An hour’s drive from Las Vegas stands America’s Hoover Dam, a commanding barrier of concrete holding back the trillions of gallons of Colorado River water held inside Lake Mead.

The dam is a proud place, built by thousands of hands and with 5 million barrels of concrete. Its golden elevator doors, Gotham-esque pillars, and stoic guardian angel statues line the lofty walkways atop the structure. A U.S. flag beating patriotically over the desert gets swapped out every few days, and then put out for sale in the visitor center.

Yet, in the 80 years since the great dam’s completion, the 1,450-mile Colorado River – which sustains some 40 million Americans in places like Phoenix, Las Vegas, and Los Angeles — has been gradually growing weaker, and the water level beyond the noble dam has fallen considerably over the last two decades. The writing is easily spotted on the steep rocky walls of the Lake Mead reservoir, where a bathtub-like ring shows where the water once sat during more fruitful times.

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Today, however, the water sits 150-feet below that line, and human-caused climate change is a major reason why.

Over the last century, the river’s flow has declined by around 16 percent, even as annual precipitation slightly increased in the Upper Colorado River Basin — a vast region stretching from Wyoming to New Mexico.

The "bathtub" ring in Lake Mead behind the Hoover Dam, showing the low level of water in 2015. Image: Jae C. Hong/AP/REX/Shutterstock

New research published in the journal Water Resources Research argues that over half of this decline is due to sustained and rising temperatures in the region, which ultimately means more water is evaporated from the river, diminishing the flow.

But it’s really been in the last twenty years that matters have deteriorated into a major drought, edging the region toward a potential water-rationing crisis.

It's the worst drought in Colorado River history.

“The river since 2000 has been in an unprecedented decline,” Brad Udall, coauthor of the new study and senior water and climate research scientist at Colorado State University, said in an interview.

“There’s no analog, from when humans started gauging the river, for this drought,” said Udall.

Global temperatures compared to average surface temperatures. Blue shows colder, orange and reds warmer. Image: nasa Global temperatures compared to average surface temperatures. Blue shows colder, orange and reds warmer. Image: nasa

To be clear, tens of millions of Americans are not yet imperiled by the drought, but trouble lies in the years ahead.

If trends continue, Udall previously projected considerable declines of Colorado River flow by around 20 percent in the next 30 years. By the century's end, this number could increase to 35 percent.

The Southwestern U.S. cities dependent on the Colorado River for life and prosperity will have to adapt to a world that’s now warming at an accelerating pace.

"We need to adapt to drier circumstances," Karl Flessa, a geoscientist at the University of Arizona who studies water policy in the U.S., said in an interview.

In practice, this means using significantly less water both in communities and agriculture (which drinks some 80 percent of the Colorado River), Flessa, who had no involvement in the study, said.

The Colorado River Basin Image: usgs

Over the last 150 years, humanity has emitted enough greenhouse gases into Earth’s atmosphere to bring carbon dioxide levels up to the highest they’ve been in 800,000 years. The planet is already warmer than it’s been in 120,000 years.

“We’re locked in — we can’t go back to the way it was before,” Jonathan Overpeck, a climate scientist at the University of Michigan who has previously studied the Colorado River but had no role in this study, said in an interview.

To determine the climate’s role in the diminishing river flow, Udall and his team employed a widely used weather model with precipitation data from the last 100 years to determine how the Colorado River would have likely flowed without an increased warming of 1 degree Celsius, or 1.8 degrees Fahrenheit.

Then, they compared this model with the warming that has actually occured over the last century, showing how much influence rising temperatures have had on the river’s decline.

“It’s a compelling and extraordinarily detailed piece of work — it really nails it,” said Flessa. “The message is climate change is going to decrease the amount of water in what is already a stressed system."

The Hoover Dam. Image: Shutterstock / Palm Tinnapat

Rising temperatures, however, are only half the story.

Udall found that the other 50 percent of diminishing river flow comes from rain and precipitation falling in the wrong places, meaning areas that are less likely to feed into the Colorado River system. For instance, more plentiful rainfall in Utah deserts will simply evaporate away, said Udall.

Still, this study underscores that rising temperatures play an outsized role in the diminishing Colorado River flow, as more heat waves and longer warm seasons dry out the long, winding river. And although some climate analyses predicted more rainfall this century in the Colorado River basin, the rains haven’t yet, and might not ever, come.

“Banking on increased precipitation to bail us out is a really bad bet,” said Udall. Instead, Udall emphasized “the need to stop emitting greenhouse gases.”

“As long as we keep increasing temperatures on the planet, we’ll have less and less water in the Colorado,” said Overpeck.