Thornton, facing projections that its population will grow by well more than half over the next 50 years, is ready to claim the water it bought in the Cache la Poudre River three decades ago.

But this Denver suburb’s proposed 70-mile pipeline — skirting the north end of Fort Collins and running down the east side of Interstate 25 through fast-growing towns like Windsor, Johnstown and Frederick — is running up against stiff opposition.

A growing contingent in Larimer County claims that Thornton is missing an opportunity to improve the river’s health — factors including flow and ability to support wildlife — by starting the pipe several miles downstream from Fort Collins. They also claim that by shortening its pipe, Thornton would save itself millions of dollars and spare homeowners disruptive trenching and placement work for the 48-inch diameter pipe.

“Why not just leave it in the river and take it out further downstream?” asked Gary Wockner, executive director of Save the Poudre, a group that aims to “protect and restore” the waterway that flows from Rocky Mountain National Park to the South Platte east of Greeley. “Thornton could be a hero — they could save money and provide environmental benefits to the Cache La Poudre River.”

The city, which predicts the pipeline could be completed by 2025, counters that allowing its water to flow through Fort Collins before piping it would degrade its quality. Moreover, Thornton says it is only legally allowed to take water out of the Poudre River north of Fort Collins under its current water decree, and that diverting water further downstream would require a new federal permit.

In many ways, the backlash to Thornton’s water delivery plans reflects the wider battles over water that have long been fought in Colorado and will likely intensify as growth in the state continues to leapfrog itself year after year. Several years ago, state planners projected statewide shortfalls of 163 billion gallons of water a year by 2050, spurring adoption of a $20 billion plan that emphasizes conservation, reuse, additional storage and more sharing between farmers and cities.

“As population grows, especially along the northern Front Range, the value of water is going to keep rising,” said David Nettles, division engineer for northeast district with the Colorado Division of Water Resources. “And when it’s rising, everyone will scrutinize every drop.”

Several hundred opponents to Thornton’s water pipe are expected to show up at a Larimer County Planning Commission meeting Wednesday evening to decry the city’s proposal. The commission is set to decide on whether to recommend for or against a permit for the $450 million project, which would pass through 26 miles of the county.

The final decision on the pipeline permit will be made by the Larimer County commissioners this summer.

Aside from preserving river health, concerns over the project center largely on Douglas Road, north of Fort Collins, where Thornton has proposed ripping up several miles of the road so it can bury its pipe in the public right-of-way. Karen Wagner, a former county commissioner and member of the grassroots group No Pipe Dream, said it’s important that Thornton tread lightly in an area that will derive zero benefit from its water pipe.

“We want Thornton to do the right thing by Larimer County,” Wagner said. “We think that Thornton should not destroy any county neighborhoods for its pipeline.”

Thornton says it is trying to tread as lightly as it can by getting ahead of planned housing developments along the I-25 corridor. Mark Koleber, Thornton Water Project director, said the city is trying to negotiate with landowners for right-of-way now to bury its pipe before rooftops and storefronts go in.

“If they put in roads and landscaping, they’ll have to tear it up right after (for the pipe),” Koleber said.

And it’s not like Thornton is springing its project on Larimer County and other communities to its north without warning, he said. Thornton bought its water shares in the Poudre River in the 1980s in anticipation of growing steadily over the years and cemented those rights through hard-fought and sometimes bitter court battles.

The city even bought 20,000 acres of farmland out by Eaton, Ault and Pierce to which to send the Poudre water it purchased while it waited for growth in the city’s population to justify shifting the water from agricultural purposes to municipal ones. That time has come, Koleber said.

“The city looked forward, saw the reality and planned for it,” he said.

That reality was a spurt in population from around 50,000 people in 1985 to nearly 140,000 people today. By 2065, Thornton expects to have a population topping 240,000. By then, its Poudre shares will deliver 14,000 acre-feet of high-quality water — an acre-foot, generally speaking, can serve the needs of two families of four for a year — to its residents.

Thornton spokesman Todd Barnes argues that the city’s water pipe isn’t necessarily more expensive than leaving the water in the Poudre longer. The city worked up an alternative a few years ago that showed a savings of approximately $75 million by making the pipe shorter, but he said that was simply a “back of the envelope” calculation that was never fully vetted.

The main impediments to Thornton taking its water out after the river passes through Fort Collins, Barnes said, is water-quality degradation and the need to obtain a new federal permit to extract Poudre water anywhere other than where the city is permitted to take it out now — north of Fort Collins.

Wockner said his side is “just offering a positive alternative” for how Thornton moves its water, one that would fortify a critical river that is already heavily tapped by various water users before it even reaches Fort Collins’ city limits.

“They could not only keep the river alive, they could improve its health,” he said.

Nettles, with the division of water resources, said fights like the one Thornton finds itself in today are not likely to subside as long as Colorado remains a magnet for people on the move.

“With the development of both commercial and residential, I think things are becoming more difficult than when this was primarily agricultural land,” he said.