The US government has proposed a plan for the drought stricken Colorado River that could cut up to 40% of current supplies to Arizona, California and Nevada, as the waterway’s reservoirs continue to plunge to critically low levels.

A top Arizona water official shared details of the Trump administration’s plan at a state meeting on Wednesday.

Under the 10-year plan, which will be finalized in June, the annual amount of water delivered to Arizona, California and Nevada could be slashed by up to 3m acre-feet, according to Tom Buschatzke, director of the Arizona department of water resources. The reductions would be evaluated every two years.

Three million acre-feet of water is enough to supply 6 million to 9 million households for one year, more than the number of homes in Arizona and Nevada.

Buschatzke said the federal plan would be either implemented under existing Colorado River law or through agreements among the states. He said federal officials had indicated that water cuts across the three lower-basin states would be based on the “priority of the law of the river”. That law, the 1922 Colorado River Compact, gives California the highest priority for water use.

Buschatzke described the proposed federal cuts as “sobering”.

“That’s us, that’s Arizona, and potentially CAP going to zero,” said Buschatzke, referring to water flows on the Central Arizona Project, a canal that transports Colorado River water to central and southern Arizona.

The Colorado river supplies water to some 40 million people in the American west. The plan comes months after the seven states that depend on the river’s dwindling supply missed a February federal deadline to agree upon how water cuts would be divided. The river has lost about 27.8m acre-feet of groundwater in the last 20 years, largely owing to overuse. A record snow drought this year further exacerbated the issue.

The river’s upper basin states, Colorado, Utah, Wyoming and New Mexico, have been resistant to water reductions. The states maintain that those downstream, California, Arizona and Nevada bear responsibility for the water’s shortages and thus should carry the burden of cuts.

Doug Burgum, the US interior secretary, indicated earlier this year that the US Bureau of Reclamation would step in to manage the protracted dispute.

Two weeks ago, California, Arizona and Nevada announced their own proposal for voluntary water reductions up to 3.25m acre-feet through 2028. Under their offer, Arizona’s water flow would be slashed by 760 acre-feet, California by 440 acre-feet and Nevada by 50 acre-feet.

However, it’s unclear if the states’ plan will go ahead, and it would still require cooperation from state water agencies and the federal government. Alex Smith, an employee with the US Bureau of Reclamation’s Phoenix office, told AZ Central the agency is evaluating the risks and benefits of the lower basin states’ plan.

During public comment on Wednesday, Patrick Adams, senior water policy adviser to Katie Hobb, the Arizona governor, said “things are moving very quickly”, referring to the federal government’s proposal.

“The risk of 3m acre-feet of reductions only in the lower basin is something that’s quite alarming to us. So we need to grapple with that,” he said.

Reuters contributed reporting