The San Pedro River, the last free-flowing river in the American Southwest, in 2013.

Only one undammed river in the American Southwest still flows freely, and it begins just south of the border, in Sonora, Mexico.

From there, the San Pedro River courses north into Arizona, a rare and unbarricaded corridor that is a haven and vital water source for a vast array of plants and wildlife, including beaver, javelina, jaguar, the hog-nosed skunk, the Southwestern willow flycatcher, migrating songbirds, and the cactus ferruginous pygmy owl.

But plans this summer to construct a bollard wall across the San Pedro, part of a broader project to put new walls across the southern border of Arizona, are threatening this crucial ecosystem. Environmentalists say that such a wall would wreak environmental and hydrological havoc on a riparian corridor that is already damaged by excessive groundwater pumping.

“The construction of 63 miles of border walls in the locations proposed would cause severe and irreversible damage to the environment and harm the culture, commerce, and quality of life for communities and residents located near the project areas,” the Center for Biological Diversity said in comments submitted Friday, July 5, to U.S. Customs and Border Protection.

An example of bollard wall. U.S. Customs and Border Protection

That agency announced this latest border wall project on May 6, and it gave local communities, organizations, and people until today, July 5, to comment on the proposal. Yet even as it supposedly awaited that input, the agency moved forward with plans and sought to waive dozens of federal laws that require stringent reviews before major construction projections.

In mid-May, the federal government awarded $646 million to Southwest Valley Constructors to design and build the wall south of Tucson, and it scheduled construction to begin as early as July 1, although that date was held up June 28 after a federal judge blocked funding for those projects.

Meanwhile, Border Patrol has offered very few details to the public regarding the proposed wall. The May proposal described replacing about 63 miles of existing fencing and barriers with new bollard wall — “18- to 30-foot, concrete filled steel bollards” — in Pima and Cochise counties. New or improved roads, lights, and “other detection technology” were listed as part of the project as well.

A spokesperson for the Border Patrol was unable to respond to a request for comment by deadline.

Advocates for the San Pedro and the surrounding area are not just deeply concerned about the environmental and hydrological havoc that such a barrier would wreak on the riparian corridor, which is one of the few areas where wildlife can still cross a man-made border marked by walls extending some 40 miles to the east and 10 miles to the west.

“It’s an environmental catastrophe, but it’s also an engineering nightmare.” - Laiken Jordahl, Center for Biological Diversity Facebook

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They are also furious about the way the federal government has abandoned all but the pretense of asking communities for input.

“We must note immediately that this comment process is deeply flawed and seemingly meaningless, as Customs and Border Protection has already awarded a $646,000,000 construction contract for this project just days after sending the public a request for comment,” the Center for Biodiversity said in its 20-page letter, which was co-signed by other environmental groups. Clearly, CBP had "no intentions" of considering the input received, the center added.

Enabled by the 2005 Real ID Act, Kevin McAleenan, the acting Secretary of Homeland Security, is attempting to waive dozens of laws that would ordinarily require agencies to go through careful review processes and consider public input before starting large infrastructure projects.

Among those laws are the National Environmental Policy Act, the Endangered Species Act, the Clean Water Act, the Archaeological Resources Protection Act, the Safe Drinking Water Act, and the Native American Graves Protection and Repatriation Act.

Critics of the proposed barrier also say that a wall across a river like the San Pedro is highly impractical, a point backed up by history.

Even if water could flow through a new fence, debris — dead branches, leaves, car tires, and any other solid things that find their way into rivers — would inevitably pile up and soon block water from flowing. Come monsoon season and heavy rains, the barrier would either give out or effectively become a dam, flooding the surrounding area, they warn.

During a flash flood in 2008, accumulated debris turned the 5.2-mile wire mesh fence along the southern border of Organ Pipe Cactus National Monument, about 200 miles west of the San Pedro, into an impermeable barrier. Pools of water rose up to seven feet deep in some places. The waters damaged property in Lukeville, Arizona and Sonoyta, Sonora, and water that usually flows north to south began spreading east and west instead, the Arizona Daily Star reported.

