Beaver Survey

Beaver Survey

A small sapling that has fallen over after being chewed through by a beaver.

Mikal Jakubal

February 27, 2023


U.S. Customs agent: “What were you doing in Mexico?”


Me: “Counting beavers! We’re trying to find out how many of them live in the San Pedro River on both sides of the border.”


Agent: “Are you with the other guy?”


Me: “Probably, since there were a few of us who came down from Tucson.” (I mean, really? How many people independently tell border guards they went to Mexico to count beavers?)


Agent (returning my passport card): “Have a nice day.”


I rarely get sent to “secondary” to be searched when coming back to the U.S., probably because I have such weird reasons for being across the border that they figure no drug smuggler would bother to make up such a goofy story.


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I was, in fact, returning from the second Binational Beaver Survey on the San Pedro River, a project of multiple NGOs and government agencies on both sides of the border. 

The approximately 18 participants met at a hotel in Cananea, Sonora on Wednesday, February 22nd for an orientation by biologist Carlos Valdez Coronel from Mexican environmental organization Naturalia, then surveyed several tributaries of the San Pedro just south of the border on Thursday and Friday.

Inside a conference room with a curved window, a man stands and points to a computer screen with a picture of a beaver and some facts about beavers written in Spanish.
Biologist Carlos Valdez Coronel and a slide with a brief timeline of beavers in the San Pedro River. After the orientation we all met up at a restaurant for an informal dinner.

Highway 2 to Cananea cuts through an uninviting landscape of dry, amber-colored grass speckled with scrubby mesquite trees, looking lifeless until they green up in spring. It is the last place most people would think of as beaver habitat, but the rolling topography hides a network of perennial streams where beavers were once abundant. 

After complete extirpation by trapping and hunting in the 19th century, they are beginning to return. How many beavers are in this basin, where they are, the manner in which beavers and ranchers interact, and the effects of beavers’ presence on riparian habitat are some of the questions this survey will help answer.

A wide view of dry grasslands with scattered, leafless mesquite bushes under cloudy skies. A few black cattle graze in the distance, with low brown mountains on the far horizon.
Not what most people think of as beaver habitat. Mesquite or oak studded grasslands like these dominate much of the landscape of northern Sonora in the San Pedro watershed just south of the U.S.-Mexico border.

In 1999, the U.S. Bureau of Land Management (BLM) began beaver reintroductions in the San Pedro Riparian National Conservation Area, an officially protected corridor along the river starting at the border and ending 37 miles north near St. David, Arizona. By 2003, fifteen beaver had been released. These were all “problem beavers” transplanted from other areas of Arizona. 


By 2005, beaver population had increased to 40 individuals in 13 different sites. Ten years later, for reasons unknown, the population had nearly collapsed. Informal surveys indicate their numbers are slowly rebounding. 

At least some of the beavers migrated upstream, across the border, into the northward-flowing San Pedro’s headwaters basin, establishing populations in small perennial tributaries. 

With the exception of the Los Fresnos nature preserve where our four-person team surveyed on Thursday, this region is mostly privately owned cattle ranches. For beavers to flourish in this area will require developing an understanding of how beavers and ranching can coexist to one another’s mutual benefit.

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Rolling hills covered in dry grass with a lone tree in one of the small ravines. Sun brushes one of the low ridges, turning the grass amber. The sky is cloudy and troubled, obscuring the tops of mountains in the background. Snow can be seen on the upper slopes.
Los Fresnos nature preserve in Sonora, Mexico looking north to the Huachuca Mountains in the U.S. en route to our starting point for the beaver count. The snow on the Huachucas will melt and flow into the upper San Pedro Basin in Mexico, eventually flowing back north through Arizona.


The team I’m on today consists of Joaquin Murrieta, Binational Beaver Survey founder from Borderlands Restoration Network; Lupita, a student at Universidad de la Sierra in Moctezuma, Sonora; and Red, an intern with the U.S. National Park Service at Coronado National Memorial. We’re surveying Los Alisos Creek (known as Bear Creek on the U.S. side), looking for any signs of beaver activity: chewed trees or sticks, dams, tunnels or vent holes in the stream banks, lodges, tracks or the distinctive water-entry trails known as “slides”. 


An extremely muddy road with two four wheel drive trucks on it, their tires caked in thick globs of brown clay.
After the previous night's rain, the road in to the survey sites was...challenging.

Our reach starts at the barbed wire fence and “Normandy barrier” where the U.S.-Mexico border crosses the creek, here flowing southward out of the Huachuca Mountains in the Coronado National Forest. We are on The Nature Conservancy’s Los Fresnos (“Ash Trees”) nature preserve. Other teams are surveying streams lower in the watershed. We’ll follow this stream for a little under four miles to the large reservoir near the former ranch house where we’ll rendezvous with the other teams before heading back to town. 


