CLARK COUNTY, NV (CBS Newspath) – A punishing drought is gripping much of the western U.S. Scientists are calling it a “mega-drought” brought on by climate change.

It’s taking a dramatic toll on the Colorado River system that provides water to 40 million people in seven states. For more than eight decades, the Hoover Dam has relied on water from Nevada’s Lake Mead to cover up its backside.

But now at age 85 it finds itself uncomfortably exposed. Much of the water the dam is supposed to be holding back is gone.

Pat Mulroy is the former head of the Southern Nevada Water Authority. She says Lake Mead, the nation’s largest reservoir, is on track to soon hit its lowest level ever recorded.

This part of the Colorado River system is a crucial water supply for Las Vegas, Phoenix, and southern California. It makes the vast agricultural land of the desert southwest possible. “This landscape screams problems to me. I mean just look at the bathtub rings, to me that is an enormous wake-up call,” said Mulroy.

Lake Mead is at just 37% of its capacity. It hasn’t been full since back in 2000 when the water came right up to the top of Hoover Dam. Since 2000, Lake Mead has dropped 130 feet.

“We’re at a tipping point. It’s an existential issue for Arizona, for California, for Nevada. It’s just that simple,” said Mulroy. For the first time ever, the federal government is expected to declare a water shortage on the lower Colorado River later this summer.

That will force automatic cuts to the water supply for Nevada and Arizona starting in 2022. Homeowners have higher priority and at first, it won’t feel the pain as badly as farmers. Dan Thelander is a second generation family farmer in Arizona’s Pinal County.

The water to grow his corn and alfalfa fields comes from Lake Mead. “So next year we are gonna get 25% less water, means we’re going to have to fallow or not plant 25% of our land,” said Thelander.

In 2023, Thelander and other farmers in this part of Arizona are expected to lose nearly all of their water from Lake Mead so they are rushing to dig wells to pump groundwater to try to save their farms. “The future here is, I hate to say it, pretty cloudy,” said Thelander.

Pat Mulroy says a rapidly retreating reservoir may be the new normal and the millions of people who rely on this water supply will have to quickly learn to live with less of it. “We don’t change unless we absolutely have to. Well, when you look out at this lake, I think that moment of it’s absolutely necessary has arrived,” said Mulroy.

At the Hoover Dam, engineers have installed new turbines designed to keep electricity flowing even as the water levels drop.