Researchers Find Beavers in Unexpected Places

Greg Hood wants you to know that, despite their reputation as freshwater critters, beavers can live in—and engineer—areas where rivers meet the sea. Hood is an estuary ecologist who works on coastal restoration for several tribes in Washington state’s Skagit River basin. He’s spent years walking miles in tidal marshes and up delta creeks, noting the flora and fauna along the way. 

It’s a habitat that encourages movement; the seemingly bottomless mud sucks you in the longer you stand in one spot. It was during his outings that Hood spotted something unusual amid the sweet gale and driftwood. “I was walking so much, crossing all these tidal channels, and every now and then I noticed something that looked like a beaver dam,” Hood said. 

At first, he thought it might be a chance collection of sticks, detritus from the tides. He’d never heard of beavers living in tidal river deltas. But as he spotted more dams, and eventually beaver lodges, his skepticism faded, replaced by tentative excitement.

Despite the evidence in front of his eyes, he wanted reassurance, so he turned to the scientific literature to read up on tidal beavers. But he found no studies, only found a few anecdotes.

Now, Hood is working to fill that gap. He has put out two papers exploring tidal beavers—essentially the only ones to do so. His work has been met with some doubt because beavers are so strongly associated with freshwater, but others are enthusiastic and wholly unsurprised. 

“There are really beavers in just about every single environment in the United States,” said Emily Fairfax, a beaver ecologist and hydrologist at the University of Minnesota who was not involved in the research.

Beavers are freshwater animals; this is indisputable. If they spend too long in pure saltwater, they can get salt poisoning. The tidal marshes Hood studies face tides that can change three meters or more twice a day, every day, and have brackish waters.

But beavers are engineers capable of creating the habitats they need to survive. Their dams form calm pools of freshwater; their lodges, cozy safe havens. And because freshwater is less dense than saltwater, when it runs out of rivers into the deltas, it floats on top, providing beavers with a chemical respite from the sea. 

This quirk of water flow creates an opening for flexible beavers. And that adaptability has enabled them to live in every US state except Hawai'i. They’re also found everywhere from forested valleys to alpine meadows—and, researchers now know, tidal zones.

“We’ve always just assumed that beavers don’t like these extreme environments,” Fairfax said. Yet they’re making a living there. 

If there’s anywhere tidal beavers would be spotted, the Pacific Northwest is a prime target. Although there are plenty of rivers and deltas up and down the West Coast, much of the coastline is heavily developed, with tidal flats converted to agriculture or housing and rivers whipped into rectilinear forms.

That habitat loss and land-use change kicked beavers out, said Chris Jordan, a biologist studying riverscapes at the National Oceanic and Atmospheric Administration’s Northwest Fisheries Science Center. “Humans are really good at simplifying nature,” Jordan said, who was not involved in Hood’s studies. “That makes it not great for beavers,” who need variability in their habitat to survive. 

Remove the variability, remove the habitat, remove the beavers. 

But in northwestern Washington, with the mighty Skagit and Snohomish Rivers coursing out of the Cascades to Puget Sound, the areas are wilder, less populated. Settlers converted much of the tidal marshes and Sitka swamps to farms, but pockets remain, such as the shorelines Hood works in. His first study focused on the habitat creation Jordan finds so important, but not for beavers: for salmon.

Chinook salmon are culturally and economically important to the Sauk-Suiattle, Upper Skagit, and Swinomish Tribes, whose ancestral homelands are in the Skagit Basin. Juvenile Chinook spend their time in calm pockets of water in the Skagit River before making their way out to the sea.

Hood found that where the tidal beavers are present, their dams create pools at low tides and increase connectivity between pools, giving juvenile salmon more habitat and options. The pools had about four times the density of juvenile salmon as in the shallows. “Beaver are having a huge impact for juvenile salmon,” Hood said.

His newest study, which came out in July, paired on-the-ground surveys of beaver dams and lodges in the Skagit Delta with mapping of those structures in Oregon, Washington, and British Columbia using satellite imagery. The dams tend to be in spots where they can best hold water at low tide, suggesting there’s plenty of habitat at high tides. For beavers, building dams to create brackish low-tide pools is about “making life easy for themselves,” he said.

With satellite imagery, he found there are dozens of locations with tidal beavers, many of which he then explored on foot. The more dams he finds, the more variability he sees in how these beavers are making it work. However, their dams can look a little different from those of their inland cousins. They’re often shorter than nontidal beaver dams, and some are built with reeds and mud rather than wood.

“The findings are really important in the world of beaver science,” Fairfax said. “This is someone who went out and rigorously proved that yes, beavers live here, and it’s not a fluke.”

What Hood wants to know going forward is how widespread tidal beavers are and in what numbers. They’ve been spotted in Virginia and along the Hudson River, and Hood hopes to find out about more. Both he and Fairfax are also curious about how tidal beavers behave.

To that end, Hood is teaming up with the Tulalip Tribes to tag and track the beavers’ movements. The earliest results suggest that tidal beavers’ ranges are similar in size to other beavers’, indicating good-quality habitat: They can afford to stay close to home, and they’re not living in the tidal zones out of desperation.

In addition to learning about tidal beaver range and behavior, researchers wonder about the ecological role the beavers play and what that could mean for coastline restoration efforts. Changing water flow can affect salmon and other fish, as well as vegetation and shoreline erosion. “It's all going to change when beavers show up,” Fairfax said. “A single beaver is maybe not a particularly significant force, but a population of beavers in a tidal environment will be.”

While some researchers think that reintroducing beavers to tidal environments might be a conservation goal down the road, they caution that more research is needed to know how beavers affect their environment, Jordan emphasized. 

For now, “It seems like they’re doing just fine,” Fairfax said. The only other species as “bullheaded” about living everywhere and shaping their environments to their will, she added, are humans.