In 2000, Lake Mead was full of deep, midnight-blue water that flooded the banks of the rivers that fed it. But 20 years later, it has shrunken drastically. And its basins are lighter, too, almost teal in places, a sign of increasingly shallow waters connected by extraordinarily skinny canyons.

In new images from this month, the lake is encircled now by a puckered shoreline and a white shadow, the so-called bathtub ring, remnants of salts and minerals left behind on the canyon walls by receding water.

“These reservoirs were stunningly full 20 years ago,” said Jennifer Pitt, the Colorado River program director for the National Audubon Society, referring to Lake Mead and Lake Powell, the two large reservoirs on the Colorado River. The low levels at Lake Mead are indicative of dangerously low levels throughout the entire Colorado River basin. Now the basin “finds itself perilously close to a Day Zero situation,” Ms. Pitt said, referring to the point at which the reservoir goes dry.

The satellite imagery underscores how acute the Southwest drought has become. Lake Mead, the largest reservoir in the United States, is a critical source of water for 25 million people across seven states as well as some of the country’s largest agricultural valleys.

In response to the growing crisis, the federal government has taken steps to conserve the water in the Colorado River basin. Last summer, the federal government declared a water shortage at Lake Mead for the first time. In June, in response to worsening conditions, the Bureau of Reclamation, which manages water and power in the West, issued an emergency request to states to propose immediate cuts for 2023 in order to prevent the reservoirs from falling further.

A Boat In A Lake Is Dwarfed By Cliffs, Which Show White From Mineral Deposits Left From The Shrinking Lake. Above The Hard Horizontal Line Of White Deposits, The Cliffs Are A Dark Brown.
An area of Lake Mead near the Hoover Dam on Tuesday.David Becker/Reuters

The images, taken by NASA’s Landsat program in 2000 and 2022, showcase the driest two-decade period since A.D. 800, according to a recent analysis of tree-ring data.

Researchers have determined that human-caused global warming has played a role in the continuation of this current drought, which has persisted despite some years with good precipitation over the past two decades. One reason for that may be that rising temperatures, more than rain and snow conditions, are driving this drought.

The images of Lake Mead provide “a stark illustration of climate change and a long-term drought that may be the worst in the U.S. West in 12 centuries,” NASA wrote in a statement accompanying the images.

The lake is just 27 percent full, its lowest level since the reservoir was filled in 1937. But Ms. Pitt warned that the available water supply downstream was much lower because of the “dead pool,” which occurs when water in the reservoir is too low to pass through the dams.

In the two decades that separate these images, the lake’s water level as measured at the Hoover Dam has fallen 158 feet to 1,041 feet, the Bureau of Reclamation said. The lake levels must stay above 1,000 feet to continue operating the dam’s hydropower turbines.

Usually, the reservoir is replenished by snowmelt in the Rocky Mountains that flows down into the Colorado River watershed. But snowpack this year has been below average.

Henry Fountain contributed reporting.

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