Jack Ethredge could see the future. It was 1985, and Mr. Ethredge, then the city manager of Thornton, Colo., understood that sooner or later, the Denver suburb would need more water.

The population was booming, businesses were flocking to the Mountain West, and Thornton had no major lakes or rivers of its own, nor any meaningful amount of groundwater to draw upon, a fluke of geology and geography. The city had drilled a dozen or so wells over the years, but the groundwater’s limited supply and high mineral content meant it wasn’t fit for drinking.

So at Mr. Ethredge’s behest, Thornton went shopping. The City Council bought about 17,000 acres of farmland 60 miles to the north, near Fort Collins, along with the associated water rights. When the time was right, Thornton would divert the water from the Cache la Poudre River that irrigated that farmland, put it in a pipeline and send it downstate.

“In the water business you have to be years and years ahead of the game,” Mr. Ethredge, now retired, said in an interview.

In theory, Thornton’s water woes were solved. In practice, the problems were just beginning. Today, almost 40 years later, the water Mr. Ethredge secured for his city remains out of reach.

Plans to build the pipeline have been stymied by bureaucracy, lawsuits and ferocious debates over who is entitled to use the snowmelt from the Rocky Mountains.

Jack Ethredge, Thornton’s former city manager.

Across the country, tensions over water are mounting as climate change, drought and development strain an increasingly scarce natural resource. The aquifers that supply 90 percent of the nation’s drinking water are being severely depleted. Housing and other uses like agriculture are draining finite supplies of groundwater. And counties and cities are being forced to hunt for new sources of water, setting up clashes between neighboring communities.

Another proposed pipeline in Colorado that would have brought water from the San Luis Valley to Douglas County ran into fierce resistance from lawmakers in Denver and was abandoned. In Texas, the city of San Antonio built a 150 mile pipeline to bolster its water supply, only to leave residents in the rural area where aquifers were being tapped complaining that their wells were running dry. And in Thornton, there is no end in sight to the pipeline battle.

“It’s a classic economic problem,” said Jeni Arndt, the mayor of Fort Collins. “How do you distribute a scarce and shrinking resource when there’s growing demand?”

After years of legal rancor, most of Thornton’s neighbors grudgingly agree that the city has a legal right to the water from up north. But no one can agree on precisely how Thornton should access it, and a fight is raging over the city’s plans to move that water down to the Denver suburbs. With the pipeline stalled, Thornton is forced to limit its growth, with all kinds of negative fallout.

City officials recently told a fast-growing company that makes a meat alternative using mushrooms that it had to pause expansion plans for lack of water. A major affordable housing project is on hold for the same reason. In total, the city says 18,000 housing units, which could accommodate about 54,000 people, are not being built because the pipeline is tied up in red tape.

Drilling for groundwater isn’t an easy option, either. The city’s few wells have produced a consistent but small amount of non-potable water that is used to irrigate parks and other public spaces. But the water level in one monitoring well in Thornton has fallen by about 100 feet since measurements began, and other wells in the Denver Basin aquifer system have had similar drops in the past few decades.

“It’s frustrating,” said Brett Henry, Thornton’s executive director for infrastructure. “We’ve had the engineering solutions all along, but we’ve got these political problems getting in the way. It doesn’t add up.”

Back when Mr. Ethredge secured the water rights up north, Thornton planners figured they would need that water by 2000. But a series of conservation measures and the construction of new reservoirs in the 1990s held off that deadline for more than a decade.

During those years, Thornton also overcame the first legal obstacles around its plan. Fort Collins and other cities and water districts sued Thornton, arguing it couldn’t move water that was currently used for agricultural purposes downstream to be used for municipal needs, a process derisively called “buy and dry” by critics. The Colorado Supreme Court ultimately decided in Thornton’s favor, appearing to clear the way for the construction of a pipeline.

Then, about a decade ago, it became clear Mr. Ethredge’s prediction was coming true. Thornton’s population had expanded by nearly 50 percent, from less than 100,000 residents in 2000 to about 150,000 today, and water supplies were getting tight. If the city was going to continue to grow, it would need to tap its reserves up north.

So in 2014, Mr. Ethredge, who was still city manager at the time, began working with the new City Council to develop plans for a pipeline that would carry water directly from the Poudre River to Thornton, bypassing the farms that had relied on that water for decades.

In nearby Weld County, construction began. About seven of the planned 72 miles have now been built, at a cost of about $20 million.

But in Larimer County, home to Fort Collins and some of the land that Thornton owns, the resistance was immediate.

“Of course I don’t want it here,” said Barry Feldman, who owns a restored ranch house on an expansive property where he keeps horses. “They will have to spend time digging, disrupting our lives, and there’s the possibility of repairs. Go bother someone else.”

Mr. Feldman, an active member of an opposition group called No Pipe Dream, said he believed Thornton had a right to access the water. But he doesn’t want the pipeline in his backyard.

Other opponents said they, too, were fine with Thornton accessing the water, but they wanted it to run down the Poudre River, where it could help revitalize an ecosystem long taxed by the region’s agriculture, then be put into a pipeline many miles downstream.

“We have one single mission, which is to protect and restore,” said Gary Wockner, an environmentalist and the founder of another opposition group, Save the Poudre, as he walked along the river bank in downtown Fort Collins. “We’re not trying to stop them from getting their water, we’re not trying to stop growth.”

But letting pristine snowmelt blend into the Poudre River and flow past urban areas and farmland, where it would mix with wastewater runoff, fertilizers and other contaminants before it would reach Thornton, was not acceptable to Mr. Henry and other city officials. The pipeline, they said, was the only viable option, and Thornton went ahead with plans for construction.

In 2018, under relentless pressure from the public, Larimer County officials denied Thornton the permits to build the pipeline, saying that the plans for the route were not specific enough.

Thornton sued Larimer County, seeking to override its objections. But a district court and an appeals court ruled in favor of Larimer. Last year, Thornton said it would not appeal to the state Supreme Court.

Instead, Mr. Henry and his team have gone back to the drawing board and are working on a new, more detailed route for the pipeline. They expect to submit new plans in late September.

In the meantime, Thornton businesses are fuming. Late last year, the Home Builders Association of Metro Denver published an open letter calling for a swift resolution that would let Thornton access its water.

“By refusing to allow Thornton to access the water it has owned for nearly 40 years, Larimer County is impacting the livelihood and ability for families and individuals to live closer to where they work and inadvertently exacerbating Colorado’s housing attainability crisis,” wrote Ted Leighty, the group’s chief executive.

Maiker Housing Partners, a real estate developer, has suspended its plans to erect hundreds of units of affordable housing near a newly opened commuter rail station in Thornton.

“Thornton staff has informed Maiker that no new residential projects will be able to start construction until the water issue is fully resolved,” Peter LiFari, Maiker’s chief executive, said in a statement.

If the pipeline is built and the water from the Poudre starts flowing to Thornton, city officials say the city’s population could expand to 242,000 in the decades to come, from about 150,000 now.

Meati, the plant-based food company that Thornton officials said could not expand, declined to say how that was impacting its business strategy. But the company said it was trying to conserve water.

“We take conservation and sustainability seriously, and we have designed a purpose-built production system that minimizes reliance on, and efficiently uses, precious resources including water,” Christina Ra, a Meati spokeswoman, said in an email. “We are confident in our ability to scale.”

City officials are working to conserve water. During the winter, Thornton uses about 13 million gallons of tap water per day. That spikes to more than 40 million gallons a day during the summer, largely because of people watering their lawns with water that is suitable for drinking, a number Thornton is working to reduce.