More Than Just Dams: Beavers Give River Systems a Boost

The dull glow from my headlamp bobs in time with each crunching footfall. It’s all I have to guide myself in the thick brush, except for the barest hint of light on the horizon. Despite its impressive wealth of animal life, the Greater Yellowstone Ecosystem can be eerily quiet on cold mornings like this. Any noise I make seems to be swallowed up by the frosty air. I stop myself occasionally — sometimes to regain my bearings, and other times just to pause and listen. These willow-covered creek bottoms are a likely place to encounter wildlife, including a couple of species — moose and grizzly bears — that I’d rather not stumble into, especially in the dark. 

The willows finally break at a chain of beaver ponds, each one reflecting the rapidly receding starlight. The pond in front of me is massive. It’s propped up by one of the largest beaver dams I’ve ever seen. It’s so large that the game trail I’m following continues straight across it to the other side. Its surface is littered with the tracks of others who have used this crossing point before me — otters, raccoons, and elk. It is, after so much pushing through tangled willow, a welcome sight. 

I don’t sense anything on the other side, so I start making my way across the dam. Halfway across I see a flash of movement and hear a resounding “THWACK!” at my feet. It is so sudden and unexpected — I can feel it as much as hear it — that I leap away from its the source…and directly into the frigid beaver pond.  

The culprit of the “THWACK.” A beaver gnaws on a branch in Grand Teton National Park. (Photo NPS/Adams) 

Admittedly, this encounter with Castor canadensis, the North American beaver, was far more cinematic than usual. I enjoy watching beavers work from a distance, or marveling at the effects of their work while casting a fly line. I, like the many species that make our ecosystem home, find myself drawn to beaver-formed wetlands and riparian habitat in general. Riparian habitat is the type you find right alongside water, and is some of the most productive and diverse habitat around. While occupying just a fraction of the landscape, it is a biodiversity hub that supports 70 to 80 percent of species during some portion of their life cycle.  

But beaver-formed wetlands aren’t nearly as ubiquitous as they once were. With six to 12 million spread out in North America today, versus anywhere from 60 to 400 million just a few centuries ago, beaver populations are a shadow of their historical selves. And while declines in the Greater Yellowstone Ecosystem weren’t as drastic as elsewhere, historical accounts and the environmental record highlight a steady decrease in numbers and distribution throughout the 19th Century. 

A beaver pond in Yellowstone National Park. (Photo NPS/Neal Herbet) 

Trapping, the primary driver of this decline, eliminated beavers from entire watersheds in places like the Lamar Valley as early as the 1830s. Other factors like a changing climate, disruptions to the food chain, and habitat deterioration are hypothesized to have further reduced beaver numbers and impeded recovery. Changes in land use and social tolerance have further slowed beaver reestablishment in places where they’ve been absent for a long time. As a result, large swaths of the Greater Yellowstone Ecosystem that once supported thousands of beavers now just contain scattered and isolated colonies. In other locations, they’re absent entirely.  

As a keystone species — organisms that play an outsized part in shaping their ecosystem — beavers and the wetlands they maintain serve a unique and irreplaceable role in the Greater Yellowstone Ecosystem. Beavers are also ecosystem engineers, second only to humans in their ability to alter a landscape. By cutting wood and building dams, beavers do more than just create ponds. Their activities provide a wide range of benefits by creating and maintaining diverse habitat types.  

This proves especially important when a landscape experiences drought or wildfire. By storing surface water and recharging groundwater, beaver-formed habitat tends to persist and offer sanctuary when conditions are tough elsewhere. On the whole, beaver presence can be the key for maintaining vibrant plant and animal communities along rivers and streams through time.  

An Idaho wildfire burned everything but a beaver-occupied stream and surrounding riparian habitat. (Photo Joseph Wheaton/Utah State University) 

On the flip side, removing beavers and the services they provide can have the opposite effect. If beavers are truly gone — and don’t return — stream systems can become less diverse, less resilient to change, and can start to physically unravel. Formerly beaver-dominated portions of Yellowstone National Park have done just that after beaver populations collapsed. Streams quickly eroded downwards, leaving plants along the banks high and dry. By the time beavers began to return, the habitat no longer could support them.  

While that sounds dire, a broad coalition of federal, state, and nonprofit partners are working to ensure that the benefits of beavers can persist across the Greater Yellowstone Ecosystem. In some cases, beavers can be transplanted from an area of abundance to one of scarcity. The policy (and legality) behind such efforts are complicated, and vary from state to state. But sometimes beaver relocation isn’t appropriate — beavers aren’t always the easiest neighbors, and as mentioned above, habitat quality may no longer support a return. 

As the senior climate conservation associate, one of my goals is to figure out how to bring back the beneficial impacts of beavers to landscapes – including places where beavers can’t simply be reintroduced. Across Greater Yellowstone, there are countless streams that have lost beavers, and we are working closely with our partners to not only implement restoration projects, but to best prioritize where we should focus our efforts and maximize restoration impacts. 

We expect this year to be a busy one in the water and climate world. Our work involving beavers and stream restoration is ongoing and will see us out in the field once summer rolls around. To stay up to date with our progress, consider signing up for our emails.  

—Kurt Imhoff, Senior Climate Conservation Associate (Lander, Wyoming)