ELEPHANT BUTTE, N.M.—Three decades ago, a rare event drew hundreds of people from miles away to the Elephant Butte reservoir, this state’s largest lake.

The reservoir, which supplied drinking water to thousands and nourished vast acres of farmland, had swelled from the Rio Grande and begun to overflow. Jim Paxon remembers the roar as water cascaded down the dam’s spillway, tossing mist into the air and churning the ground below.

Those days are long gone. One morning last month, Paxon, now a 79-year-old county commissioner, pointed to where the diminished reservoir lapped at the base of the 301-foot dam. “The lowest I’ve seen in decades,” Paxon said.

Elephant Butte reservoir earlier this month. In July, the reservoir had shrunk to 1.4% of its original capacity.

Spanning about 1,900 miles and sustaining farms, towns and wildlife, the Rio Grande is the lifeblood of much of the Southwest—and it is running dry, in what experts describe as one of America’s most severe water crises. This has depleted reservoirs at Elephant Butte and elsewhere in Colorado, New Mexico and Texas, imperiling the many communities that depend on the river for drinking water and irrigation.

The Rio Grande has been sapped by more than two decades of drought and a century of heavy water use. Researchers estimate the river has lost roughly 85% of its natural flow, and some experts say it is more endangered than the Colorado River.

That depletion has accelerated in recent years. Snowpack in the Upper Rio Grande basin, which helps replenish the river, fell to near-record lows last winter, and in July Elephant Butte reservoir dwindled to 1.4% of its original capacity. By September, a stretch of the Rio Grande in Albuquerque had become a cracked riverbed dotted with shrubs and grass.

The shrinking river has pushed many farmers to scale back their agricultural production or shut it down. It has also inflamed tensions among states and between the U.S. and Mexico, which have a water-sharing agreement. With no end to the drought in sight, some environmental experts warn that many acres of farmland might disappear and cities may be forced to curb water use.

“The guillotine is on top of our heads,” said Samuel Sandoval Solis, a specialist in water-resources management at the University of California, Davis.

“It hasn’t fallen, but it’s just a matter of time.”The Rio Grande’s decline poses an existential threat to the town of Elephant Butte and nearby farms, where life revolves around the reservoir. In August, Elephant Butte’s city leaders declared a local disaster and requested emergency state funding. Sierra County Commissioner Jim Paxon

Boats depart from a dock at the reservoir, which made Elephant Butte a vacation destination for fishing and swimming. Mountains tower over homes in Elephant Butte. Surrounded by the windswept desert and towering mountains, locals are keenly aware of how improbable it was that a bustling town and verdant fields arose in this unforgiving terrain—and how swiftly it all might disappear.

Victoria Franzoy, a fourth-generation farmer in the nearby Hatch area, worries she might be the last of her family to farm chilies.

“If this drought continues,” Franzoy said, “it will kill our little valley.”

Early in the 20th century, Franzoy’s great grandfather arrived from Italy, cleared the desert land and planted chilies, onions and potatoes, nurturing the plants with water from the Rio Grande.

As a child, Franzoy often went tubing on the river or splashed with playmates in gurgling irrigation canals. Decades later, on a scorching afternoon last month, she maneuvered her pickup to a bridge overlooking the Rio Grande’s sandy riverbed. A rivulet of water wormed its way below. Nearby, some of her land, once brimming with rows of leafy chili crops, had become naked plots of dirt.

Victoria Franzoy and her granddaughter take fresh green chilies from a driver on a truck bound for Albuquerque.

Chilies have grown since Franzoy’s great grandfather cleared the land in the early 20th century. Franzoy’s fallow fields and a dry irrigation ditch.

Franzoy has turned more than a third of her land into fallow fields under a state program that pays farmers to reduce their water consumption. “That’s what the valley’s going to look like if we lose all our water,” she said.

The Rio Grande has cycled through periods of drought before. During a particularly severe one in the 1950s, many farmers in New Mexico and Texas drilled wells to supplement their river water.

But some experts say the current megadrought, which has lasted more than two decades and is intensified by rising temperatures, is part of a long-term shift toward drier conditions in the Southwest. While occasional heavy storms cause flash flooding, as occurred in late September, researchers say the underlying drought still continues.

Even as the water supply has contracted, human demand has persisted; experts say more than 50% of human water use is unsustainable, meaning people are depleting the river, underground aquifers and reservoirs faster than they can be replenished.

In the harsh environment of the American West, choices about water are rooted in politics: Who comes first? What is fair? Who is deserving?

Over a century, a patchwork of laws, treaties and irrigation systems sprang up to address those questions, allocating the prized water and governing how much must be passed downstream. But as the river shrinks, competition over what remains has intensified.

For 13 years, New Mexico and Texas battled in courts over water rights, with Texas blaming New Mexican farmers for pumping too much groundwater and reducing downstream deliveries. In May, the Supreme Court approved a settlement requiring New Mexico to trim groundwater use over the next decade. Farther downstream, farmers in South Texas accuse Mexico of hoarding water and shirking the obligations of a 1944 water-sharing treaty between the two countries.

The paucity of water has pressured communities along the river to make painful decisions about where to cut back their water usage. Sugar-cane farmers in South Texas have closed their operations. Onion and chili growers in New Mexico have let swaths of land lie fallow. And communities from Albuquerque to Laredo, Texas, have grappled with where to find alternate sources of drinking water.

In Elephant Butte, population just over 1,400, officials and residents fear the dwindling reservoir spells doom for their way of life. The town grew up around the reservoir, created in 1916, as a destination for fishing, swimming and boating. Many businesses still depend on tourists, but the shrinking reservoir has turned many away.

During Labor Day weekend, the town’s foot traffic plummeted compared with the year before, according to Mayor Richard Holcomb. Russ and Edna Trager, who moved to Elephant Butte from Los Angeles and own a kayak-outfitting shop, said that over the summer, revenue dropped 63% compared with average sales from several prior years. “This place is going to turn into a ghost town,” Russ said. Russ Trager at his kayak-outfitting shop.

The community is divided on how to resolve the water-scarcity crisis. Residents say farmers are consuming too much water for irrigation, while farmers accuse towns and cities of unsustainable population growth. Several experts say communities throughout the region, particularly farmers, need to drastically reduce their water use.

Franzoy and other farmers say their only recourse is to pray for rain. In September, Franzoy shared photos of the river after a torrential overnight downpour.

For a moment, the river appeared restored to its former state, the water rippling from bank to bank. A rainbow arced overhead. But days later, amid unrelenting sun and parched soil, the riverbed was dry again. The Rio Grande near Franzoy’s farm. Additional images from National Aeronautics and Space Administration.