Beaver Dams Help Wildfire-Ravaged Ecosystems Recover Long after Flames Subside

Oregon endured the third-largest wildfire in its recorded history last summer. The Bootleg Fire tore through the Upper Klamath Basin, an ecologically sensitive area that is home to multiple threatened and endangered species including the northern spotted owl and two fish—the koptu and c’waam (shortnose sucker and Lost River sucker)—that are culturally vital to the area’s Klamath Tribes. The fire left behind a charred landscape more than twice the size of New York City.

After the local fire season ended in autumn, Bill Tinniswood, a fisheries biologist with the Oregon Department of Fish and Wildlife, went out to survey the damage. Ash from the fire, which burned for more than a month, had clogged formerly pristine tributaries and turned them into black slurries. Thriving trout populations had disappeared, presumably choked to death by waterborne debris particles that deprived the fish of oxygen. “I was in total shock,” Tinniswood said. “It just looked like devastation.”

Then Tinniswood and his team stumbled upon something even more surprising, and somewhat encouraging: roughly five acres of pristine greenery amid an otherwise burned-out area along Dixon Creek, a tributary in the Sprague River watershed. At the center were roughly eight active beaver dams. But this was more than a refuge from fire, which hundreds of beaver dams are known to have afforded to other riparian areas. Whereas fish seemed to have disappeared upstream of the Dixon Creek dam site, the downstream water was crystal clear—and trout were thriving as though the fire had never happened. The dams and ponds appeared to have altered the hydrology of the landscape around them, Tinniswood says. The beavers had effectively built something like a water treatment plant that staved off fire-related contamination.

Similar dam-driven refuges have been documented from Colorado to California, Idaho to Wyoming. Now, scientists are discovering that these green sanctuaries are part of a larger story of how beaver dams contribute to fire resilience. Along with deterring the flames themselves, beaver dams and ponds also function as filters for ash and other fire-produced pollutants that enter waterways—thus maintaining water quality for fish, other aquatic animals, and humans—emerging evidence suggests.

Tinniswood isn’t the first to observe that beaver dams protect streams from the toxic effects of postfire runoff. In the past several years, as climate change has ramped up wildfire frequency and intensity throughout the western U.S., similar accounts have come in after fires across the region. These range from the 2018 Sharps Fire in Idaho to the 2020 Lefthand Canyon and Cameron Peak fires in Colorado. Ecohydrologist Emily Fairfax of California State University Channel Islands, who personally made such observations in Colorado, says such findings support efforts to conserve and reintroduce beavers in the West, and to establish human-made structures that mimic beaver dams—a growing movement in riparian restoration.

As many as 200 million beavers once engineered ecosystems throughout North America. When European settlers moved across the continent, they brought the fur trade with them. “They proceeded to trap as much as they possibly could … and then they would move on to other locations,” said Alex Gonyaw, a fisheries biologist for the Klamath Tribes in southern Oregon. Between the mid-17th century and late 19th century, North American beaver populations plummeted to between 10,000 and 100,000 individuals, according to some estimates. After the fur trade largely ended, beaver numbers have bounced back to around 15 million throughout the continent. Still, some urban officials and rural landowners consider beavers a nuisance when their activity floods roads and neighborhoods, blocks culverts or fells trees.

After the Bootleg Fire in Oregon, conservation nonprofit Trout Unlimited rushed to build “beaver dam analogues” at Harmony Preserve, a private wildlife refuge along the North Fork Sprague River, before fall rains washed ash and other debris into the watershed. Here, contractors help to build post-assisted log structures, or PALS, which function much like beaver dam analogues. Credit: Charles Erdman, Trout Unlimited

Beavers have good evolutionary reasons for their aquatic lifestyle. On land they move awkwardly, standing out “like giant chicken nuggets,” Fairfax says. But in water they can nimbly hunt food and hide from predators, which gives them scope to build complex dam systems that allow rivers to connect and spread out over their floodplains, expanding the footprint of watery sanctuaries.

The filtration provided by dams is crucial for the surrounding ecosystem. In the aftermath of wildfires, autumn rain and spring snowmelt wash sediment into waterways—including ash and other debris, and soil that vegetation normally would hold in place. This pulse of pollution can be deadlier to aquatic life than the fire itself, Tinniswood said. Just as humans struggle to breathe air that’s thick with smoke, fish can’t take in enough oxygen from water laden with sediment that their gills are not designed to block.

