PHOENIX (AP) — California communities exposed to hazardous dust by a drying lake bed have found themselves at the center of tensions between Arizona and California over how to conserve water along the overtaxed Colorado River.

U.S. Sen. Mark Kelly, an Arizona Democrat facing reelection, wants the federal government to withhold money for environmental cleanup at the Salton Sea until California agrees to use less of its share of the river. He also faulted the U.S. Bureau of Reclamation for not being clear about when and how it will act if the seven Western states that rely on the river fail to significantly lower their use.

“We are out of time,” Kelly wrote Tuesday in a letter to the U.S. Department of the Interior. “The longer the Department waits to press for an agreement … the more difficult this crisis will be to solve, leading only to tougher choices and litigation.”

Federal officials in June said the states must dramatically cut usage as key reservoirs risk dropping so low they can’t produce hydropower or supply water users. But the states blew through an August deadline without a plan. Congress has dedicated up to $4 billion in part to pay farmers and cities to use less water, but its impact remains unclear.

Much attention is on California, the largest holder of the river’s water and the last to lose in times of shortage. The state’s users said recently they would cut use up to 9% contingent on federal money and a plan to clean up toxic dust around the Salton Sea.

The lake formed in 1905 when the river overflowed and is mainly fed by runoff from southeast California farms. As it dries, wind kicks up particles that worsen air quality. When the farms use less river water, less excess flows into the sea.

California officials and the community group Alianza Coachella Valley were surprised by Kelly’s letter. They said it’s unfair to use communities exposed to environmental harms as a bargaining chip.

“The Colorado River system is in crisis, what we need is less finger pointing and more actual water conserved,” Wade Crowfoot, secretary of California’s Natural Resources Agency, said.

Kelly’s letter comes amid a tough reelection bid against Republican Blake Masters, a contest that will help determine control of the U.S. Senate.

As farmers and cities across the West face diminishing water supplies, anxiety about Arizona’s future water access has become a major issue particularly in cities like Phoenix and Tucson.

Nevada, Utah, New Mexico, Wyoming and Colorado, tribes and Mexico also hold rights to the river’s water. It helps supply drinking water to an estimated 40 million people plus countless farms that grow vegetables and crops for the nation.

Latest federal projections show that the dam at Lake Powell — a critical reservoir on the Arizona-Utah border — won’t be able to produce power by the end of next year if rain and snowfall is minimal.

Already, Arizona, Nevada and Mexico have endured mandatory supply cuts. California eventually would be looped in to those cuts if Lake Powell and Lake Mead continue to drop as projected.

The savings California offered in October — 400,000 acre-feet of water annually — make up about about one-fifth of the minimum amount federal officials say needs cutting across the basin.

Tom Buschatzke, director of the Arizona Department of Water Resources, said Wednesday he didn’t consider California’s offer a firm commitment. Arizona has saved some water beyond its mandated cuts since June but hasn’t settled on how much more it would offer, he said by email.

Any federal money given to California to address the Salton Sea should be tied to a promise by the state to keep water in Lake Mead, Buschatzke said.

Withholding money for cleanup projects around the lake “would affect real communities already suffering from higher rates of asthma and other health problems,” Silvia Paz, executive director for Alianza Coachella Valley, said in a statement.

Kelly said Wednesday it’s wrong for California to demand money for the Salton Sea to resolve a crisis on the Colorado River.

“I’m not going to let California get away with that,” Kelly said after a campaign stop in Phoenix. “You can’t hold the Colorado River hostage with funding for something else. Doesn’t matter what it is. I mean, this is water we’re talking about.”

Kelly also wants specifics about when California will be limited from taking out water it stores in the lake.

The Interior Department declined to comment on Kelly’s letter, spokesman Tyler Cherry said.

Masters, Kelly’s Republican rival, calls for an even more aggressive confrontation with California, saying during a recent debate, “we can solve this problem with technology and sharp elbows.”

“Why is California even putting its straw into the Colorado River?” Masters said, suggesting the state instead should be removing salt from ocean water to increase its supply.

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Ronayne reported from Sacramento, California. Associated Press writer Felicia Fonseca in Flagstaff, Arizona, contributed.