The crew fans out along a riverbank, boots sinking into the mud as they follow the San Pedro’s patchy channel when someone suddenly yells, “BEAVER!”
From his side of the channel, Brick Cummins can see only ripples. But across the bank, a small face pops out from beneath a log — a beaver, small paws gripping a stick as it munches contentedly in the shade.
“I honestly think this is the only time anyone on these surveys has ever seen a beaver — like, officially,” Nadira Mitchell later said.
Mitchell is the restoration ecology coordinator for Watershed Management Group, a Tucson-based nonprofit environmental organization that has been running a binational beaver survey to estimate the number of beavers in the San Pedro River since 2021.
The survey is run entirely by volunteers, with each group assessing at least a 2-mile stretch of river. In 2024, multiple groups surveyed 77.5 miles of river on both sides of the border. This year, they’re covering closer to 50 miles.
“This year we aren’t doing the middle San Pedro — kind of in the Cascabel area — because it was so dry last year and again this year, and there wasn’t really any activity last year,” Mitchell said.
Traces of a beaver
It’s rare to see a beaver during daylight hours. They are crepuscular, meaning they are most active at dawn and dusk. So volunteers look for evidence of beaver activity — the small traces they leave behind as they prepare for winter.
“We’re not actually seeing beavers, but we’re creating a population estimate based on their activity,” Mitchell said.
As Mitchell and the group waded through the water and navigated the brushy banks, they searched for beaver gnaw marks on cottonwood and other trees.
If chewing activity is fresh enough that wood chips are still scattered nearby, Mitchell collects the chips in a clear red plastic bag and sends them to Cochise College for genetic analysis.
But eating wood isn’t all beavers do.
“They create wetlands that increase vegetation and biodiversity in these areas,” Mitchell said. “They basically create these awesome ecosystems.”
She called them habitat engineers, as beavers are known to modify their surroundings to support their survival. Beavers build dams out of sticks, mud and other natural materials to stop flowing water.
Get involved
While this was the last survey of the year, those interested in helping with the organization’s beaver initiatives can sign up for upcoming events.
“They basically build these dams to stop the flow of the river or to ease it a little bit and make it kind of more of a ripple or a trickle,” Mitchell said.
By stanching the flow of water with dams, they flood the surrounding area, creating a beaver pond and making it easier to swim through the terrain and increasing protection from predators.
“It’s easier for them to have their lodges — their homes — and it’s easier for them to transport branches back and forth,” she said.
For these reasons, other indicators like beaver lodges, slides and scent mounds are sought as signs of beaver activity in the area.
The retained water also helps replenish groundwater supplies. A United States Geological Survey study on the effects of beaver reintroduction in the upper San Pedro River, Southeast Arizona and Northern Sonora, Mexico found that USGS wells “adjacent to beaver ponds showed a measurable increase in groundwater levels.”
Beaver-inhabited areas are wetter and more biodiverse, attracting far more wildlife than just beavers — especially in dry Arizona. The same USGS study also found an increase in abundance and richness of bird species in the areas due to beaver presence.
A history of beavers in Arizona
Today, beavers are rare in Arizona, having been decimated in the 1800s due to fur trapping. But they were once so plentiful that the San Pedro River was named the “Beaver River” by Anglo-American trappers in 1825.
In the early 1900s, beaver culling continued due to damage concerns, further diminishing the population. Throughout the rest of the century, the Arizona Game and Fish Department worked to reintroduce beavers across the state. In 1999, Arizona Game and Fish and the Bureau of Land Management reintroduced 40 beavers to the San Pedro Riparian National Conservation Area, established in 1984 near Sierra Vista.
“Arizona Game and Fish and BLM were able to monitor the population for a bit, but obviously they have to prioritize so many different things and so many different areas, so that population became less of a priority to continue monitoring,” Mitchell said.
While the beaver population eventually reached 100 beavers by 2010, according to Watershed Management Group’s 2024 binational survey results, Mitchell said residents later began noticing a decline. That’s when citizen science stepped in.
“In 2015, a community member — his name is Mike Foster — started doing his own beaver surveys because he noticed the beavers weren’t as abundant,” Mitchell said.
Watershed Management Group formalized the process in 2021 and began organizing the surveys annually. Now, the results are confirming residents’ suspicions.
“What we’re seeing based on last year’s results is that there are fewer than 40 beavers on the San Pedro River,” Mitchell said.
Though the year’s beaver surveys are over, people can mark their calendar for next year’s volunteer surveys in the fall.
“People can look out for other restoration project volunteer opportunities as well like helping with those post-assisted log structures,” Mitchell said. These projects involve building structures that mimic beaver dams to help restore the river habitat.
All of these activities form part of the organization’s River Run Network program that is open for interested community members to sign up here.
Mitchell said she was grateful to have seen a beaver for the first time during the survey but remains concerned by the lack of fully constructed lodges or dams in the river channel.
She still has miles of data to sort through to reach a new population estimate, but if the trend continues, she expects the number to drop further.
“Last year, based on the evidence, we estimated there were two family groups in this stretch, which I’m not sure is correct now,” she said. “I think there’s less than two family groups now.”
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