The Colorado River is evaporating, and climate change is a big culprit

An hour's drive from Las Vegas booths 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.

Today, however, the water sits is a major reason why.

Over the last century, the river has dropped by around 16 percent, even increased slightly in the Upper Colorado River Basin – a vast region stretching from Wyoming to New Mexico.

New research published in the journal Water Resources Research argues that this is one of the most important causes of water loss in the world, which decreases the flow.

But it has really been in the last twenty years that matters have deteriorated into a major drought, edging the region towards 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 people start gauging the river, for this drought," said Udall.

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

If trends continue, Udall has already been projected to be significant in Colorado River by 20 percent in the next 30 years. By the century's end, this number could increase to 35 percent.

The Southwestern U.S. cities depend on the Colorado River for life and prosperity.

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 water and agriculture (which drinks some 80 percent of the Colorado River), Flessa, who had no involvement in the study, said.

Over the last 150 years, the world has grown to 800,000 years. The planet is already warmer than it's been in 120,000 years.

"We're locked in – we've been here before," Jonathan Overpeck, a climate scientist at the University of Michigan, who has previously studied in the Colorado River. interview.

To determine the climate and the role of the river in the flow of water and climate change, he said: degrees Fahrenheit.

Then, they compared this model with the warming that has actually occured over the last century, showing how much influence has 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 moving towards a deeper economy."

Rising temperatures, however, are only half the story.

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

Still, this study underscores that rising temperatures in the Colorado River flow, winding river. And though some climate analyzes predicted more rainfall this century in the Colorado River Basin, the rains have not yet, and might not ever, come.

"Banking on increased precipitation 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're in Colorado," said Overpeck.