Arizona and California are arguing over Colorado River water again — this time over whether it should be inscribed in law that California can’t take Arizona’s share of river water that’s left in Lake Mead to prop up lake levels.

The dispute is over whether it would benefit the troubled river system to guarantee in writing that one state can’t take another state’s water that’s left behind in the lake — or whether such an effort could disrupt already delicate negotiations over the river’s future.

Arizona water officials and U.S. Sen. Jeff Flake are interested in seeing such language go into a Western drought bill that’s being negotiated in the U.S. Senate under the direction of Sen. Dianne Feinstein, a California Democrat.

“My No. 1 priority is that if we have a voluntary agreement to leave water behind the dam, that the water stays with the user who put it there,” Flake told a meeting of Arizona and Tucson-area water officials last week.

Speaking at a water conference in Phoenix, he said, “We want to make sure it doesn’t disappear behind a California canal.”

He said later that “it stands to reason” that water users who voluntarily leave some of their share in the lake should have firm assurance that it will stay there.

California water officials say such language would be unnecessary and counterproductive, leading to future conflicts at a time when all seven river basin states are trying to manage the ailing river collaboratively.

“We don’t see the need for legislation. If he moves forward with something that we object to, that would be problematic,” countered Tanya Trujillo, executive director of the Colorado River Board of California, which represents that state’s interests in Colorado River matters.

“The traditional way we’ve done things in the basin is that we work among the states in trying to reach agreement. If we do need legislation, we let our representatives know what we’re looking for and move forward in a collaborative fashion,” said Trujillo, adding that she is working with Arizona and Nevada to come up with language in an agreement on this matter that no side would object to.

“I don’t think Arizona is wrong” in seeking legal language to protect itself, said UA Law professor Robert Glennon, a water expert who has written two books on water.

The 1922 Colorado River Compact that divided river water rights among states is one of many poorly drafted elements in the legal framework known as “the Law of the River,” he said.

But Arizona’s effort poses a political risk, he said, since once you get one issue tucked into law, another state could bring up another issue.

“There was a lot of contention over the river from the 1930s to 2000, but in the last 15 years there’s been remarkable conciliation and cooperation,” Glennon said. “It would be a shame if anyone is perceived as going it alone.”

Today, the river basin states are trying to negotiate an agreement to conserve additional water to store at Mead. The goal is to prevent lake levels from dropping below 1,020 to 1,025 feet, which could trigger severe shortages that could reduce Central Arizona Project water deliveries to Arizona cities and tribes.

Currently, the U.S. Bureau of Reclamation predicts that Mead will drop to 1,077 feet at the end of 2016. That level is 2 feet above what would trigger a milder cutback of deliveries to Central Arizona farms and groundwater recharge facilities.

At stake in the Arizona-California dispute is water that states agree through interstate negotiations and other agreements to leave in the river. So far since the drought intensified in recent years, Arizona has left about 380,000 acre-feet in the river in two programs. Those supplies are protected from transfer to another state due to language written into the agreements. But as more water is saved in future agreements, Arizona wants to make absolutely sure that none of it could ever be transferred.

This dispute is surfacing about nine months after Arizona Gov. Doug Ducey warned in a speech that this state must be vigilant against efforts by California to take Arizona’s river water. California officials denied any such ambitions, and several outside experts said they thought it was very unlikely. Arizona also has long felt vulnerable because this state has a lower priority than California for use of river water during shortages.

But there is a legal way California or any state could get another state’s river water without legislation. It’s outlined in a 1964 U.S. Supreme Court decree, carrying out its landmark Arizona v. California decision of a year earlier.

The decision was a legal victory for Arizona because it for the first time guaranteed this state access to 2.8 million acre-feet of river water — or almost two Central Arizona Projects’ worth. But the 1964 decree said that if a state doesn’t use its allocation in a given year, the Interior Secretary can transfer the water to another state.

That language allowed California to take more than 5 million acre-feet of river water for many years until 2002, even though its allocation is for 4.4 million, said Tom Buschatzke, director of the Arizona Department of Water Resources.

Like California officials, Arizona officials would rather have these issues resolved in negotiations rather than turning to legislation, said CAP general manager Ted Cooke . But so far, some parties in California haven’t been willing to agree on this point, added Cooke, who declined to be more specific.

“It’s a backstop,” Cooke said of the use of legislation. “If we fail to reach agreement on the water left behind, we have to have this other approach to be absolutely certain.”

While the states are having some success with the collaborative approach now, “there are some folks who would like to convince us that this is proceeding so swimmingly, so why don’t we abandon this language and everything would be fine,” Cooke said. “We really can’t do that.”

But putting guarantees into law takes away the Interior Secretary’s flexibility, and that’s needed in unanticipated circumstances, countered Bill Hasencamp, Colorado River resources manager for the six-county Metropolitan Water District of Southern California. The state has been willing to commit every year in writing to not seeking other states’ water that’s been stored in Mead, but doesn’t want to make a permanent commitment, he said.

“If there is an emergency in California, we might argue to the secretary that ‘You should allow some water to go to California,’” Hasencamp said. “The secretary would consult with Arizona and Nevada and come up with a plan. We don’t know what contingencies would arise in the future, so tying up the secretary’s hands doesn’t seem like a good idea to us. It could be an emergency in Arizona or Nevada that we don’t know about.”

Interior officials have sought to assure Arizona that their unused water stored in Mead isn’t going to be transferred.

Terry Fulp, the Bureau of Reclamation’s Lower Colorado Regional Director, made that promise nearly a year ago in a letter to then-CAP General Manager David Modeer involving 40,000 acre-feet that CAP agreed to leave in Mead.

Last fall, Deputy Interior Secretary Michael Connor made a similar promise in an exchange with Flake in a Senate hearing. He noted that since 2007, the secretary had never used the department’s legal right to reallocate unused river water.

But he added, “I can give you my word until January 2017 (when a new administration takes office). It’s not going to mean much until we lock it into agreements.”

Contact reporter Tony Davis at tdavis@tucson.com or 806-7746. On Twitter: @tonydavis987.