Arizona’s Beloved San Pedro River Is about to Meet the Rising Wall on the US-Mexico Border

As crews begin work on a 30-foot fence across one of the Southwest’s last free-flowing rivers, conservationists worry about cost to ecosystem.

THE BIRDS ARE the first sign, their scattered song piercing the quiet. A breeze skips through the grass as the arid land slopes downward and soil begins to soften underfoot. Water can’t be far.

photo of San Pedro River
In a region where water is scarce and highly managed, the San Pedro is rare because it has no dams. Photo by Russ McSpadden.

Tangled mesquite trees come into focus, then towering cottonwoods, unveiling a canopy of green. They lead at last to the San Pedro River, a cradle of life in the Arizona desert.

The San Pedro River appears in satellite images as a narrow band of moisture slithering against a rugged and dusty terrain. It’s been called a “ribbon of green” and a “national treasure,” and is one of the most studied and beloved rivers in the Southwest.

In a region where water is scarce and highly managed, the San Pedro is also rare because it has no dams. The free-flowing river changes with the seasons, sustaining life for hundreds of species that have adopted the river’s cycles as their own.

Juliet Stromberg, a retired plant ecologist who made her career studying the San Pedro, remembers the first time she saw it nearly 30 years ago.

“I had this foreboding about the river,” she said. “I was just so desperately worried about that river.”

Though it has remained undammed, the San Pedro has not remained unscathed by human development. It has been harmed by decades of groundwater pumping, cattle grazing, and climate change. Now it faces yet another test. President Donald Trump’s border wall is on track to cut across the riverbed where the San Pedro flows from Mexico to Arizona, threatening to disrupt the river’s course and with it, its vibrant ecosystem.

Border officials have evaluated how to build the barrier through a floodplain even as 30-foot fencing rises elsewhere along the southern border. The US Border Patrol rejected an initial design for the San Pedro, saying it didn’t account for the river’s powerful floods, and in June officials announced a new design involving a series of gates.

Bulldozers tore down cottonwoods months ago in preparation. By July, construction was underway.

The wall will bisect what Stromberg, who has studied dozens of waterways across the Southwest, sees as the last, best example of a wild river.

She looks at the San Pedro and sees the landscape’s potential. “This is how these rivers work and function,” she says. “Here’s what these rivers could be.”

THE SAN PEDRO RIVER begins as water trickling down mountainsides in northern Mexico. It flows north, crossing the US border through grassland, forest, and prickly desert — three biomes converging. It curves past mining outposts and cities eager for water.

The river has fended off threats since the days when conservationist Michael Gregory floated down its waters in an innertube. Those were the 1980s, when the river’s southern reach was owned by Tenneco Oil Company and ranchers regularly let cattle graze along the banks.

Gregory, a northerner unaccustomed to the desert, began advocating for the river soon after he moved to Arizona to work for the Forest Service.

“I like green. The San Pedro was green,” he said.

After years of negotiations with landowners and activists like Gregory, Congress made the San Pedro River the nation’s first Riparian National Conservation Area in 1988. Some 56,000 acres of river habitat hugging the US border with Mexico were newly protected for the sake of science, recreation, and education. The protections largely ended cattle grazing near the river and gave activists further power to challenge pumping from the groundwater beneath it.

Today, the San Pedro River is a nesting ground for hundreds of bird species, from violet-crowned hummingbirds to Mexican spotted owls. An estimated 40 percent of all North American birds use the San Pedro River at some point in their lives.

The river is also home to rare fish and frogs and wetland plants. Before Covid-19 forced parks to close, dozens of visitors departed each week on nature walks from a white cottage called the San Pedro House. Volunteers carefully marked wildlife sightings in a daily log.

Photo of wildlife log The wildlife log at the San Pedro House.Photo by Helen Wieffering.

The San Pedro has risen in importance as similar habitat in the Southwest has been lost to drought or development. In 1999, the North American Commission for Environmental Cooperation declared the river’s importance on the continent “beyond dispute.”

The San Pedro “supports one of the richest assemblages of biodiversity in North America,” it said. “Indeed, there is mounting evidence suggesting that more birds use the upper San Pedro now than ever before.”

More than 65 species in the San Pedro’s upper and lower watersheds are considered vulnerable by the Arizona Game and Fish Department. But only 47 miles of the river — about a third of its length — are federally protected within the Riparian National Conservation Area. The wall now threatens the southern edge of that sanctuary.

