OAKLEY, Utah — Across the western United States, a summer of record-breaking drought, heat waves and megafires exacerbated by climate change is forcing millions of people to confront an inescapable string of disasters that challenge the future of growth.

Groundwater and streams vital to both farmers and cities are drying up. Fires devour houses being built deeper into wild regions and forests. Extreme heat makes working outdoors more dangerous and life without air-conditioning potentially deadly. While summer monsoon rains have brought some recent relief to the Southwest, 99.9 percent of Utah is locked in severe drought conditions and reservoirs are less than half full.

Yet cheap housing is even scarcer than water in much of Utah, whose population swelled by 18 percent from 2010 to 2020, making it the fastest-growing state in the country. Cities across the West worry that cutting off development to conserve water will only worsen an affordability crisis that stretches from Colorado to California.

In the little mountain town of Oakley, about an hour’s drive from Salt Lake City, the spring that pioneers once used to water their hayfields and filled people’s taps for decades dwindled to a trickle in this year’s scorching drought. So town officials took drastic action to preserve their water: They stopped building.

During the pandemic, the real estate market in their 1,500-person city boomed as remote workers flocked in from the West Coast and second homeowners staked weekend ranches. But those newcomers need water — water that is vanishing as a megadrought dries up reservoirs and rivers across the West.

So this spring, Oakley imposed a construction moratorium on new homes that would connect to the town’s water system. It is one of the first towns in the United States to purposely stall growth for want of water in a new era of megadroughts. But it could be a harbinger of things to come in a hotter, drier West.

“Why are we building houses if we don’t have enough water?” said Wade Woolstenhulme, the mayor, who in addition to raising horses and judging rodeos, has spent the past few weeks defending the building moratorium. “The right thing to do to protect people who are already here is to restrict people coming in.”

Farmers and ranchers — who use 70 to 80 percent of all water — are letting their fields go brown or selling off cows and sheep they can no longer graze. Gov. Spencer Cox of Utah said all but one of the fields on his family’s farm had dried up.

“It’s just brutal right now,” said Mr. Cox, who also asked the faithful to pray for rain. “If we continue to grow at the rate we’re growing now and have another drought like this in 10 years, there will be real drinking-water implications. That’s the thing that worries me the most.”

For now, most places are trying to stave off the worst of the drought through conservation instead of shutting off the spigot of growth. State officials say there is still plenty of drinking water and no plans to stop people from moving in and building.

“A huge consideration for many politicians is that they don’t want to be viewed as a community that has inadequate resources,” said Katharine Jacobs, who directs the University of Arizona’s climate adaptation research center.

In states across the region, Western water providers have threatened $1,000 fines or shut-offs if they find customers flouting lawn-sprinkler restrictions or rinsing off the driveway. Governments are spending millions to rip up grass, reuse wastewater, build new storage systems and recharge depleted aquifers — conservation measures that have helped desert cities like Las Vegas and Tucson reduce water consumption even as their populations exploded. In California, Gov. Gavin Newsom has called for 15 percent cuts in water use — but so far those are largely voluntary.

But water now looms over many debates about building. Water authorities in Marin County, Calif., which is contending with the lowest rainfall in 140 years, are considering whether to stop allowing new water hookups to homes.

Developers in a dry stretch of desert sprawl between Phoenix and Tucson must prove they have access to 100 years’ of water to get approvals to build new homes. But extensive groundwater pumping — mostly for agriculture — has left the area with little water for future development.

Many developers see a need to find new sources of water. “Water will be and should be — as it relates to our arid Southwest — the limiting factor on growth,” said Spencer Kamps, the vice president of legislative affairs for the Home Builders Association of Central Arizona. “If you can’t secure water supply, obviously development shouldn’t happen.”

Late last month, the state water department announced that it would not approve any applications for developers seeking to use groundwater within the area. The decision has raised concerns from local developers, who said that these restrictions would make it harder to meet the needs of Arizona’s voracious housing market.

In Utah, Oakley and the nearby farming town of Henefer are vowing not to grow until they can secure new, reliable sources of water through drilling or pumping — an expensive and uncertain prospect.

“These towns are canaries in the coal mine,” said Paul D. Brooks, a professor of hydrology at the University of Utah. “They can’t count to go to the tap and turn on the water. Climate change is coming home to roost right now, and it’s hitting us hard.”

In the 1800s, water was one of the main draws to Oakley for white settlers. The town sits beside the Weber River, and its water and other mountain springs irrigated farmland and supported dairies that once speckled the valley.

It is still a conservative farming community where tattered 2020 Trump flags flutter and the mayor is dubious of human-caused climate change. Its beauty and location a half-hour from the ski-town glitz of Park City have made it an attractive bargain for out-of-staters.

Utah law allowed Oakley’s City Council to pass only a six-month moratorium on building, and the city is hoping it can tap into a new water source before deciding whether to re-up the moratorium or let it expire.

One project that would build as many as 36 new homes on tree-covered pasture near the town’s ice-cream parlor is on hold.

“You feel bad for the people who’ve been saving up to build a house in Oakley,” Mr. Woolstenhulme, the mayor, said as he drove around town pointing out the dusty fields that would normally be lush with alfalfa. The distant mountains were blurred by wildfire haze. “I hate government infringement in people’s lives, but it’s like having kids: Every once in a while you got to crack down.”

Oakley is planning to spend as much as $2 million drilling a water well 2,000 feet deep to reach what officials hope is an untapped aquifer.

But 30 miles north of Oakley, past dry irrigation ditches, rumpled brown hillsides and the Echo Reservoir — 28 percent full and dropping — is the town of Henefer, where new building has been halted for three years. Right now, Henefer is trying to tap into new sources to provide water for landscaping and outdoor use — and save its precious drinking water.

“The folks in town don’t like it,” Mayor Kay Richins said of the building moratorium. “I don’t like it.”

Experts say the smallest towns are especially vulnerable. And few places in Utah are as tiny or dry as Echo, a jumble of homes squeezed between a freight railroad and stunning red-rock cliffs. Echo was already struggling to hang on after the two cafes closed down. Then its spring-fed water supply hit critical lows this summer.

Echo’s water manager has been trucking in drinking water from nearby cities. People worry that the water needed to put out a single brush fire could deplete their tanks.

At their house, J.J. Trussell and Wesley Winterhalter have let their lawn go yellow and take showers sparingly. But some neighbors still let their sprinklers spray, and Mr. Trussell worried that the little community his grandparents helped build was on the brink of drying up and blowing away.

“It’s very possible we’ll lose our only source of water,” he said. “It would make living here almost impossible.”