DENVER >> A vital reservoir on the Colorado River will be able to meet the demands of Mexico and Southwestern U.S. states for the next 13 months, but a looming shortage could trigger cutbacks in late 2019, officials said today.

A forecast from the U.S. Bureau of Reclamation echoes previous warnings that a nearly 20-year trend toward a drier regional climate coupled with rising demand could drain so much water from the Lake Mead reservoir that cutbacks would be mandatory.

The report increases the pressure on seven U.S. states that rely on the river to finish a long-delayed contingency plan for a shortage.

“If these projections materialize, we’re very quickly going to lose control of how to manage the deteriorating conditions on the Colorado River,” said John Entsminger, general manager of the Southern Nevada Water Authority, which serves 2.1 million people, including the city of Las Vegas.

The Colorado River system — including the giant Lake Mead and Lake Powell reservoirs — serves about 40 million people and 6,300 square miles (16,300 square kilometers) of farmland. Arizona, California, Colorado, Nevada, New Mexico, Utah and Wyoming rely on the river, along with native American reservations and northwestern Mexico.

The water is divided under international treaties, court rulings and interstate agreements. If there’s not enough water to go around, Mexico, Arizona and Nevada would be the first to see their shares reduced.

The Bureau of Reclamation forecast says all the users will get their usual share through September 2019. But the report projects that by October 2019, the surface of Lake Mead could fall below 1,075 feet above sea level, the agreed-upon point that would trigger cutbacks.

The bureau, which manages major dams and reservoirs nationwide, operates on a water year running from October through September.

Lake Mead has never had a shortage before.

The Colorado River states agreed to come up with contingency plans to conserve water and avoid mandatory cutbacks in the event of a shortage. But negotiations have been slow and difficult, in part because Arizona’s largest river users are still trying to agree on a unified state position, water experts said.

“Right now the thing that’s holding it up is Arizona and the inability to come together,” said John Fleck, director of the University of New Mexico’s Water Resources Program. “The whole system is at risk.”

Officials of the Arizona Department of Water Resources and the Central Arizona Project, the state’s largest water supplier, did not immediately respond to phone calls and emails seeking comment.