Gregory gave a speech at a protest against the wall after the cottonwood trees were tagged for removal this winter. In a yellow construction vest, his white hair pulled back, he spoke of the protections he helped win for the river more than 30 years ago, and which President Ronald Reagan signed.

“Now another president is trying to override dozens of laws and regulations,” he said, “to build a wall that will serve no purpose but political ambition.”

SANDY ANDERSON HAS LOVED cottonwoods since her childhood growing up on the Lower Colorado River. Since 1991, she’s cared for a grove of trees on the banks of the San Pedro, just a short walk from her front door.

“I usually try to always see the bright side,” Anderson said of the chance the wall might be defeated. “But they’ve already cut the trees.”

Anderson makes her living on the wildness of the river. She runs the Gray Hawk Nature Center, so named because she once planned to be a bird guide. Now, as an educator — “and a naturalist and a tracker and a nature lover and a tree hugger” — she makes the river her classroom.

In a regular school year, when busloads of visitors arrive for field trips, she sets students and volunteers off to pull invasive weeds, care for reptiles in her “snake house,” or journal on the banks of the river. She believes in letting young children run free — muddy and happy.

Anderson does what she can to help the river thrive. Just beyond her porch, she’s restored the native sacaton grasses that decades of cattle grazing nearly wiped out before the river was protected. She’s more proud of that feat than any other of her projects on the river.

“Destroying grassland is easy; anybody with a plow or a torch can do it,” she said. “Bringing it back is hard.”

Two-hundred miles north in a Phoenix lab, botanist Liz Makings stores samples of sacaton grass in metal cabinets. Makings oversees an herbarium of 24,000 plant species, each dried and pressed between pages of archival paper, which botanists call a “flora.” Six-hundred of those plants belong to the San Pedro’s conservation area; Makings collected most of them herself.

“It’s a labor of love,” she said.

Twenty years after starting her collection, Makings still returns to the river at least once a year to look for vegetation washed in from Mexico or the Huachuca Mountains in Arizona. “When you do a flora, it’s kind of like your area,” she said. “You never, I don’t think ever, stop looking for plants.”

Countless other scientists have spent their grad student days wading through and studying the river. The reason is simple: No other river in the region has the freedom to respond to rains and floods like the San Pedro. Most are controlled by dams that limit their flood potential. Others are withered from groundwater pumping as cities and suburbs expand.

Seven hydroelectric dams known as the Salt River Project manage water in Arizona’s Verde and Salt Rivers, and the Gila River meets at least four dams as it flows west. The Colorado River, a major source of water for cities and industry in both the US and Mexico, rarely meets the ocean as it once did.

“The story of rivers in the Sonoran Desert is largely a story of ‘rivers no more,’” the Arizona-Sonora Desert Museum wrote in its annual magazine in 2014. The same issue includes a requiem for the Santa Cruz River, once free-flowing like the San Pedro until farming and irrigation drained it of water.

Makings already oversees a collection of plants that no longer grow along the rivers they were sourced from. Floras, she said, can serve as a living record of a place — or as a record of plant life that’s been lost.

When the Bureau of Land Management sought public comment over plans to reintroduce grazing in the San Pedro’s conservation area, Makings emailed four thousand words arguing against it. “The grasses on the San Pedro are just that — grasses. They are not ‘forage for cattle,’” she wrote.

The plants she’s studied “provide untold benefits to the ecosystem,” she added, “from the soils they stabilize to the microbes and invertebrates that make them their home, to the animals that depend on lower organisms.” But the BLM went forward with lifting some restrictions on grazing last year. Conservation groups have since sued the agency over the move.

Grazing and the border wall both impose a human footprint on a place Makings wishes to be left alone. For her, the San Pedro River is all the more vulnerable because its beauty and importance aren’t obvious to the casual eye. “It’s not going to blow your socks off if you’ve never been there before,” she said in an interview. “You’re not going to kayak the San Pedro.”

“It’s really, really a subtle place,” she added quietly. “It’s a subtle beauty, but it’s amazing.”

WATER LEVELS IN THE San Pedro River have declined to a worrying degree for many who cherish it as a desert oasis. In 2018, the Nature Conservancy found just 23 percent of the river had water in peak summer heat, the lowest percentage on record since the survey began in 1999.

Hydrologist Holly Richter started that effort to map the dry sections of the river channel. At the time, Richter said the San Pedro’s health was awash with controversy: Some stakeholders thought the river was dead already while others found nothing wrong.

So she paired councilmembers with scientists or realtors and set them off tracking the river’s surface water with a GPS device. She lives in the river’s backyard, in Palominas, and still spearheads the effort every June, coordinating with volunteers and agencies to map all 150 miles of the river from its Sonoran headwaters to the Gila River confluence. The data from this year’s survey is still being processed.

