How do you draw a single borderline through three separate, overlapping nations? While that might sound like a Zen koan—the geopolitical version of the sound of one hand clapping—it’s the actual predicament facing the Tohono O’odham Nation, a Native American tribe whose territory includes both the United States and Mexico. The tribe’s members are infuriated by the prospect of a border wall further dividing their ancestral lands.

Tohono O’odham means “desert people.” The nation historically stretched from the Sonoran Desert southeast of Tucson, Arizona as far south as the Sea of Cortez, which today is the Mexican state of Sonora. There are about 34,000 members of the tribe, most of them living in the United States, and an estimated 2,000 Tohono O’odham living in Mexico. Traditionally, Tohono O’odham would routinely move back and forth across the U.S.-Mexico boundary, often for cultural events at the tribe’s religious sites scattered across the desert. The border militarization of the past 20 years has restricted tribal members’ movements, and today there is just a single authorized border crossing on the reservation. Large sections of the Tohono O’odham territory are bisected by what the U.S. Border Patrol calls “Normandy barriers”—criss-crossed steel bars that can block vehicles from making an illegal border crossing. Tohono O’odham members are concerned that President Donald Trump’s border wall, if constructed, would create new problems for the tribe, as well as for the ecosystem they call home.

“We have so many concerns regarding the wall,” tribal chairman Edward Manuel told me in an interview last spring. “If they are looking at a 30-foot-high fortified wall, that is going to cut our traditional lands in half. The wall would interfere with our way of life, our ceremonies, with our traditional and our cultural activities that we do on both sides of the border, because we still have communities on the Mexican side.” 

Manuel said that further border tightening would make it difficult for tribal members in the United States to “do religious pilgrimages into Mexico” and, similarly, for Mexican tribal members to reach Baboquivari Peak, the Tohono O’odham’s most sacred site, located north of the border. Family gatherings also could be interrupted. For example, Tohono O’odham members who were born in Mexico and now live in the United States often want to be buried at their birthplace, and border militarization makes that difficult. The wall, Manuel said, “will have a direct effect on our members.”           

Chairman Manuel and other Tohono O’odham leaders also express concerns about the environmental impacts of border militarization. A solid wall could “impede the migration of animals,” Manuel said, “and will unbalance the ecology in the region.” Manuel also pointed to an issue often overlooked in debates over the border wall: the way in which impassable barriers could disrupt the sensitive hydrology of the desert. 

The desert is normally arid, but it’s also punctuated by bouts of rain—winter storms and the region’s signature summer monsoons that can drop several inches of water in the space of an afternoon. If border walls were to be built across normally dry arroyos that can turn into raging rivers during a flash flood, the barriers would act as de facto dams that would either shift water courses unpredictably or damage the wall itself.

This has already occurred in the Santa Cruz River watershed around the twin cities of Nogales Arizona, and Sonora, where bollard-style fences stretch about 20 miles to the east and west of the bi-national metropolis. The Santa Cruz River starts in the United States and flows south into Mexico, then makes a U-turn and flows back north into the U.S. and toward Tucson, a kind of hydrological horseshoe. In 2015, storm-related flooding knocked out a 60-foot section of steel border fencing. It cost more than $700,000 to repair. At one of the Santa Cruz River’s border crossings, east of the city of Nogales, the 20-foot-high bollard fence gives way to Normandy barriers in the riverbed. But even those get clogged with debris, threatening the health of the riparian zone. When I visited the site with a couple of activists from a group called Friends of the Santa Cruz River last March, the steel jacks were piled high with flotsam, even though the monsoon season was months away.

“What are they thinking, that they’re going to stop the water flows?” Sheri Sass, a member of Friends of the Santa Cruz River, told me as a Border Patrol SUV rolled past. “Are they going to create dams at the drainages? And if not, then people will just cross the drainages. How could someone even imagine that this is a good idea? It’s just stupid.”

In February, the Tohono O’odham Legislative Council passed a resolution stating that “…while the Nation coordinates closely with [the Border Patrol] and ICE [Immigration and Customs Enforcement] and has supported the construction of vehicle barriers, the Nation opposes the construction of a wall on its southern boundary with Mexico.” Within weeks of that vote, the Inter Tribal Association of Arizona passed a similar resolution against the wall, as did the National Congress of American Indians, which said it opposes “the construction of a physical wall on the southern border of the United States on tribal lands without the consent of affected tribes.” 

For now, the Tohono O’odham are withholding that consent and biding their time. Manuel said that officials from the Department of Homeland Security and the Interior Department’s Bureau of Indian Affairs have told tribal leaders that “nothing will happen in a two-year time frame.” In the meantime, the tribe has consented to the construction of at least eight new remote surveillance towers on their territory that could serve as a kind of “virtual wall”—helping to apprehend illegal border crosses without further impacting Tohono O’odham travel or the desert landscape. “They said that if anything is going to happen here, they will sit down and do a consultation with us,” Manuel said. 

Tribal leaders have met with officials from the Department of Homeland Security and the Interior Department, and they are in contact with members of Congress, who, experts on Indian law say,  would have to approve any seizure of the nation’s territory for border wall construction. The Tohono O’odham leadership also have invited federal officials to their nation to see the border for themselves. They’ve even extended that invitation to the wall’s biggest backer, Donald Trump. “We want them to come and take a look at the area, especially the president,” Manuel said.

So far, the White House hasn’t accepted the offer.