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.

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