Arizona is threatening to sue the Trump administration over its plan to limit the state’s access to the drought-stricken Colorado River, which supplies Phoenix — America’s fifth-biggest city — with about 40 percent of its water.

Officials in the Grand Canyon State say the cutbacks are based on a misunderstanding of Arizona’s water rights and faulty legal reasoning, The New York Times reported Sunday.

Short-term reductions announced Friday would cost Arizona about one-third of the water it draws from the river over the next two years, with the possibility of a complete shutoff if drought conditions worsen, according to the NYT.

“They are threatening to shut down the Southwest,” said Brenda Burman, general manager of the Central Arizona Project aqueduct system. “It’s not possible to picture it.”

About 40 million people in seven western states rely on water from the Colorado River and its two largest reservoirs, Lakes Mead and Powell, which are now at record lows. Under the administration’s plan, four upper-basin states — Colorado, New Mexico, Utah and Wyoming —wouldn’t face mandatory federal cutbacks that target Arizona, California and Nevada.

Hardest hit, the NYT said, would be the canals of the 330-mile Central Arizona Project, which supplies water to Phoenix, population 1.6 million, and the surrounding region.

The potentially dire situation resulted from an agreement under which the federal government spent billions of dollars to build the system, in exchange for it being among the first to face supply cuts during water shortages.

“It was the price we had to pay,” former Phoenix water director Kathryn Sorenson told the NYT. “The chickens have come home to roost.”

Earlier this month, the director of the Arizona Department of Water Resources sent a stinging letter to officials at the Department of the Interior and the Bureau of Land Reclamation, saying the state reserved its right to take the matter to “an appropriate judicial forum,” the Los Angeles Times reported last week.

“Arizona does not accept a framework that gives the federal government the discretion to select from a wide range of alternatives — including catastrophic cuts,” director Tom Buschatzke wrote.

Neither the Department of the Interior, the Bureau of Land Reclamation nor the White House immediately returned inquiries Sunday from The Independent.

Historian Thomas Sheridan, who’s written about Phoenix’s Central Arizona Project origins, described the system as “Arizona’s holy grail” to continue the state’s growth after the Bureau of Reclamation built its first major dam outside Phoenix in 1911.

Water from the Colorado River began flowing into Phoenix in 1985, nearly two decades after Congress authorized the government’s most expensive water-supply project in 1968.

But even back then, some warned that the tap would run dry one day.

“The time is going to come in the not-too-distant future when we are going to be looking for water again, and today is the time to start looking for it,” former U.S. Rep. Eldon Rudd of Arizona said during a ceremony to mark the system’s opening, the NYT reported at the time.

Water experts told the NYT that Phoenix and other Arizona cities that rely on the river weren’t in immediate danger of running out of water.

But losing access to river water will likely drive up the cost of replacing it and lead to more pumping of already-depleted groundwater that would threaten future generations, the NYT said.

Phoenix Mayor Kate Gallego wants the federal government to help fund new, multi-billion-dollar purification plants to make wastewater safe to drink, saying the city faced a disaster similar to those suffered by coastal states ravaged by hurricanes.

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

But the problem facing Phoenix is dwarfed by that in tiny Cave Creek, which gets 95 percent of its water from the Central Arizona Project.

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

Morris said the town has hired a water consultant and has made deals to buy water from nearby municipalities that should keep the taps running for the next five years, and he’s warned his 5,200 constituents to brace for higher water bills and stricter conservation rules.

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

Although Native American tribes hold the state’s highest-priority water rights, leaders worry about federal cutbacks, with the Ak-Chin Indian Community unsure where it will get water to grow crops if the Central Arizona Project runs dry.

The tribe is still smarting from the 1970s-era draining of an aquifer under its reservation by non-tribal farmers, which led Congress to pass a law mandating that it would always have access to water.

“It’s deeply concerning if this promise to us will once again be broken,” the tribe’s chairman, Gabriel Lopez, told the NYT. “We always say ‘water is life,’ and that can’t be more true than it is today.”