Greg Hood’s fieldwork takes him deep into the Salish Sea’s gnarliest habitats—often waist deep. 

“The trick to falling in a marsh is to just let yourself go,” he says, shortly before unintentionally demonstrating the technique by slipping on a muddy slope and splash landing in a tidal channel. 

“If you try to fight it,” he grunts, while struggling free of the suckling muck, “you’re more likely to break your leg.”

Perhaps Greg’s finest fall came when he stumbled upon something totally unexpected in the Skagit River estuary. While hiking transects to study the link between nurse logs and shrubs in tidal marshes, he noticed a pile of sticks that looked purposely woven across a small channel. He wondered if it might be a beaver dam. 

The area Greg was surveying as senior research scientist for the Skagit River System Cooperative, which studies and manages natural resources for the Sauk-Suiattle and Swinomish Tribes, offered nothing like what’s considered traditional beaver habitat. Instead of a green, thickly wooded valley wetted by a slow-moving freshwater river, this tidal marsh was a harsh, low-lying tangle of shrubs, thorns and cattails. Not to mention it was exposed to huge sea tides. 

“I calculated that the dam would be submerged by at least a meter of water during high tides,” he says. 

What good is an underwater dam? Greg called a top beaver expert. “He’d never heard of them living in tidal marshes, and said he’d be shocked if they were.” 

Hit hard by humans

Beavers are semi-aquatic critters, not marine mammals. They’re North America’s largest rodent, second only to capybaras for the world title—third if you count Mickey Mouse. Before Europeans arrived, more than 60 million (some estimate as high as 400 million) beavers busied themselves in every suitable river system on the continent and shaped much of its ecology. As far as keystone species go, it’s hard to find one that’s more key than beavers. Or one that was hit harder by humans.

By the early 1900s, the fur trade had driven the population of North American beavers (Castor candadiensis) nearly to extinction. Since then, the U.S. has been forced to spend billions on ecosystem services that beavers always provided for free. Fortunately, we’re now living in the big beaver rebound, with scientists and a growing number of enlightened wildlife managers realizing how much was lost just to feed a fetish for felt hats. Today, beavers are valued for their ability to raise water tables, refill aquifers and repair incised streams, mitigating wildfire risk and helping to satisfy thirsty farmers and forests. Their ponds and meadows also support everything from important insects and bats to plants, waterfowl and moose.

Here in the Salish Sea watershed, the loss of beaver-engineered habitat was also a big contributor to salmon decline. One study concluded that on just the Stillaguamish River, removing beavers caused a 700 percent decline in Coho smolt-rearing habitat.    

Beavers were missing for so long that we forgot how hardy and wide-ranging they are as a species. It became common knowledge that they only lived in freshwater. “It’s true that they can’t tolerate high salinity,” says Greg. 

But when he measured salinity in the tidal shrub marsh, Greg found that it was only about a sixth as salty as Puget Sound seawater. “And there’s also a freshwater lens effect,” he says. As tide-driven brackish water rises in the spongy marsh soil, there’s enough less-dense freshwater floating on top of it to sustain salt-intolerant plant life as well as populations of briny beavers.

The big tides were a tougher riddle. Why would beavers expend energy to build dams that are submerged half the time? 

The answer came to Greg—or more precisely came at Greg—as he was leaving the marsh after a long day of fieldwork. “It was low tide,” he says, “and the sea level had dropped a couple meters below the bottom of the channel I was walking in.” But he was still wading in calf-deep water held back by the small, beaver-built barricade of sticks and mud artfully erected across a narrow section of channel. 

“I look up and here comes an adult beaver swimming right towards me.” The beaver was as shocked as the scientist and turned tail before Greg could get a photo. But the encounter was a revelation.

Greg realized that dams in these tidal channels weren’t failed attempts at holding back all the water that floods in. “They’re engineered just to stay deep enough at low tide that the beaver can still use them for transportation.”

Beavers are ungainly on land and  make easy meals for bears, wolves and cougars. But they thrive surrounded by all these predators by creating ponds as moats, by digging canals when they need to reach new food sources, and by building their lodges like diving bells, with moon-pool underwater entrances.

The tidal marshes won’t support large ponds, but by ensuring that tidal channels stay swimmable even at low water, beaver can safely range around the estuary without having to waddle onto dry land like a stubby legged meatloaf served up to predators. 

Refuges for salmon

Once he knew where to look, Greg found more beaver clans inhabiting tidal shrub marshes in other Salish Sea river deltas. Wading into history, he also unearthed evidence that they’ve always been there, with at least one local tribe traditionally setting up seasonal camps to hunt tidal beavers. 

