Two large lakes in the western United States have white rings around them. These circles show how sharply water levels are dropping. They are a warning that the 40 million people who use water from the Colorado River face a much drier future.

The Colorado River supplies seven U.S. states and Mexico. The seven states are Colorado, Arizona, California, New Mexico, Nevada, Utah, and Wyoming. People use its water in large U.S. cities such as Denver, Las Vegas, and Phoenix.

Colby Pellegrino is with the Southern Nevada Water Authority. He told The Associated Press: "The future of the river is going to be drier than the past. All the climate models and the current drought suggest that," he said. "Every sector is going to have to learn how to do more with less."

Many laws and agreements govern the use of the Colorado River's water. In 1944, for example, the United States signed a treaty with Mexico to guide water sharing between the two countries.

But even within the U.S., there are different agreements on how water is used. Twenty-two tribes in the Colorado River Basin have water rights that have been confirmed by courts, notes the Congressional Research Service. These agreements allocate – or divide and give out – amounts of water.

The Colorado River supplies enough water for 15% of total U.S. crop output and 13% of total livestock production. It also supplies Lake Mead and Lake Powell – the two largest man-made reservoirs in the United States.

The U.S. Bureau of Reclamation oversees much of the Colorado River's water supplies. It recently released estimates that suggest Lake Powell and Lake Mead will drop 5 meters and 1.5 meters, respectively, by January 2021. The numbers are in comparison to water levels in January 2020.

Even with the drop, Lake Mead's levels will likely stay above the point at which water cuts take place in Arizona and Nevada. In other words, officials will have more time to prepare for when such cuts may be necessary.

Since 1990, the population of the Las Vegas area has more than tripled. Las Vegas gets nearly 90% of its water from the Colorado River. But the area has been able to use far less water than it is allocated.

The Las Vegas area has done this by reusing almost all water used indoors and replacing nearly 790,000 square kilometers of grass with desert-friendly plants.

In other areas, officials are working to find different water sources for growing cities and farms. In Arizona, where nearly 40% of the water supply comes from the Colorado River, officials need to find other water sources, notes Ted Cooke of the Central Arizona Project.

Once Lake Mead's water levels sink low enough, Arizona will have the most painful cuts of any state based on a priority list — first rural farmers, then eventually cities.

"It's at least a couple of decades until we're saying, ‘We don't have one more drop for the next person that comes here,'" he said. "But people certainly ought to be aware that water... is expensive and, with climate change, going to get even more expensive."

I'm John Russell.

Sam Metz reported on this story for The Associated Press. John Russell adapted it for VOA Learning English. George Grow was the editor.

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sector – n. an area or part of something; a part of an economy that includes certain jobs

livestock – n. farm animals

reservoir – n. a large lake that is used to supply water

triple – v. to become three times as great or as many

priority – n. something that is more important than other things and that needs to be done or dealt with first

decade – n. a period of 10 years

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