A scene of a small, flowing, boulder-lined stream overhung with white-barked sycamore trees. Crossing the stream is a low barrier made of welded together railroad rail iron.
Looking north up Los Alisos Creek (Bear Creek on the U.S. side) in the Los Fresnos nature preserve. The "Normandy barrier" you see was built along the border around 2008. For reference, the shipping container wall built by Arizona's previous governor and dismantled in January of 2023 ended about a mile east of here.
Three people walk down a shallow, rocky stream bed lined with leafless shrubs.
Red, Lupita and Joaquin scouting Los Alisos Creek for any signs of beaver activity.


While our hike is lovely, the only beaver sign is a single chewed stick a quarter mile upstream of the reservoir. It’s weathered surface and location far off the present stream channel indicates it likely washed down from elsewhere in past years, possibly across the border during a flood event. While mildly disappointing to find no evidence of active beaver presence, this is still very useful baseline data.



Someone out of the frame is holding a weathered gray stick about one foot long and one inch across. The end shows distinctive bite marks of beaver teeth.
Weathered beaver chewed stick, our only sign of past beaver activity on this reach.

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So, what’s the big deal about beavers?


Imagine having magical watershed restoration gnomes who come out every night and work tirelessly to stabilize eroding river banks, trap silt, clarify water, slow down runoff so it can recharge the water table and repair damaged habitat for every other creature that depends on desert rivers. Beavers do this for free! And all we have to do is leave them alone. 



How is that not like something from a fairy tale?



A large pond surrounded by grasslands. Three people observe a beaver lodge at the water's edge, built of sticks and covered in mud.
One of two beaver lodges along the reservoir edge at Los Fresnos. These beavers do not build dams, but build their lodges on the shore with an underwater entrance. The open area atop the lodge with white sticks and no mud is an air vent.

While our furry, flat-tailed rodent-gnomes can live in tunnels or lodges on the banks of streams or reservoirs, it is their signature dam building that creates the most dramatic and observable transformations, a process biologists call “ecological engineering”. 

While all animals alter their habitat to some degree, beavers’ dam building has a significant affect on both the physical environment (stream channel configuration, groundwater levels, silt runoff, flood intensity, etc.) and on the entirety of the surrounding living ecosystem, from aquatic microorganisms to large land mammals, and from towering trees to tiny duckweed floating on the surface.

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A shallow pond with leafless willow trees growing out of the water. In the middle is a beaver lodge made of sticks and mud, looking something like a small hut.
Beaver-dammed pond at Rancho Las Cieneguitas. The low mound of sticks in the center is a very large beaver lodge. Without their dams, this would likely be a narrow, rocky stream channel.


On our second survey day, we visited Rancho Las Cieneguitas (“Little Marshes Ranch”), a stone’s throw south of Los Fresnos, where tiers of low beaver dams cascading down the valley bottom have created a sprawling perennial marshland (“cienega”).

These wetlands and pools slow runoff from storms and perennial springs, giving it time to seep in and recharge the water table. As water slows, sediment and organic matter settle to the bottom, clarifying the water and creating a nutrient rich medium that supports lush vegetation. The abundant grasses, shrubs and trees in turn provide habitat and food for the entire ecosystem.

A log partly submerged in a pond. On the end nearest the camera is a pile of beaver poop. It is a mixture of round balls and elongated "turds", with tiny wood chips visible where they have weathered and started to fall apart.
Beaver poop on a log forming part of a dam at Rancho Las Cieneguitas. Note the tiny wood chips visible.


Recent climate models suggest the U.S. Southwest is likely to experience drier winters and more flash-flood prone summers. By stabilizing and modulating water flows, beavers increase water security during droughts and lessen flood impacts. 


A broad, shallow stream lined with leafless cottonwood trees. In the foreground and across the creek are blackened, burnt grass clumps.
The managers of Rancho Las Cieneguitas burned this native forage grass along the edge of this beaver pond to remove old, dead biomass and encourage new growth.

If you were to remove the beavers and their dams, the marshes of Las Cieneguitas would likely become a narrow, rocky channel more like the Los Alisos creek bed we surveyed the day before. 

This is what happened to most of the San Pedro River in the U.S. after beavers were trapped out and their dams dynamited. Instead of the narrow river channel you now see when visiting the lower San Pedro in the U.S. (assuming it’s even flowing), the pre-colonial San Pedro was a vast extended wetland, its water lazily wending its way through innumerable mud-and-stick dams and shallow pools lush with emergent water plants.

A long, low beaver dam made of mud and small sticks holds back water to create a marsh. A large almost horizontal tree trunk stretches over the dam.
A beaver dam and fallen willow tree at Rancho Las Cieneguitas. These dams are mostly mud, reinforced with small sticks and grass. Another survey team found a dam made from mostly interwoven sticks further down in this watershed.

The owners of Rancho La Cieneguita seem proud of their beavers. Their Facebook page features a photo of “presa de castores” (beaver dam) and their other ecological conservation efforts are featured prominently.

There is a growing movement across the country to bring back beavers, letting them apply their habitat engineering expertise to degraded streams. It seems every month I hear about a new beaver book coming out, so a quick search will turn up lots of reading. If you’re in Arizona and want to get involved, the River Run Network has a Release The Beavers campaign



Also, beavers are adorable.

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