Although Congress has yet to provide the money for Mr. Trump’s wall, preparations for its construction are underway. Officials with Customs and Border Protection recently held a meeting in the valley at which they displayed a map of the wall’s proposed route.
A United States-Mexico treaty prohibits building a wall or levee in the Rio Grande floodplain, so the map shows the wall being built well north of the river. But that means it will slice through countless pieces of private property and bisect several major wildlife refuges. When asked about the map, officials emphasized that it was only a proposal, and that the wall’s construction was dependent upon the federal budget for 2018.
But that, as Ms. Wright and others have learned, does not mean that the officials tasked with building the wall cannot start making plans.
Because of the anticipated resistance from private landowners, the first sections of the border wall most likely will be built on federal land. In the valley, that includes the Santa Ana National Wildlife Refuge near the city of Alamo, where officials and contractors have been taking soil samples.
Known as the crown jewel of the national system, the lush, 2,088-acre refuge was established in 1943 and is one of the most popular bird-watching destinations in the country, attracting about 165,000 visitors a year.
The threat to the area’s natural habitats like the birdlands and butterfly haven has prompted letters to members of Congress, lawsuits and protests — the most recent of which was Saturday in this remote South Texas city in the Rio Grande Valley.
The protesters began gathering before dawn at Our Lady of Guadalupe Catholic Church to demonstrate against President Trump’s proposed border wall.
Father Roy Snipes delivered a prayer in English and Spanish, then climbed into a station wagon — outfitted with a large figurine of the church’s namesake strapped to the roof — to lead the approximately 1,000 protesters in a four-mile march to the Rio Grande.
Ken Merritt, a former federal Fish and Wildlife Service official who oversaw Santa Ana from 1997 to 2008, said building an 18-foot-high wall through the refuge would most likely destroy it.
“If you put the wall there, you basically cut off all the trails, all the habitat,” he said. “The office might stay open, but I don’t see how there could be any more public use of the refuge.”
Keith Hackland has operated nearby Alamo Inn B&B, which caters to bird-watchers, since 1999.
“We put a lot of effort into getting the word out about the valley,” he said. “Recently I’ve been getting calls from around the country asking what’s going on. People have heard that Santa Ana’s being bulldozed.”
The Rio Grande Valley is one of the most biologically diverse regions of the country. It is a major bird migration corridor, with over 500 species, more than half the total number in North America. It is also the last remaining habitat for the endangered ocelot, a medium-size spotted cat hunted nearly to extinction for its fur.
For decades, naturalists have worked to build a corridor of public and private sanctuaries along the Rio Grande to allow the ocelot and other wildlife to roam semi-freely. That effort is now under threat.
“If you take the habitat that was set aside for those animals and put walls across it, they can no longer find sufficient food, water and mates,” said Scott Nicol, the co-chairman of the Sierra Club’s borderlands campaign. “Drug smugglers can get over the wall. They just use ladders. Ocelots don’t have ladders.”
Thanks to its natural riches, South Texas enjoys a robust ecotourism industry that pumps an estimated $350 million each year into one of the poorest parts of the country. But that, too, is now under threat.
“If the wall is built through the refuge, that will be very bad publicity,” said Mr. Hackland. “It will reduce nature tourism, no doubt about it.”
Representative Filemon Vela Jr., whose district includes a nearby section of the valley, made headlines last year when he wrote an open letter to Mr. Trump suggesting that the then-presidential candidate “take your border wall and shove it….”
When contacted earlier this month, Mr. Vela, a Democrat, doubled down on his defiance. “I meant it, and I would repeat it,” he said. The problem, he said, is that since Democrats are in the minority in Congress, “it could be the other way around.”
Saturday’s protest march ended at La Lomita Chapel, which was built in the 19th century on a rise overlooking the Rio Grande. Father Snipes explained that it was originally intended as a sign of friendship between America and Mexico. But if the border wall is completed, the chapel will be equally inaccessible to both countries.
“The chapel is a symbol of the call of God to be hospitable and neighborly,” Father Snipes said. “With the wall between us and the chapel, it’s just kind of obscene.”