They Built the Wall. Now Some in Texas Fear It May Fall Down.

MISSION, Texas — Along a bend in the Rio Grande, shorn of all brush except for an occasional palm, looms an 18-foot fence of galvanized steel a few feet from the muddy water’s edge.

The fence, constructed three years ago with private funds, was once at the center of a bitter national debate over border security, its builder touted by President Donald J. Trump and promoted in a fraudulent scheme by Steve Bannon known as “We Build the Wall” that resulted in criminal indictments and convictions.

Now, the three-mile-long barrier is essentially orphaned, functionally useless — because of a federally constructed border barrier a short distance behind it — and, according to an engineering report commissioned by the Justice Department, at risk of falling over in a major flood and floating away.

And because of its location and construction along the water’s edge, federal officials worry that the fence could end up redirecting the Rio Grande in such a way that the land it sits on would end up as part of Mexico.

The fence has been opposed in litigation brought by the nearby National Butterfly Center, which attracted threats of such vitriol last year that it briefly closed, and by the Justice Department, which accused the private builder of the fence, Fisher Sand & Gravel Company, of violating an international treaty.

The Justice Department reached a settlement with Fisher last year that allowed the fence to remain in place and required a subsidiary of the company to maintain it. The butterfly center, which sits just upriver, is continuing its effort to force the demolition of the fence; a trial could take place this year.

“The whole thing was stupid,” Ryan Patrick, the former U.S. attorney for the Southern District of Texas, said of the fence, whose construction on the edge of the river, he argued, was a violation of a treaty with Mexico. “The erosion began almost immediately,” Mr. Patrick said. “I would not be happy if I lived in the vicinity of this thing.”

Despite its size, the fence is mostly invisible to the residents of the border community in Mission, Texas, where it rises at the edge of a sugar cane farm on the outskirts of town. But it is impossible to miss when looking out from José Alfredo Cavazos’s property a short distance down the river.

The reedy riverbank land has been in the Cavazos family for generations, said Mr. Cavazos, 73, who recalled working there for his grandmother on what was then a farm and described jumping into the river to cool off. Mr. Cavazos, who once ran a local grocery store, now gets around in a motorized wheelchair and his family rent plots along the water to local residents, including four members of the Border Patrol, so that they can have access to the river for fishing and boating.

Mr. Cavazos was dismayed when the fence was built on his neighbor’s property upriver, concerned about the impact it would have on the river and the land around it.

“He never even bothered to come and talk to his neighbors,” Mr. Cavazos said, speaking of the owner of the sugar cane farm, Lance Neuhaus. “He probably knew it was going to damage his neighbors because he’s not dumb.”

Mr. Neuhaus, reached by phone, did not express concern about the fence, which sits on land he sold to Fisher. “The wall is still standing,” he said. “It’s a good project.”

Before the private fence was built, the Cavazos family spent years fighting against the existing federal barrier. Mr. Cavazos’s cousin, Reynaldo Anzaldúa Cavazos, expressed dismay at having had to watch the private fence go in right along the riverbank, an area that they thought everyone knew was ill-advised to build.

“We’ve lived here all our lives so we know what a flood does,” said Mr. Cavazos, 77, a retired U.S. customs agent. “You don’t build on the riverbank.”

Engineers who studied the fence’s construction on behalf of the Justice Department reached a similar conclusion. Among the issues outlined in the 400-page report from the engineering firm Arcadis were that, in the event of a major flood, the fence “would effectively slide, overturn and become buoyant.”

The firm concluded: “The fence is likely not fit for use under all reasonably anticipated service loads,” meaning environmental conditions, such as snow, wind, rain, earthquakes and floods.

But the government did not take its case to trial, choosing instead to reach the settlement with Fisher. Among the stipulations agreed to by the Justice Department and Fisher were that copies of the engineering report be destroyed. Its conclusions were instead reported by ProPublica and the Texas Tribune. A copy was obtained by The New York Times.

A spokeswoman for the Justice Department declined to respond to questions about the settlement.

