It is thought there could be fewer than half the beavers in the Bighorn National Forest now compared to about 15 years ago, and to make up for their work, wildlife officials have built more than 150 artificial dams, or “beaver dam analogs.”
The Wyoming Game and Fish Department and the U.S. Forest Service have also worked together to reintroduce beavers to the area.
It’s not quite clear why the beaver population has been declining.
Surveys in watersheds on the forest yielded a population estimate of 171 beavers in 2010, 113 in 2013 and 63 in 2021.
Surveys are done about every five years, or as funding is available, said Travis Cundy, Game and Fish Sheridan Region aquatic habitat biologist, in an interview with Cowboy State Daily.
‘Fake Beaver Dams’
Bighorn National Forest Supervisor Andrew Johnson was asked about drought mitigation during a meeting of the Wyoming Legislature’s Select Federal Natural Resources Committee last week.
Beaver dams in meadows have played a key role in slowing the water flow, allowing for the growth of willows and forage for wildlife and livestock, Johnson told legislators.
The decline in the beaver population has had notable effects, he said.
“We don’t have the beaver population anymore that we used to, so we replicate that by making fake beaver dams. It’s actually proven to be really simple and effective,” he said.
The population of beavers on the Bighorn National Forest in Wyoming has declined by more half in recent years. Officials have built roughly 150 artificial beaver dams to make up for it. (Courtesy Wyoming Game and Fish Department) 
This beaver, trapped elsewhere, was released on the Bighorn National Forest in 2024. The Wyoming Game and Fish Department and the U.S. Forest Service are trying to boost the struggling beaver population there. (Courtesy Wyoming Game and Fish Department) 
The population of beavers on the Bighorn National Forest in Wyoming has declined by more half in recent years. Officials have built roughly 150 artificial beaver dams to make up for it. (Courtesy Wyoming Game and Fish Department) 
The population of beavers on the Bighorn National Forest in Wyoming has declined by more half in recent years. Officials have built roughly 150 artificial beaver dams to make up for it. (Courtesy Wyoming Game and Fish Department)
Why Are the Numbers Down?
The recent decline in the beaver population is troubling, Cundy said.
“The beaver population in the Bighorns has experienced a decline over the past several years,” he said. “There is not a numerical population objective for beavers in the area, but Game and Fish is concerned about the decline and is working with the Bighorn National Forest and other partners to identify causes of the decline, conduct habitat improvement projects and increase the population where possible.”
Officials haven’t pinpointed a cause for the decline, but they have some ideas.
One might be the encroachment of conifers (such as pine trees) into riparian areas – the vegetation-rich habitats along creeks and streams.
“Increased conifers in an area can compete for sunlight, water and nutrients with more desirable riparian vegetation such as willows and aspens,” Cundy said.
Other factors might include long-term drought and grazing and browsing by livestock and wildlife affecting willow and aspen growth, he said.
“It is possible that disease, such as tularemia, may be impacting the population as well,” Cundy said.
Tularemia, also known as “rabbit fever,” is a bacterial disease that can sicken or kill numerous animal species. It’s rare in humans, but can be serious if left untreated.
Transplants Boost Population
There have also been efforts to boost the beaver population in the forest, mostly in the Bighorn Mountains, by transplanting beavers, he said.
Since 2000, Game and Fish has transplanted about 250 beavers to the Bighorn National Forest.
“Transplants are done periodically, not every year,” Cundy said.
Beavers are live-trapped in areas where they might be causing a nuisance, and then transported for release in the Bighorns, usually in September.
Live-trapping targets mated pairs, with or without kits (young beavers),” Cundy said.
“Relocating a family unit together, as well as during the fall timeframe, has been shown to have the most success in encouraging beaver to remain in the area where they are released. In the fall, beaver are more likely to begin caching winter food, rather than relocating to new areas,” he said.
“In recent years, relocations have taken place on Owen, Sourdough and Muddy creeks,” Cundy added.
Population Surveys
Beaver population surveys are most effective when done from the air, and October is the best time to do them.
By then most of the trees have lost their leaves, making it easier to see what’s on the ground in beaver habitat, Cundy said.
And that’s usually when beavers start caching food for the winter.
“During the surveys, observers look for active beaver dams as well as cached food sites. Inactive or historic beaver dams are also noted during the survey. These are dams that have been used in the past but are not in active use and have been grown over with vegetation,” Cundy said.
During the 2021 survey, beaver activity was documented in eight watersheds on the Bighorn National Forest.
“Several active dams were documented, and six food caches were seen - 4 in the northern Bighorns and 2 in the southern Bighorns. In the 2013 survey, 15 food caches were seen - 5 in the north and 10 in the south,” he said.
Each food cache in willows is estimated to support 4.5 beavers. It’s thought that 35-to-40% of caches aren’t visible from the air, he said.
Artificial Dams Might Lure Beavers Back
The beaver dam analogs are built by sinking posts vertically, then weaving in willow and packing the structure with mud and rocks.
“The structure mimics the activity of a beaver dam, by slowing water flow, raising the water table and encouraging plant growth such as willows,” Cundy said.
Beavers might also put the structures to use, he added.
“Over time, the changes these structures make may entice beaver to naturally repopulate the area and also serve as potential transplant locations in the future,” Cundy said.
Mark Heinz can be reached at mark@cowboystatedaily.com.