The dry segments that she and volunteers have tracked over time are largely the result of climate change — fewer inches of rain — and groundwater pumping for drinking and industry. Without rain, groundwater is the only supply of water to the river.

Environmental groups have repeatedly sued federal agencies for allowing pumping to deprive the San Pedro of water. Fort Huachuca, a nearby US army base that is one of the biggest strains on the aquifer, has been a frequent target of litigation despite reducing its water use by 65 percent from 1993 to 2014, according to the University of Arizona.

“Every gallon of water that we pump out of the valley is one less gallon of water that would have eventually made it through that underground aquifer to the river,” Richter said. Yet desert communities have no other place to source their water than underground.

Some stretches of the San Pedro now only flow when heavy monsoon rains send it churning in late summer. The floods are sudden and, in the undammed river, uncontained. Damp sediment is deluged by rushing water and whole trees can be washed away.

photo of San Pedro River
Only 47 miles of the river — about a third of its length — are federally protected. The wall now threatens the southern edge of that sanctuary. Photo by Michael Foster.

Richter wonders how long a border wall could withstand that force. A highway bridge close to the border was rebuilt and raised in 2016 to account for the San Pedro’s unpredictable floods.

Even if the wall remained standing, a dense barrier could create a stagnant pileup of debris, halting the floodwaters that replenish the river’s ecosystem. The chaotic downpours are critical to plants and wildlife, scientists say, irrigating cottonwood and willow seeds and sprouting plants that feed insects, birds and fish.

Can a desert river still be free-flowing and flood-driven across a bollard fence? “That question is a newer, more complicated question than we’ve ever had with these border issues before,” Richter said. Future months could bring a test of strength between the San Pedro River and the border wall, each one endangered by the other.

IT’S BEEN MORE THAN a year since the Trump Administration announced plans for the border wall over the San Pedro River, part of a larger effort to enforce border security between the US and Mexico. The river had no label on the Border Patrol’s maps last May — it was the unnamed reason for 0.3 miles of new wall highlighted in green.

The low-hanging fence that once crisscrossed the riverbanks will soon be replaced by swinging gates topped by 30-foot fenceposts to match the wall stretching east and west of the river. A spokesperson for US Customs and Border Protection (CBP) confirmed that crews have started digging trenches for the new wall’s foundation.

The former fencing, which was built of old railroad rails and removed every flood season, was a compromise between Department of Homeland Security (DHS) officials and federal scientists during an earlier round of border construction in Arizona. According to documents obtained by the Center for Biological Diversity, the Bureau of Land Management wrote to DHS in 2007 that the river’s floods were “essential” to the ecosystem, and that bollard fencing could lead to “a wall of sediment,” not to mention additional maintenance and cost.

Public comments on the current round of border construction have raised many of the same concerns. In a June 12 webinar that was recorded and shared with Earth Island Journal, CBP officials said they rejected an initial design based on feedback that it could block debris and be damaged in a flood. Though officials say the new design will allow floodwaters to pass, similar gates at other places on Arizona’s border have sometimes failed. In 2011 near Lukeville, and in 2014 near Nogales, sudden rains toppled sections of the barrier when gates were left closed. (In June, the Supreme Court declined to hear a case brought by environmental groups challenging certain sections of the border wall.)

Former acting secretary of homeland security Kevin McAleenan waived a host of environmental laws to expedite construction. He said he did so for the sake of national security, citing border apprehensions in the Tucson sector — 52,000 in 2018 and 63,000 last year — that put the San Pedro in one of the busiest corridors for illegal immigration.

Anderson used to frequently spot migrants following the river near her land. “I was constantly picking up backpacks, with like, life in them. Pictures of kids, pictures of families,” she said.

“Chokes me up — literally life in these backpacks,” she said.

She thinks the wall will only push migrants to further extremes. That it will stop everything — water, branches, animals — but what it’s intended to stop.

Stromberg, the ecologist, said it’s too soon to say for certain how the wall could change the river and the wealth of life around it. Even without the barrier, groundwater models predict the San Pedro will be drying out for decades to come.

It’s not a question of whether the river survives, Stromberg said, but of what will be lost in the process. “It’s an ecosystem,” she said. “Some species will not survive, others will.”

Construction has started midway through Arizona’s wettest season, when the river typically awakens from rains. The floodgates will determine if the San Pedro River can still rush north across the border, connecting two countries and one ecosystem by water.

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