Beaver dams and ponds filter out sediment by slowing the rate at which water flows, says researcher Sarah Koenigsberg at the Beaver Coalition, an Oregon-based nonprofit organization that promotes conservation. When water lazily drifts through a beaver pond rather than rushing in a torrent down a narrow channel, suspended sediment has time to settle on the bottom where it poses less risk to fish and other aquatic animals. “You can almost think of it like a coffee filter,” Koenigsberg said.

Koenigsberg observed these effects firsthand in the aftermath of the 2020 Almeda Fire, which destroyed two towns along the Bear Creek watershed in southern Oregon. In one burned portion of the creek, sediment from the fire had formed a deep sludge just above a beaver dam. “That’s homes, that’s tires, that’s asbestos,” Koenigsberg says. But below the dam, the water was crystal clear.

Anecdotes about beavers’ potential to preserve water quality and aquatic life after wildfires have generated excitement in the scientific community over the past several years. “The idea that this is an important postfire function is very new,” Fairfax says. Formal research on the subject has only just begun. Outside of the fire context, it is well-established that beaver dams could act as sediment filters, Fairfax says—after all, erosion doesn’t only happen after wildfires. When biologists compared sediment flow above and below a complex of beaver dams in Devon, England, in 2017, they found that for every 112 milligrams of ordinary erosion sediment flowing into the complex, only 39 milligrams flowed out. This study’s results, published in Science of the Total Environmentsuggest that nearly two thirds of the material had settled to the bottom of the beaver pond above the dams, keeping the water column and the riverbed below the dams clear. It is not a huge leap to suggest beaver dams would have a similar effect on fire-related sediment, Fairfax said.

For riparian habitats in the Klamath Basin, some of the worst effects of the Bootleg Fire could still lie ahead, Tinniswood says. Heavy snows have blanketed the region this winter. When all that snow melts in spring, new pulses of ash and other sediment could wash into the rivers. “It could be real bad,” Tinniswood adds. 

A solution could come in the form of human-made structures called beaver dam analogues (BDAs) and post-assisted log structures (PALS). The latter are designed to mimic the natural buildup of woody debris in a healthy river system and function similarly to BDAs. The U.S. Fish and Wildlife Service and nonprofit conservation group Trout Unlimited, in collaboration with private landowners and river restoration firm Anabranch Solutions, has already placed 41 BDAs and 66 PALS on the North Fork of the Sprague River, abutting Bootleg Fire’s burn scar. Before that fire, the project had been planned for 2022, with the goal of creating healthy wetland habitat. But then Bootleg happened. “Everything upstream of the property burned,” says Charles Erdman, a restoration project coordinator for Trout Unlimited. Erdman and his colleagues worried that autumn rains would soon bring an influx of sediment rushing down the North Fork. “We decided we need to do this project now.” So last November his team rushed to build and install the structures, using juniper branches, sod, pine boughs and other on-site natural material.

Erdman also plans to place five new BDAs on the Sycan River, a tributary to the Sprague. For two years following this installation, Trout Unlimited will collect data on water quality, fish populations, vegetation, and groundwater quantities around the Sycan site for comparison with data collected before the installment of the BDAs.

After the 2018 South Sugarloaf Fire scorched 230,000 acres in northeastern Nevada, the U.S. Forest Service installed BDAs in anticipation of postfire runoff. That same year in Idaho, a coalition of government agencies and nonprofit organizations collaborated to build 100 BDAs along a five-mile stretch of Baugh Creek, in part to prevent runoff from infiltrating the aquatic ecosystem after a fire there.

BDAs are certainly not a replacement for beavers, notes Joe Wheaton, a fluvial geomorphologist at Utah State University and one of the scientists who developed the analogues. They are “leaky sieves” that should be thought of only as a temporary solution, he adds. “If they’re not maintained, they don’t have the same benefit.” Without beavers around to do this work, the structures will eventually wash away.

The hope is that by creating a riparian area that more closely resembles beaver habitat, BDAs will attract the animals to the area or allow them to be readily introduced, the Klamath Tribes’ Gonyaw says. The Tribes recently received $20,000 from the Oregon Conservation and Recreation Fund to initiate such a project in the spring. The first step will involve planting native trees for future beavers to gnaw on. Then, construction of BDAs and lodgelike structures will begin. Gonyaw estimates the full project will be completed in three to five years. “It’s a work in progress,” he says. And it’s a step toward a riparian landscape that more closely resembles the one that existed before the fur trade, he adds—one that evolved alongside wildfire, and was resilient to it.