The water stored in Lake Mead, the Colorado River reservoir held back by the Hoover Dam outside Las Vegas, is absolutely vital to San Diego and the Southwest.

Last year, 75 percent of San Diego’s water came from the Colorado. It keeps our lawns green, our faucets flowing and our businesses thriving.

But the reservoir has been under sustained pressure from drought and demand, and it’s been dropping since 1997. Last November, it hit its lowest point since it was being filled in the 1930s.

And as we noted today, there’s no emergency plan if severe shortage strikes the river.

Interested in the reservoir that sates our region’s thirst? Here are some facts and figures about the vital waterbody.

How high is Lake Mead headed this year?

Federal officials expect it to hit 1,105 feet above sea level by Sept. 30. That’s well above 1,000 feet, the level where Las Vegas gets cut off or 1,075 feet, where a mild shortage is declared.

The reservoir is currently at 1,096 feet and rising because of a major release of water upstream from Lake Powell, on the Utah-Arizona border, and from snowmelt after a wet winter.

When is Lake Mead empty?

The actual bottom is around 700 feet above sea level, according to a 2001 federal study.

But because of the way it’s built, the reservoir no longer yields water once it hits 895 feet. That’s commonly referred to as “dead pool,” and yes, it’s as ominous as it sounds. The Hoover Dam, which provides a massive amount of electricity to the Southwest, would stop functioning before dead pool. About 2 million acre-feet of water would be left in the reservoir, but it would be of crummy quality and would have to be pumped out (instead of letting gravity do the trick). By comparison, Lake Mead can hold 27 million acre-feet (enough to supply a whopping 54 million homes for a year) when it’s full.

What happens if a shortage is declared on the Colorado?

This is triggered when Lake Mead drops to 1,075 feet or below. The cut primarily affects Arizona and grows larger as the reservoir approaches 1,025 feet.

If a shortage is declared, Arizona’s supplies would be cut as much as 17 percent under current law. Nevada would be cut a maximum 6 percent. Arizona right now doesn’t use its full slice of Colorado water, so it’s storing some underground in case that happens.

“There’s acute concern at 1,075,” said Chuck Cullom, Colorado River programs manager for the Central Arizona Project, which supplies that state’s major cities. “The tension and anxiety ratchets up the further the reservoirs drop.”

Those rules are in place until 2026. They address what happens until Lake Mead hits 1,025 feet. If the reservoir falls beneath that, no one knows what happens to stop the decline.

Is the Colorado River close to coming up short?

Not today. Lake Mead is rising right now. Cullom said Arizona believes the river wouldn’t drop to 1,075 until 2015 or 2016 at the earliest because of the wet winter.

The federal Bureau of Reclamation, which manages the river, says there’s a:

• 20 percent chance of hitting 1,075 by 2020.

• 40 percent chance by 2030.

• 50 percent chance by 2050.

A shortage would be remarkable — none have been declared in the river’s modern history, though one was almost triggered late last year.

The bureau says there’s a less than 5 percent chance that Lake Mead will drop to the ominous dead pool by 2050.

Just how big is the reservoir?

Gigantic. The largest in the country. When full, it covers 254 square miles, an area five times the size of Chula Vista.

That’s good for the Southwest. The reservoir’s trillions of gallons of storage act as a major buffer against drought, capturing high runoff from wet years to carry the region through dry ones. Lake Mead has been dropping for more than a decade, but it’s still had enough water to meet demand. That’s one reason water managers say they aren’t concerned about the lack of an emergency plan for severe shortage.

“You can see it coming. That’s the beauty of the Colorado,” Roger Patterson, assistant general manager of the Los Angeles-based Metropolitan Water District, told me. “You can see things coming and you have time to put things in place.”

How much water evaporates from Lake Mead each year?

A lot.

The lake loses almost 900,000 acre-feet a year to evaporation. That’s enough for 1.8 million homes. It’s two times more than what San Diego’s urban users consumed in 2010.

Do you have questions about Lake Mead, the Colorado River or San Diego’s water supply? Contact Rob Davis directly at or 619.325.0529 and follow him on Twitter:

Rob Davis was formerly a senior reporter for Voice of San Diego.

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