Under cover of darkness, beavers are swimming through tranquil pools in the San Pedro River. They're gnawing on tree trunks. They're building dams.
We know this because of the work of volunteers who have recently walked miles along the river searching for signs of beavers.
Next to the piled branches of one beaver dam, a volunteer strapped a wildlife camera to a tree several months ago. And in the middle of the night, two beavers repeatedly turned up in the infrared images, their eyes glowing in the darkness.
"It's so cool to see the beavers," said Mike Foster, a member of the group Friends of the San Pedro River. For years, Foster has walked the river seeking out the animals and filming educational videos that show them swimming, grooming and chewing on branches.
"When you're walking down the river, you'll see what looks like maybe a ridge, almost like a platform," Foster said. "You'll see the river flowing and then all of a sudden, you'll see a step, and then the water above the step is very calm." He's found beaver dams ranging from several inches to 5 feet high.
He's seen how some beavers start building by plowing up mud and rocks like a foundation, and how they'll drag tree branches to continue erecting their dam. Mike Foster stands by a beaver dam on the San Pedro River in November 2020.
The creatures are generally nocturnal, so Foster has learned to spot their structures and then return at dawn, when they sometimes are still out floating across the water.
Beavers were once plentiful along the San Pedro River. When fur trappers rode through in the early 1800s, they found so many that some called the San Pedro "Beaver River."
Trappers killed them to sell the prized fur, decimating their populations in streams across the country. By the late 1800s, beavers had been wiped out along the San Pedro.
More than a century later, in 1999, wildlife officials released eight beavers in the San Pedro Riparian National Conservation Area in southeastern Arizona. Five more beavers were reintroduced in 2000, and two more in 2002.
The goal was not only to bring back the beavers but also to improve the watershed's ecological health, because when beavers block the water and slow the flow, they also help boost groundwater recharge, hold back erosion and nourish the riparian habitat.
Beavers are considered a keystone species that helps the overall health of stream ecosystems, so the reintroduction plan was carried out by the federal Bureau of Land Management with the involvement of Arizona’s Game and Fish Department.
Over the next decade, the beaver population flourished and growing numbers of dams appeared. The population was estimated to have risen to more than 100 animals in 2010, when surveys confirmed a total of 39 dams. In a 2014 article, wildlife biologist Marcia Radke of the Bureau of Land Management wrote that she and others had mapped and documented the expansion of active beaver dams and had found an average of about 20 dams per year from 2000 to 2013.
Foster began walking the river and keeping an eye on the beavers years ago when he worked as a technician at Sierra Vista public schools, where he shared his videos with students. He helped Radke with the surveys over the years, but then other work came up and he stopped doing surveys.
In the mid-2010s, for reasons that aren't known, the population of beavers in the conservation area plummeted. Foster said he guesses some beavers could have been preyed upon by mountain lions or killed by humans, or might have succumbed to a combination of factors, such as lower water flows or bacteria-contaminated water.
When Foster resumed doing surveys on his own a couple of years ago, he and others were concerned about the drop in the population.
In 2019, three active dams were found in the conservation area. In 2020, Foster and others found six dams.
"The numbers went way down and now they seem to be inching back up," Foster said.
That's good news for the whole ecosystem, he said, because as beavers topple trees and build barriers in the water, they help the water soak in and give a boost to the vegetation, which helps other animals. "It's almost like gardening. But here in this case, you have an animal that comes in and does the gardening for you," Foster said. "The beaver comes in and makes the environment better for all the other members of the environment."
The North American beaver — scientific name Castor canadensis — is the largest rodent in the United States. The webbed-footed herbivores typically grow 2 to 3 feet long, not including the tail.
Beavers have been widely recognized as a species that, when they return to a habitat or are returned by humans, can help restore healthier streams, wetlands and floodplains — bringing resilience to habitats that are increasingly stressed by water withdrawals and the changing climate.
Journalist Ben Goldfarb described the vital role beavers play in shaping watersheds in his book "Eager: The Surprising, Secret Life of Beavers and Why They Matter," writing: "Beavers are agents of profound change, responsible for sculpting streams' forms and dictating their functions." Two beavers appear in front of a wildlife camera in the San Pedro River in December.
Steve Merkley, an instructor who teaches ecology and environmental biology at Cochise College in Sierra Vista and Douglas, has joined Foster on surveys and has brought along students.
In November, Merkley strapped one of his wildlife cameras to a tree by the beaver dam, and he's been collecting images of the beavers over the past several months.
In some images, he saw one beaver swimming with twigs in its mouth and placing them on the dam. Other photos showed a beaver rearing up on its hind legs in the water, looking toward the camera.
