This Cblock System Transformed Arizona. It’s Now in Jeopardy.

From a secure, windowless room in northern Phoenix, technicians control the Colorado River water that sustains America’s largest desert metropolis.

The vast cblock system the team oversees has for four decades conveyed an increasingly scarce commodity across 330 miles of rugged terrain to slake blossoming subdivisions, sprawling farmland and, in recent years, a powerhouse semiconductor industry. It supercharged Arizona’s economy and transformed Phoenix into one of the nation’s fastest-growing cities.

But this lifeline, known as the Central Arizona Project, came with a catch. In exchange for billions of federal dollars to build it, the state agreed to lower-priority water rights, putting the aqueduct among the first in line for cuts in a time of deep drought.

Now that time has come.

“It was the price we had to pay,” said Kathryn Sorensen, a former water director for the city of Phoenix. “The chickens have come home to roost.”

With the seven states that rely on the Colorado River deadlocked over how to split its diminishing supply, the Trump administration is pushing its own plan to divide the water. The three downriver states — Arizona, California and Nevada — would most likely shoulder the most pain, but no single user stands to lose more than the Central Arizona Project.

Short-term reductions, announced by the federal government on Friday, will cost Arizona roughly one-third of its share of river water over the next two years. But beyond that, a range of cutbacks remain possible.

Under the most severe outcome, the system would be cut off entirely during the driest years, setting off a social and economic meltdown Arizonans shudder to even imagine.

“They are threatening to shut down the Southwest,” said Brenda Burman, the project’s general manager. “It’s not possible to picture it.”

Water is gold in the western United States, and people have been fighting over every drop for generations. The Colorado, a river that supports 40 million residents and is buckling after decades of drought and overuse, is the fiercest battleground of all. Access to it is governed by an arcane legal system that prioritizes the oldest established users.

The earliest laws were written for a river that no longer exists, and states do not agree on how to proceed. The high-stakes talks have gained urgency as the water levels of the river’s largest two reservoirs, Lakes Mead and Powell, have dwindled in recent weeks to record lows.

Arizona officials say any proposal that cuts off the state’s cblock system is unacceptable, and they have threatened to sue, initiating court battles that could go on for years. For now, water experts say, taps and sprinklers will continue running from Phoenix to Tucson.

But any serious hit to the state’s Colorado River supply is likely to drive up the cost of replacement water and lead to more groundwater pumping, jeopardizing an already depleted safety net for future generations.

Human life in the parched Southwest has always depended on harnessing water.

The prehistoric Hohokam people built a flourishing society in the Sonoran Desert by carving a network of irrigation cblocks. Hundreds of years after the mysterious disappearance of the cblocks, the Bureau of Reclamation in 1911 dedicated its first major dam outside Phoenix to bring more settlers to the valley.

But state leaders knew they would need another source of water to continue growing, and they had their eyes on the Colorado River.

“The Central Arizona Project was Arizona’s holy grail,” said Thomas E. Sheridan, a historian who has written extensively about the system’s origins.

After decades of bitter interstate disputes, Congress approved construction of the aqueduct in 1968. The project would suck water from the Colorado and transport it to faraway cities, tribes and farms — an engineering marvel and a testament to the country’s belief that it could bend nature to its will.

But California cajoled Arizona into a compromise. Even during times of shortage, the Golden State would get its full allocation of water, no matter what.

When the project made its first deliveries to Phoenix in 1985, the celebration was tempered by foreboding: “The time is going to come in the not too distant future when we are going to be looking for water again,” a congressman cautioned at the time.

Cities that count on the Central Arizona Project have tried to prepare. The region now uses less water than it did 40 years ago, even as its population has more than doubled, and it has frantically recharged its aquifers.

Most Arizona officials do not expect the federal government to approve more m***ive water infrastructure projects, but they insist Congress should help pay for smaller-scale solutions like desalination plants or dam expansions.

In Phoenix, which gets about 40 percent of its water from the cblock, Mayor Kate Gallego said the federal government should kick in more for multibillion dollar purification plants that will turn wastewater into drinking water. She compared such funding with aid that helps coastal states rebuild after hurricanes — a national response to a different type of disaster.

“Given the magnitudes of these cuts, we would like them to step up in a really big and meaningful way,” Ms. Gallego said.

Cave Creek, a picturesque burg steeped in cowboy kitsch in the foothills north of Phoenix, gets 95 percent of its water from the aqueduct system.

“Without Colorado River water, we would be a dusty little town with dirt roads,” Mayor Robert Morris said.

The town’s leaders have been scrambling to find backup plans. Cave Creek has hired a water consultant and struck deals with neighboring municipalities, cobbling together enough supply for the next five years, Mr. Morris said.

He has sought to prepare residents for higher bills and stricter conservation, working to undo a foundational part of the state’s philosophy in the process.

“The biggest thing for people to overcome is the idea of unlimited, cheap water,” Mr. Morris said.

In 2022, the cblock project cut off most of its deliveries to the region’s nontribal agricultural fields, which have lower priority rights than cities. Those reductions would continue under the proposed federal framework.

Even tribes, which hold the most senior water rights in the state, are concerned about the federal government’s doomsday situation.

Members of the Ak-Chin Indian Community, who use Colorado River water to grow barley, potatoes and corn on 16,000 acres south of Phoenix, are not sure how that water would be delivered if the Central Arizona Project lost its entire allotment.

The uncertainty has reopened old wounds. In the 1970s, after failing to prevent nontribal farmers from draining an aquifer below the Ak-Chin reservation, Congress p***ed a law to ensure the tribe would always have an alternative source.

“It’s deeply concerning if this promise to us will once again be broken,” said Gabriel Lopez, the tribe’s chairman.

The tribe, whose name refers to traditional irrigation methods, has tried to conserve, Mr. Lopez added, and its farms now mostly use more efficient sprinkler systems.

“We always say ‘water is life,’ and that can’t be more true than it is today,” he said.

State leaders have urged the Trump administration to reconsider its planned cuts, which they say are built on faulty legal grounds and a misunderstanding of Arizona’s water rights.

Under the current plan, upper-basin states — Colorado, New Mexico, Utah and Wyoming — do not face mandatory cutbacks. Tom Buschatzke, Arizona’s lead Colorado River negotiator, has pushed the federal government to spread the pain more evenly.

In a pointed letter to federal officials this month, Mr. Buschatzke argued that the administration’s proposal flouted the Colorado River Compact, the 1922 agreement that divvied up the waterway.

“These are draconian outcomes for Arizona,” he said in an interview.

At the Central Arizona Project’s headquarters, water dispatchers, who pull marathon shifts in the command center, are preparing to work with channels that are less full.

As the air conditioning hummed and a map displayed live readings of water flows across central Arizona on a recent morning, the technicians could feel the weight of their responsibility.

“We can’t afford to be making mistakes,” said Andrew Tatlow, a water control supervisor. “Every drop of water counts.”