Lake Mead, the largest reservoir in the US and a critical water supply for millions across the Southwest, has declined about 140 feet since 2000 and now sits at just 37% of full capacity.
CNN  — 

Thousands of people will celebrate Memorial Day this weekend on the water of Lake Mead, just 24 miles east of Las Vegas on the border of Arizona and Nevada.

What they may not realize is that the oasis they’re enjoying in the desert is entering uncharted territory, with significant ramifications for millions across the Southwest in the years to come.

On Tuesday, the water level in Lake Mead – the largest US reservoir, and fed by the Colorado River – fell below the elevation of 1,075 feet. It has hit that mark only a handful of times since the Hoover Dam was finished in the 1930s, but it always recovered shortly after. It may not this time, at least not any time soon.

The US Bureau of Reclamation (USBR) forecasts the lake’s levels to continue to decline, without any sign of recovery through at least the end of 2022. If the next major study in August from the USBR projects water levels in the lake will be below 1,075 feet on January 1, it would trigger the first-ever shortage declaration on the Colorado River, meaning some communities would begin to see their water deliveries cut significantly next year.

Lake Mead and nearby Lake Powell – the two largest reservoirs on the Colorado River – have drained at an alarming rate. Lake Mead has fallen more than 139 feet since January of 2000.

Lake Mead is currently 16 feet below where it was this time last year and the reservoir is only 37% full, while Lake Powell is down 35 feet from last year and sits at just 34% of the lake’s total capacity.

The Colorado River, which supplies water to more than 40 million people and irrigates millions of acres of farmland, has seen its supply sapped by drought and climate change.

The significance of the dwindling supplies in both reservoirs cannot be overstated. Water flowing down the Colorado River fills the two reservoirs, which are part of a river system that supplies over 40 million people living across seven Western states and Mexico.

A drought that has persisted for two decades has left the much of the Western US parched.

In addition to dwindling snowpack, which provides most of the river’s water supply, experts say dry, thirsty soils across the basin are soaking up meltwater, meaning that less makes it into the river system.

“It’s a pretty awful year,” said Ted Cooke, the general manager of the Central Arizona Project, a massive, 336-mile canal and pipeline system that carries Colorado River water to Phoenix, Tucson and farms and towns in between. “Last year wasn’t so bad as far as the snowpack went, but with the higher temperatures and drier soils that we’re still experiencing this year, we’re seeing even less runoff that actually makes it into the waterways and in the reservoirs.”

Climate change is also taking a toll on the river’s water supply. A study by US Geological Survey scientists published in 2020 found that the Colorado River’s flow has declined by about 20% over the last century, and over half of that decline can be attributed to warming temperatures across the basin.

Who would the shortage affect?

With the level of Lake Mead dipping below 1,075 feet on Tuesday and forecast to drop further, it is nearly certain that the Bureau of Reclamation will declare a Tier 1 shortage later this summer.

If a Tier 1 shortage is declared, Colorado River water deliveries would be reduced for Arizona and Nevada as soon as next year, based on the terms of the 2019 drought contingency plan signed by the lower Colorado River basin states.

The looming water cuts will have the greatest impact in Arizona.

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As part of the lower basin’s drought contingency plan, the Central Arizona Project would see its water supply slashed by about one third in 2022 due to its junior rights to the river’s water.

While Arizona’s main population centers will be spared, the effects of those water cuts will be felt most acutely on farms in central Arizona, due to their lower priority status in a complex tier system used to determine who loses water first in the event of a shortage.

California’s water deliveries would not be impacted in a Tier 1 shortage, according to the drought contingency plan.

Water officials in Arizona say that while the falling water levels and future projections are concerning, the state is prepared to absorb the cuts.

“We’re concerned … but we’ve allowed for something like this to happen, and as a matter of fact, keep on happening,” Cooke said. “Hopefully it doesn’t but if it does, we’re prepared.”

What happens if Lake Mead sinks further?

In the event of a Tier 2 shortage – which the USBR projects could happen as soon as late 2022 – the cuts would impact some cities and tribes in Arizona that receive water from the Central Arizona Project canal.

“I’m definitely concerned that the raw projections continue to go downward and that we are heading towards potentially a Tier 2 [shortage] in 2023,” said Tom Buschatzke, director of the Arizona Department of Water Resources.