of beavers proves that conservation works!

One of the most ambitious beaver restoration projects is taking place in the Methow Valley, Washington State. Describe what they are attempting.

The Methow Valley is in central Washington on the east side of the Cascade Range. It’s a pretty dry place, with lots of wildfires. Snowpack and glacial melt from the Cascades is also declining, so water is critical. It’s one of the country’s largest apple- and hop-growing regions, a critical agricultural bread basket in the middle of Washington State. Inevitably, there are lots of beaver conflicts. Usually, the knee jerk reaction is to trap them out. But the Methow Project traps the beavers and relocates them to headwater streams on public lands high up in the mountains, thus getting them off private land. By building dams and creating ponds, they keep rivers and streams in Central Washington wet throughout the entire year. So beavers function as a climate adaptation strategy, compensating for the loss of snowpack and glacial melt.

Beaver Engineering Once spring arrives, beavers get to work on the many tasks that help the vast network of wetlands survive.

On the Puget Sound, beavers are also being re-introduced to enhance salmon stocks. How does this work?

That’s a cool project! If you’re a baby salmon, you don’t want to live in the main channel; you’re going to get blown downriver. You’re looking for some nice, slow water habitat, like a pool or backwater, where you can get out of the current and find food without using too much energy. By slowing waters down, beavers create that fantastic salmon habitat.

This is important for Native American tribes in the Pacific Northwest, who have historically been dependent on salmon. Salmon runs all over America have declined as a result of dams, overfishing and habitat loss. By reintroducing beavers to recreate salmon habitat, tribes like the Tulalip can restore some of the fish they depend on and that are integral to their culture.

You did some pretty messy hands-on research for your book. Tell us about your experiences “sniffing beaver butts.”

[Laughs] In the Methow Beaver Project they like to find compatible pairs of beavers to relocate as a family. That way, they will settle down and start building dams like you want them to. Oftentimes, if you catch a beaver by itself, that beaver is going to go wandering around looking for a mate and probably get eaten by a bear or a cougar. The Methow Project tries to create compatible beaver pairs, a bit like a beaver dating service. [laughs]

But beavers make it difficult to differentiate the sexes. Male beavers don’t have external genitalia, which makes sense. If you’re an animal that spends its life swimming around logjams, you don’t want any dangling bits that can get snagged. [laughs] And, unless the female is lactating, you can’t reliably tell which sex she is.

The only way to differentiate the sexes is to use your fingers to push out the anal gland on the beaver’s underside, squeeze out a little dollop of the secretion they use to mark their territory, and sniff it. If it smells like motor oil it’s a male, and if it smells like cheese it’s female. [laughs] I got to smell two beavers but I could not reliably tell them apart. The folks at the Methow Project absolutely can, though. And they use this method to make compatible beaver matches.

You meet a colorful cast of characters along the way. Tell us about Heidi Perryman and her organization Worth A Dam.

Heidi is a fascinating person, a child psychologist who didn’t know much about beavers until 2007, when beavers showed up in downtown Martinez, California, where she lives. It’s in the Bay Area, the former home of John Muir, and when beavers showed up there the response of the city was to kill them because landowners downtown were worried they were going to cause flood damages. There’s no evidence supporting this, but the reflexive reaction was to get rid of them.

Heidi spent a lot of time going to the streams of Alhambra Creek, where the beavers lived. She filmed them and organized a campaign to save them. In so doing, she became one of the most knowledgeable beaver advocates in the country. She now organizes an annual beaver festival in downtown Martinez. As a result of her campaigning, the city has let beavers live with many generations of offspring and now Martinez is regarded as a leader in beaver coexistence.

A lot of the foremost beaver authorities are self-taught people, like Heidi. I met former real estate agents and physicians working on beaver issues—all kinds of people who aren’t trained biologists, but come into contact with these amazing animals and get transfixed. There’s a group called The Beaver Believers, an informal designation that beaver-lovers give themselves. You don’t have to be a wildlife biologist to be a beaver believer. You just have to be a person who spends time with these animals and experiences their power to transform landscapes.

Another charismatic character is Dave Rosgen, aka The Restoration Cowboy. Introduce us to him and his work.

He’s probably the most famous stream restoration practitioner in the country. He wears this big hat and belt buckle and is this swaggering guy who leads workshops around the country attended by thousands of restoration professionals. In some quarters he is a controversial figure, though, because he sometimes uses heavy-handed methods, like bulldozers, to recontour streams.

Everybody in the stream restoration community respects what he’s done, but there are some stream restoration practitioners who think of beavers as an alternative to Rosgen’s techniques. Instead of using heavy machinery, you can put in lighter, much cheaper artificial beaver dams by pounding a few logs into a stream, which induces the beavers to come and take over. Rosgen is also a beaver fan and in his own way imitates them through the use of heavy machinery. There’s this continuum of approaches when it comes to stream restoration, but beavers are increasingly at the forefront of restoring degraded streams in the American West.

Britain is also experimenting with beaver restoration. Tell us about your journey to the Highlands and the Scottish Beaver Trial.

Beavers were completely wiped out in Britain by the late 1700s. But in the last few years there have been a number of reintroduction efforts, taking beavers from Germany and Norway and relocating them to England and Scotland. Some of these efforts, like the Scottish Beaver Trial, are officially authorized by the government. Some are a little more rogue.

I went to England in the course of my reporting and saw both the official releases, as well as the unofficial reintroductions, and the picture is really bright! In Scotland, there’s still a fair amount of beaver resistance among farmers, but the Scottish government has acknowledged that they are a native species and is moving to protect them and they are gradually becoming a more integral part of the landscape again.

That’s important in Britain because it’s a rainy place with lots of flooding issues. There is some great research on reintroduced beaver colonies in Devon, southwest England, showing that their ponds and wetlands are fantastic when it comes to mitigating flood damage. As floods race downstream, the water gets stored in the ponds and disbursed outward to the surrounding wetlands. This research team from Exeter, in Devon, has shown that beavers swallow up 30 percent of the water during your typical big rain event. So, a lot of beaver reintroductions in the U.K. now are motivated by flood damage reduction, which is another cool function of beavers.

Bring it home for us, Ben, by summing up what you love about beavers, and what you think the future holds for them.

One of the things I love about them is that they’re so easy to empathize with. We humans love rearranging our surroundings to maximize our own human shelter and beavers do the exact same thing! They’re incredibly ingenious and enterprising. I find that so relatable.

As to what the future holds, it’s in many respects very bright. I grew up just upriver from New York City, which was once an incredible beaver habitat. Even Times Square used to be this beaver-filled swamp. But by the early 1900s beavers had been completely wiped out by trappers, pollution, and development. Then, in 2006, a beaver returned to the Bronx River, which had been incapable of supporting any life. It was named Jose, in honor of Congressman Jose Serrano, the local politician who catalyzed the restoration of the Bronx River. Since then, beavers have been coming down to the Bronx and that’s a hopeful thing for a lot of people. It shows that we’re capable of remedying some of our most serious environmental mistakes.

On my travels, I saw beavers in wilderness areas, like Yellowstone. But I also saw lots of beavers in places like downtown Martinez, California. I even visited a colony of beavers next to a Wal-Mart parking lot in Utah! [laughs] These are animals that do pretty well in close proximity to humans, and if we let them they can provide many wonderful services. As one beaver scientist put it: “We have to let beavers do their work, to help us solve some of our most serious environmental problems.”

This interview was edited for length and clarity.