Two and a half years after signing a deal aimed at averting a damaging crisis along the Colorado River, water officials from California, Arizona and Nevada are discussing plans to take even less water from the shrinking river and leave it in Lake Mead in an effort to prevent the reservoir from falling to dangerously low levels.

Representatives of water agencies from the three states said they are firming up the details of a deal that would leave an additional 500,000 acre-feet of water in the reservoir next year, and the same amount again in 2023 — about double the quantity of water used annually by Las Vegas and the rest of southern Nevada.