On some occasions, two beavers appeared together, clambering on the riverbank.
Merkley said he hopes it's a breeding pair and that he might eventually see young beaver kits. Though he'd known the beavers were there, he said just seeing the pictures has been exciting.
"I'm hopeful that the population will continue to increase," Merkley said.
"I think that we just need to continue to support their existence there. There's lots of room for them," Merkley said. "There's lots of good habitat that we didn't find beaver in."
For the latest survey in December, volunteers and conservation advocates from the Tucson-based nonprofit Watershed Management Group joined Foster and Merkley at the San Pedro Riparian NationalConservation Area, where they split up into groups. Walking along the banks and wading in the water, they found trees with scarred bark showing beaver-gnawing and "slides" in the riverbank where the animals had repeatedly slid into the water.
"You can see actually what looks like a little chute," said Lisa Shipek, executive director of Watershed Management Group, whose team also found other signs including chewed trees. "You do have to walk through the middle of the river to try to really see what's happening on either bank down below. So you're looking for evidence of dams. You're looking for bank lodges."
During dry times when the river is low, Foster has seen the round entrances of empty burrows exposed in the earth of the dry riverbanks.
In places, the surveyors move slowly along the river as they pick through thick vegetation. The cool weather in November and December tends to be best, Shipek said, because the rattlesnakes usually aren't out.
Shipek said this past December's outing will become her group's first annual survey, and they plan to return this coming winter with more volunteers.
Her group has also begun a larger effort to gather and document accounts of beavers along the river, which rises from its headwaters in Mexico and flows about 140 miles, in places disappearing underground, until meeting the Gila River. They've contacted scientists, conservationists and ranchers on both sides of the U.S.-Mexico border, and they've collected reports of beaver activity far north and south of the area where beavers were released two decades ago.
Shipek and her colleagues think the beavers must have moved upstream across the border into Sonora. And they've also confirmed beaver sightings along the upper reaches of the Santa Cruz River in Mexico, so they presume the beavers must have moved overland into the neighboring watershed.
"They were reintroduced into one specific area in 1999 and 2000, and today they're all over the place," Shipek said. "The beavers have been so resilient to move to so many other areas, and they've set up new populations."
Watershed Management Group has launched a campaign called Release the Beavers, advocating for more releases of animals to augment the population. The group's members plan to coordinate population surveys on both sides of the border, and they're also promoting a proposal to release beavers elsewhere in Ciénega Creek.
Trevor Hare, the group's restoration director, said he had known previously that there were some beavers in Sonora. In 2012, he had seen signs of beavers on the banks of the Santa Cruz River.
But he had no idea how many beavers there were in Mexico until he recently contacted colleagues at Profauna, a Mexican conservation group.
"The reports of beaver came flooding in," Hare said. "We now have about 22 reports of beaver in Sonora in the San Pedro watershed."
Other beavers have been found far north of the area where animals were released. A couple of years ago, Hare said, one injured beaver was found at an auto mechanic shop in Benson.
"They're resilient critters. So we're just pleased and happy that they found homes in Sonora," Hare said, and are "doing what beavers do."
The group doesn't yet have a solid population estimate for the watershed, but they hope to pull together more data in the next surveys later this year.
Hare has worked for years on stream restoration projects, enlisting volunteers to help build rock structures that slow the flow of water and help habitats recover after years of heavy cattle grazing and erosion.
"But beavers can do it a heck of a lot better than I can. They are nature's engineers," he said. And that's why he and others with the group believe beavers can play an important role in helping to restore creeks and rivers in southern Arizona.
Along the San Pedro, beavers face various threats.
The river is fed by groundwater that emerges in springs, but many segments have suffered long-term declines as wells have pulled down groundwater levels.
Foster said a live trap was found by the river at one point. Foster said he's also concerned about ranchers letting cattle wander in the river bottom, including in the national conservation area.
"When cattle get down there, they cause erosion, they eat the vegetation, and they shouldn't be down there," Foster said. "So actually practicing serious conservation in there would begin by keeping the cattle out."
When the Trump administration built new stretches of border wall over the past year, one portion of the steel fence was erected across the bed of the San Pedro River, where workers installed floodgates.
Shipek said she wants the government to ensure the border wall doesn't block wildlife along the river so that animals can move freely across the border — as the beavers presumably did at some point, moving south before their reappearance in Mexico.
She said she's convinced beavers can play a vital role in river restoration in the Southwest.
"If we're serious about restoring our desert creeks and rivers and adapting to climate change," Shipek said, beavers are "one of the best tools we have."
"You put beavers on the land and they start doing it naturally, automatically," she said. "It's what they're here to do. And so I think we need to somewhat get out of their way and let them do the work."