The water in Lake Mead, a key reservoir on the Colorado River, has slowly been disappearing.

The surface elevation of Lake Mead along the Nevada-Arizona border dipped to 1,071.56 feet last month. That level was last hit in July 2016 and it’s the lowest level since Lake Mead was filled in the 1930s, according to the U.S. Bureau of Reclamation.

That’s a scary picture for multiple states that rely on Lake Mead — and the Colorado River system that it’s a part of — for water, including California.

“When you look at kind of an average over the last 10 years, the Colorado River provides about 25% of the water that’s used in Southern California,” says Deven Upadhyay, chief operating officer of the Metropolitan Water District.

The Metropolitan Water District, which is the largest treated drinking water provider in the U.S., provides water to about 19 million Southern Californians in six different counties, much of which is delivered by the Colorado River Aqueduct.

Places like Las Vegas, Phoenix and Tucson also get water from the river. But soon, they may not be getting as much.

“We’re at the point where some serious decisions will likely have to be made,” said Doug Hendrix of the Bureau of Reclamation.

Hendrix said that in August, the record-low water levels in Lake Mead are expected to trigger the bureau’s first ever declaration of a Tier 1 water shortage on the system. That would mean cutbacks starting next year in the amount of Colorado River water sent to Nevada and Arizona — states that have already seen reductions in their share of the river’s water.

Mexico would also get less.

Thanks to senior water rights and other agreements, California’s share of water from the Colorado River won’t be cut just yet.

But if the trend continues, California could be next.