The Colorado River supplies nearly 40 million people in the U.S. with drinking water, but it’s been drying up at an alarming rate. The river runs through seven states. The Upper Basin includes Colorado, New Mexico, Utah, and Wyoming; and the Lower Basin includes Arizona, California, and Nevada.

When it comes to control, that’s largely on the federal government through the Bureau of Reclamation, which negotiates rules with each of those states. As the western U.S. faces its worst mega drought in centuries, the feds could soon step in unilaterally to cut supply.

Federal officials have announced plans to revise rules for how it operates the Glen Canyon and Hoover Dams, which form Lake Powell and Lake Mead, the country’s two largest reservoirs, which sit at 29% and 24% capacity respectively, according to Reuters. Officials have said lower levels threaten the ability to generate hydroelectric power for millions in the West.

In June, the feds gave states that rely on the Colorado River an option to come up with their own plans to cut back. They didn’t meet the deadline, and the bureau said it would stop hearing public comments on the proposals on Dec. 20. From there, one of three courses of action can be taken:

• a so-called “framework agreement” option where Colorado River basin states, tribes, cities and farmers come to agreement on how much water they can save through voluntary cuts to restabilize Lakes Mead and Powell;
• a “reservoir options modification” plan, where the federal government would take unilateral action and tell states how much water each must cut in order to save the river system; or
• a “no action” option where the current agreements controlling operations at Glen Canyon and Hoover Dam would continue.

A federal takeover could cut water supplies to California, Nevada and Arizona, according to Reuters. While officials have said they prefer a negotiated plan, they made it clear that they are willing to take decisive unilateral steps in an effort to protect the Colorado river system.

“The Interior Department continues to pursue a collaborative and consensus-based approach to addressing the drought crisis afflicting the West. At the same time, we are committed to taking prompt and decisive action necessary to protect the Colorado River System and all those who depend on it,” Secretary Deb Haaland said. “Revising the current interim operating guidelines for Glen Canyon and Hoover Dams represents one of many critical Departmental efforts underway to better protect the System in light of rapidly changing conditions in the Basin.”