The cactus ferruginous pygmy owl. Wikimedia Commons

Six years later, in 2014, monsoon runoff downed a section of barrier in Nogales, after branches and other detritus clogged unmaintained tubes and gaps intended to allow water to course through the fence.

“Water is a really powerful force,” said Louise Misztal, executive director of the Sky Island Alliance, a group based in Tucson that works to preserve biodiversity throughout southeastern Arizona and adjacent areas in New Mexico and Mexico. “With our monsoon rains, there’s a lot of water that runs through the San Pedro."

“This proposal is just outrageous,” Misztal added. “It flies in the face of the rule of law. The destruction it would wreak on ecosystems and communities along the border … it’s just horrifying.”

The Sky Island Alliance has about two dozen wildlife cameras in the borderland, some of which are directly along the San Pedro River. Over the past three years, one of those cameras has documented 1,165 instances of wildlife traveling along the river, Misztal said. The cameras showed that badgers, bobcats, mountain lions, raccoons, three species of skunks, and many other species used the corridor.

But it’s hard to specify just how destructive this particular wall would be to these species and to the river around which they live without the scientific assessments required by the laws that the Department of Homeland Security is seeking to waive, Misztal pointed out.

“Because the laws have been waived, those things aren’t being analyzed,” Mistzal said. The National Environmental Policy Act, for example, requires federal agencies to conduct rigorous assessments of the environmental impacts of construction projects like airports and highways.

To make their point, advocates of protecting the San Pedro River and the surrounding San Pedro National Riparian Conservation Area, which Congress designated in 1988, point to those cases where walls elsewhere along the Arizona-Mexico border caused flooding and destruction. They also highlight the basic hydrology of the river, and how it and the ecosystem it supports would be altered by a fence running through it.

During monsoon season, which begins in July, the San Pedro River can swell into a wide, large torrent. It rapidly expands from its lowest flow during the months from April through June, when cottonwood trees and mesquites along the banks drink their fill. These changes in flows are important for the river’s ecology, which relies on such disturbances to function.

Laiken Jordahl, the borderlands campaign manager for the Tucson-based Center for Biological Diversity, described the wall and the policy behind it as “delusional,” for reasons both practical and environmental.

“[The San Pedro River] gets churning when it rains,” Jordahl said. “If you build a wall there, it’ll get washed out.” Building a wall there would severely damage the river, he added, echoing the comments of Misztal and other advocates. “It’s an environmental catastrophe, but it’s also an engineering nightmare.”

In 2007, the Department of Homeland Security proposed to build a similar wall across the San Pedro. When the Bureau of Land Management evaluated that proposal, it warned about the risk of debris piling up and causing floods. As the Center for Biological Diversity noted in its comments to Border Patrol, memos from the BLM that year said that the Department of Homeland Security had ignored debris accumulation in its flood modeling.

“Existing structures ... have considerable problems with debris build-up,” the BLM’s memo said, according to the Center’s comments. That same memo also warned, “The timing and intensity of seasonal flood flows in the San Pedro River are essential for maintaining riparian function as well as recharging the alluvial aquifer.” A fence could “inadvertently act as a flood control structure altering natural flood sequences,” it said.

On June 28, environmental advocates were handed a minor, if temporary, victory when a federal judge blocked the administration from shifting $2.5 billion in military funding to border wall construction, including that of the proposed wall across the San Pedro River.

The next day, the Trump administration appealed that injunction, sending it to the 9th Circuit Court of Appeals. For now, the funding to build a wall across the San Pedro has been stopped, Laiken said. “But that could change in a week.”

A previous version of this story incorrectly identified the date that the Center for Biological Diversity submitted comments to Border Patrol. It was July 5, not June 28.