If these remnant beaver populations have inhabited and engineered the shrub marshes long-term, Greg wondered what associations have formed with other wildlife. He sampled pools behind the dams and found something potentially far more significant than the rediscovery of salty beavers.

“Along with sticklebacks and juvenile lamprey, there were Chinook salmon fry using the beaver pools at low-tide,” he says. 

And there weren’t just a few and they weren’t there by accident.

“Densities of Chinook salmon were three times higher there than in low-tide channels without beaver dams.”

Beavers have been creating refuges for salmon. Along with retaining water at low tide, the dams slow and swirl the outgoing currents so the tiny fry aren’t flushed out into big water and big hungry mouths. Instead, the salmon enjoy calm baby pools filled with nutritious bugs that are attracted to the plant detritus collected behind the dams. 

To make life even easier for the young of these ESA-listed salmon, the banks of the narrow tidal channels are crowded with shrubs that, Greg theorizes, make it difficult for fish hunters like great blue heron to land. Since the beavers’ pools are too deep for wading birds, the combination makes the tidal shrub marshscape a bountiful salmon sanctuary.

“Protected from predators and given more time and food to grow larger,” says Greg, “means that these Chinook fry will have higher survival rates once they head out into the sea.” 

Greg’s findings show that all along we’ve had a salty, buck-toothed silent partner in our salmon restoration efforts. If beaver still occupied all the tidal shrub marsh that once fringed our shoreline, salmon numbers might be very different today. Unfortunately, they can’t. By diking and draining, we’ve already destroyed 95 percent of the tidal shrub habit in the Skagit and even more in our other river deltas. Shoreline and riverbank armoring also mean that tidal shrub marsh will likely be unable to migrate landward to adapt to sea level rise, making these forbidding fragments of marsh the briny beavers’ last stand.

The good news is that we know they’re out there. And maybe there are ways for us to help them help us even more with critical salmon recovery. We know they’re beneficial, but we know very little about the tidal beavers themselves. To remedy that knowledge gap, Skagit River System Cooperative partnered with the SeaDoc Society and the Tulalip Tribes’ Wildlife Program last summer to place the first-ever satellite tag on a tidal beaver (video).

The Tulalip Tribes run one of the U.S.’s most successful beaver relocation programs. As beaver rebound into their historic habitat, they often find that what was once a babbling brook watering a lush meadow is now a storm sewer astride a big box parking lot. But with their famous work ethic, the beaver still get right down to business. That can lead to conflicts.  

“There are many methods we can try to prevent beaver from damaging property,” says Dylan Collins, a wildlife biologist with the Tulalip Tribes. But if the arsenal of excluders, dissuaders and pond levelers is unsuccessful, the beavers may then be labelled as  “nuisance wildlife.”

“Nuisance beavers are just doing what beavers do,” says Dylan. “They just happen to be doing it somewhere where it causes problems for people.”

The scarlet N was always a death sentence for nuisance beavers until the Tulalip Tribes became the first group in Washington State permitted to relocate instead of terminate. 

Beavers live in tight-knit clans, with parents mating for life and kits living in the family lodge for two years before striking out on their own. So, the Tulalip Tribes always trap and transport entire families together. At their release site in the Mt. Baker-Snoqualmie National Forest, the occupancy of prime beaver habitat has increased from 35 percent before relocations began to as high as 90 percent now, with a corresponding leap in stream and salmon benefits.   

Return to the wild

The experience gained on nuisance operations is one of the keys to the groundbreaking tidal beaver tagging success. It is late August, and as sunrise leaks through the morning mist, Dylan leads a field science crew into a shrub marsh that feels like the Land That Time Forgot. No one would be surprised to encounter a seven-foot-long Pleistocene beaver back here, but from a raven’s point of view we’re only a few hundred meters from a residential neighborhood just north of Everett. It’s likely that tidal beavers have continuously inhabited this spot for hundreds of years without ever being seen by humans because normal folks would turn back the first time they fell into waist-deep black mud. Or the second time. Or third. 

The VIP of the group is a 46-pound female beaver calmly riding in a cage rigged like a palanquin fit for the queen of the tidal marsh. Glued to the base of her paddle tail is a GPS capsule tag. The hope is that she’ll introduce us to the world of briny beavers and provide insight into questions like how large a range does her family need to support itself.

The cage is opened a few meters away from the lodge where we captured her days ago. After a little coaxing, she lumbers out, pauses for a moment, and then dives into the black water, slapping her tail on the surface in what is unmistakable beaverspeak for “Get your muddy butts outta my marsh!”

She disappears into a tidal pool that her family created to keep them safe. They didn’t build it to help recover salmon populations, but it is. And that makes these newly rediscovered tidal beavers the very opposite of nuisance wildlife.