“Ultimately we think what would be the best for everyone involved is just to take it down, even for Fisher,” said Javier Pena, a lawyer for the National Butterfly Center, referring to Tommy Fisher, the owner of the company. “He did not intend for this fence to stay up forever, because if he did, he would have built it better.”

Mark J. Courtois, a lawyer representing Fisher and its subsidiary in the project, said the company stands behind the design and construction of the fence, disagreed with the “assumptions and modeling” of the government’s report, and “agreed to perform routine maintenance for the project as is required for all structures.”

The fence sits just outside a relatively urbanized portion of the border in the Rio Grande Valley, which has been among the top locations for illegal crossings. Around the time of its construction, more people crossed in the Rio Grande Valley sector than anywhere else in Texas, though more recently larger numbers have been recorded farther northwest, in and around Eagle Pass and El Paso.

Almost from the moment it went up, Marianna Treviño Wright, the director of the butterfly center, has been watching for cracks in the wall and erosion along the waterfront. “The river is going to continue to reclaim its bank,” she said while motoring along the fence in a boat and pointing out areas where she says new dirt and rock appeared to have been carted in to replace what had washed away.

After working on the fence, Fisher received large federal border barrier contracts from the Trump administration.

But by 2020, Mr. Trump had distanced himself from the project after ProPublica and the Texas Tribune raised questions about its construction. “It was only done to make me look bad,” he wrote on Twitter, adding that perhaps “it now doesn’t even work.”

For the Cavazos family and others who have opposed the fence, its towering expanse along the denuded shoreline stands as a monument to a political moment. Its simple repetition of forms, particularly when catching the light at sunset, has the effect of a colossal artwork.

Indeed, it has become more symbol than substance. The private fence is effectively redundant because of a federally constructed border barrier, which also rose up during the Trump administration and runs along the levee nearby.

And the fence looks to be slowly shifting. Up close, its base appears to be separating and cracking. From afar, its evenly spaced posts are visibly misaligned in places.

On a recent visit, a red breasted hawk perched atop a post where one of its security lights had broken and fallen off. Trucks could be seen hauling in sand and gravel, which appeared to have been recently spread across the riverbank to replace what had washed away.

Among those who rent space along the water is Our Lady of Guadalupe Catholic Church in Mission, which uses the river access for a day camp. For years, Father Roy Snipes has been taking groups down to the patch of riverfront for fishing and bonfires and overnight camping.

“The river deserves respect. The river is sacred. It’s been a source of life for hundreds of thousands of years,” said Father Snipes, sitting in the parish office surrounded by several stray dogs he had taken in. “They really messed up the river. And for what?”

He said that after big rains, he has been able to see the erosion washing out from under the fence. Apart from its potential danger in a flood, Father Snipes said the fence has, from the beginning, sent a message to local residents who had long known that they were not allowed to just build whatever they wanted along the riverbank: “If you’re rich, you can do as you please.”

But not everyone is concerned about the fence. Jennifer Hart has owned the Riverside Club, a restaurant and event space, for four decades, and in that time there have been floods, including a massive one in 2010 that saw several feet of water come into the dining room and sit there for a month.

Ms. Hart said she did not worry about the fence eventually collapsing. “If it’s going to happen, it’s going to happen,” she said.

Her husband, Johnny Hart, motors by the fence several times a week, driving a tour boat from the restaurant for afternoon rides along the river and mixing commentary about the ocelots and migratory birds with talk of how smugglers use rafts to take migrants across where the water is deep. On a recent tour, several deflated rafts were visible in a particularly overgrown section of the United States river front.

As the fence approached, out-of-town visitors marveled at how imposing and impenetrable it appeared.

Mr. Hart provided some basic facts about its construction, highlighting its cheaper cost when compared with the federal border barrier construction. The silvery posts contrasted with the rust-hued metal of the federal barrier, visible at a distance behind it.

“This is a private wall, this has nothing to do with the federal government,” he told the group. A few snapped photos. No one commented on its sudden end. There, a short stretch of concertina wire spiraled off into the tall grass, held together in places by bits of metal and cords.