(Bloomberg) -- States reliant on the Colorado River are running out of time before the US government steps in to cut water supplies.

The US Bureau of Reclamation is seeking a proposal by the end of January from Western states and tribes on dividing up proposed water usage limits, Commissioner Camille Touton told attendees at the annual Colorado River Water Users Association meeting. 

The Biden administration said it would step in as soon as next summer to mandate reductions if an agreement isn’t reached, as a historic megadrought dries out the river’s biggest reservoirs, Lakes Mead and Powell. 

“We’ve got our backs against the wall,” said Ted Cooke, general manager of the Central Arizona Project, which delivers the majority of that state’s Colorado River water allocation. “We’re going to have empty reservoirs here in a year-and-a-half if we don’t do something.” 

States and groups remained at an impasse at the water conference in Las Vegas this week, though stressed the need to strike a deal and avoid what could be significantly harsher reductions if the US government acts alone. The river provides water for 40 million people from Denver to Los Angeles, and a historic drought that’s wreaking havoc on crucial US food producing hubs and threatening electricity generation is a stark reminder of what’s at stake. 

No official negotiations took place this week, though informal talks on the sidelines of the conference could help toward generating a proposal. 

Some tribes are already considering next steps if no agreement is reached, including potential litigation, according to Manuel Heart, chairman of the Ute Mountain Ute Tribe in Colorado.

Tribes “need to be at the table to help— not just as consultants but helping to make new decisions with the states” and the US and Mexico, the two countries affected, he said. 

Democratic Senator Mark Kelly of Arizona encouraged states to come together on a solution before the government acts. 

“It’s better for everyone if the federal government doesn’t make these tough decisions,” Kelly said at the conference. “No matter what basin state we represent, we all want water users to take the lead here.” 

Tommy Beaudreau, deputy secretary of the Interior Department, remained optimistic in his conference address, saying he was “encouraged by the conversations by the basin states, including this week.” 

The Biden administration in August called for states to conserve up to 4.2 million acre-feet of water from the Colorado River annually through 2026 to ensure enough can serve the region. (One acre-foot is about 326,000 gallons, enough water to submerge a US football field one foot deep.) In May, the Interior Department’s Bureau of Reclamation said it would withhold 480,000 acre-feet of water in 2022 in Lake Powell to prevent its water levels from falling too low.

(Updates with tribe, Interior Department comments starting in second paragraph.)

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