Farther downriver, in the muddy flats at the head of the Elephant Butte Reservoir, in southern New Mexico, the water would neither support their weight nor allow them to paddle, so they devised a method of pushing their boats with their hands and feet while lying on the stern. Crossing into Texas, where the river meets the Mexican frontier, the Ledyardians switched to bicycles and rode along paved roads until, a couple of hundred miles later, the Río Conchos, running out of the Mexican state of Chihuahua, replenished the ancient riverbed, so that they could saddle up their kayaks again. Because of upstream depletions, the Rio Grande is really two rivers: one that fizzles in southern New Mexico (the locals there refer to it as the Rio Sand) and one that begins in West Texas. In between is the puddled and trenched borderland east of El Paso and Juárez—the Forgotten Reach, which, prior to the big dams, had been regularly revived (and scoured) by seasonal floods from New Mexico. There had even been eels in Albuquerque—fifteen hundred miles upstream of the Gulf of Mexico.

The Dartmouth expedition, now five strong, made it through the deep canyons and riffles of the Big Bend and then entered the Lower Canyons, the river’s most remote leg, which Congress, a year later, designated part of the National Wild and Scenic Rivers System. The desert eventually gave way to a subtropical luxuriance of palms, broccoli farms, and citrus orchards, the riverbanks and wetlands teeming with wildlife. The birds and animals didn’t recognize the border. The people, though, were defined by it. The kayakers regularly encountered Mexicans crossing the river with burlap bundles. Near Eagle Pass, they came across a bloated male corpse, with a noose around the neck. (“We tried to report him, but neither side was terribly interested,” Reicher recalls.) At night, burrowing into the invasive wild cane to make camp, they set off seismic sensors installed by the U.S. Border Patrol.

After four months on the river, they reached the Gulf. They posed on the beach, five gringos, tan and lean, brandishing the Ledyard flag. Relations among some of them had frayed, amid a clash of egos—endemic to such expeditions. Reicher and Anella have hardly spoken since. But the trip remains a highlight of their lives. To Anella, it was a religious experience. “One-half of the hydrologic cycle—it reached something deep in my soul,” he says. He likes to cite Ecclesiastes: “All streams flow into the sea, yet the sea is never full. To the place the streams come from, there they return again.”

Reicher prefers Heraclitus: “No man steps in the same river twice, for it is not the same river and he is not the same man.” Since 1977, he has been back to the Rio Grande six times; the river may have changed more than he has. Four years ago, a young newspaper reporter in San Antonio named Colin McDonald set out to duplicate the source-to-sea trip, using Reicher’s journals as a blueprint. He dubbed it the Disappearing Rio Grande Expedition. He soon discovered that the river was in even worse condition than it had been forty years earlier. Groundwater depletion, suburban sprawl, periodic droughts (attributable, probably, to climate change): every year, people were asking more of less water. He wound up having to walk a third of the river’s length. Reicher, who had helped McDonald raise money and get attention for the trip, joined him for a couple of actual-water segments—in the Big Bend and then the last miles, where the river limps into the Gulf. When McDonald did a slide show in Albuquerque, Anella approached him afterward and said simply, “That was my trip.”

After Donald Trump was elected, he pursued his campaign promise to build a wall along the nearly two thousand miles of border between the United States and Mexico. The Rio Grande’s “disappearance” took on fresh meaning. As imagined, such an undertaking would be devastating to life along an already threatened river.

Having been determined by the 1848 peace treaty that ended the Mexican-American War, the border traces the river’s deepest channel—the thalweg—which, because the riverbed frequently shifts according to the water’s whims, is in some respects notional. Of course, no one is proposing that a wall be built in the middle of the river, or for that matter on Mexican soil, even if Mexico is going to pay for it. So the wall would go on the American side, some distance from its banks—miles into U.S. territory, at times. It would cut people off from their own property and wildlife from the main (and sometimes the only) water source in a vast upland desert. The Center for Biological Diversity has determined that ninety-three listed or proposed endangered species would be adversely affected. The wall could disrupt the flow of what meagre water there is, upon which an ecosystem precariously depends. And it would essentially seal the United States off from the river and cede it to Mexico: lopping off our nose to spite their face. It would shrink the size of Texas.

The wall would cut people off from their own property, cede the river to Mexico, and shrink the size of Texas. Photograph by George Steinmetz for The New Yorker

There is also the matter of efficacy. The wall would probably delay a hypothetical crossing by a few minutes, depending on its design and the manner of the breach. There are videos of Mexicans deploying ladders, ramps, ropes, welding torches, and tunnels to get over, through, or under border fences. (There are about seven hundred miles of fence already, most of it in California and Arizona.) For a great deal of its length, the river is insulated on both sides by hundreds of miles of desert—inhospitable terrain that does more to discourage smugglers and migrants than a wall ever could. (The vast majority of hard drugs intercepted on the southern border is coming through so-called points of entry—the more than forty official crossings—hidden in vehicles and cargo.) And, while the banks of the river, for much of it, are free of impediments, except for thick stands of invasive cane and salt cedar, which can make life miserable for the Border Patrol, about a hundred miles of it cut through deep canyons far more imposing and prohibitive to a traveller on foot than a slab of concrete or steel. The canyons don’t require funding from Congress.

This winter, Reicher put together a trip on the Rio Grande, with American Rivers, an advocacy group, of which he’s a board member, to celebrate the fiftieth anniversary of the Wild and Scenic Rivers Act, and to begin to articulate, in an informal but pertinent setting, a response to Trump’s wall. (Last week, American Rivers, for the first time since 2003, included the Rio Grande in its annual list of the ten most endangered rivers.) This wasn’t so much an expedition as a floating Chautauqua, with a missionary bent. He and Bob Irvin, the president of American Rivers, invited me along. Among the guests were two grandees with dynastic connections to environmental conservation: Senator Tom Udall, Democrat of New Mexico, whose father, Stewart Udall, spearheaded the protection of vast tracts of American wilderness and was a crucial proponent of the Wild and Scenic Rivers Act; and Theodore Roosevelt IV, whose great-grandfather, the twenty-sixth President, used his bully pulpit, and hundreds of executive orders, to turn the federal government into a force for, and an enforcer of, land and wildlife conservation. Before American Rivers got involved, Reicher had invited Rob Portman, who has the kayak from the 1977 expedition mounted in his office on Capitol Hill, but his schedule was too tight, and he’d been back to the river a year earlier, with his family. “Last thing a Republican needs now is to be seen spending a week on a river with a bunch of tree huggers,” Irvin told me with a chuckle.