All through the 20th century, the United States built a concrete army of dams around the West to tame rivers, generate electricity, and store up water in reservoirs for cities and farms.

This intricate water system is why metropolises like Los Angeles, Las Vegas, and Phoenix have been able to flourish in what's basically a desert. It's what makes farming possible in California's Imperial Valley or central Arizona.

Now, however, rising demand and a prolonged dry spell are putting a serious strain on this whole system. Dean Farrell recently created a great interactive map that shows how key reservoirs around the West have seen a big drop in capacity after years of drought:

Notice California, where a lack of rainfall and record heat have pushed many reservoirs below 50 percent capacity. Farmers in California's Central Valley have already seen cuts to water deliveries, and the governor has ordered cities to pare back water use.

But then there are those two huge red circles near Arizona: Lake Mead and Lake Powell. These gigantic reservoirs, which help supply water from the Colorado River to farms and cities throughout the Southwest, are also reaching historically low levels.

That's a huge deal, as Eric Holthaus nicely explains at Slate. If water levels at Lake Mead keep dropping in the months ahead, Arizona could soon face a water crunch of its own.

Lake Mead, a crucial reservoir, is at record lows

Back in 1936, the federal government completed the massive Hoover Dam, which blocked the flow of the Colorado River and created Lake Mead, a giant reservoir meant to fill up during rainy years and supply needed water during dry years. Later, in the 1950s, the government built the Glen Canyon Dam even further upstream to create Lake Powell, to help store and regulate water supplies.

Las Vegas relies on Lake Mead for 90 percent of its water

Water from these reservoirs gets divvied up between California, Nevada, Arizona, and Mexico according to a longstanding set of agreements.* California has built aqueducts to transport water to Los Angeles and San Diego and farms in the Imperial Valley. Meanwhile, Las Vegas gets 90 percent of its water from the lake.

Then there's Arizona. In 1968, Congress approved the Central Arizona Project, a series of aqueducts that would transport about 1.6 million acre-feet of Colorado River water down into central and south Arizona and help nurture farms and cities like Phoenix and Tucson. This elaborate, expensive system laid the foundation for modern-day Arizona.

But as journalist John Fleck explains, there was a catch. In order to win support from California politicians, the project's backers had to agree to a key condition — in the event of a water shortage, California would get first dibs on the Colorado River's water. Arizona would have to move to the back of the line.

Now, for the first time, it looks like that shortage might actually arrive. Water levels at Lake Mead had already been dropping for years, as more and more users have been overdrawing its water. More recently, drought and extreme heat have been adding to the pressure. With less snow in the mountains, there's less water flowing into Lake Powell, which in turn is delivering less water downstream to Lake Mead.

Add it up, and Lake Mead is down to just 38 percent capacity:

When Lake Mead was at its fullest, the water reached an elevation of 1,220 feet. By early May 2015, it was down to just 1,079 feet — the lowest level since the dam was built.

The US Bureau of Reclamation is currently conducting a study to figure out what the future might portend. If the study concludes that Lake Mead's water levels are likely to stay below 1,075 feet by January 1 of next year, then the US government could declare an official shortage. Arizona, in particular, would have to start cutting water.

What a water shortage would mean for Las Vegas and Arizona

So what would an official "shortage" mean in practice? On this, everyone should read this post by John Fleck, a terrific writer on water issues who has been covering the Colorado River for many years. There are a few key places to look at — although Arizona is the biggest story for now:

First, California won't see any cutbacks in water from the Colorado River anytime soon — again, because of that 1968 agreement. Los Angeles, San Diego, and the Imperial Valley will continue to get their full share of Colorado River water ahead of Arizona. California still has its own water crisis, but at least it won't have to deal with this.

Arizona has been developing a plan to deal with a shortage in an orderly fashion

Second, Las Vegas — which gets 90 percent of its water from Lake Mead — is likely to survive for the time being. City officials have been watching the lake's levels drop for years and have already taken steps to curtail water use 30 percent over the past decade, not least by providing incentives for homeowners to replace the grass on their lush lawns with less-thirsty native plants.

Las Vegas has also been building costly new water-intake systems so that it can keep drawing water from Lake Mead even if water levels keep dropping further. (When the original intake systems were built, the pipes were essentially too high up; few imagined that reservoir levels would drop this far.)

Finally, there's Arizona, which will face cuts in deliveries if a shortage is declared. But, Fleck explains, the state does have a plan. The Central Arizona Project would continue to keep water flowing to cities like Phoenix and Tucson, Indian tribes, and high-priority agriculture. Instead, it would cut back on "low-priority" agriculture and delay refills to groundwater storage:

Summer Pauli of the Tucson Sentinel has a good piece detailing what these cuts might look like in practice. Some Arizona farmers have been trying to conserve water by switching from flood irrigation to more efficient drip irrigation (though installing these systems can cost more up front). Others may try to pump more water from underground aquifers (though, unlike in California, Arizona has stricter controls on groundwater pumping). Others may just cut back on growing crops like alfalfa.

Even if it's not apocalyptic, it won't be easy. "It’s going to be a very painful cutback when we start losing our water, but we’ll do what we can to survive and that’s all you can do," one farmer told the Sentinel.

But what happens if water shortages get worse in the future?

There is, however, a much more difficult question that keeps coming up as the West endures its 15th year of drought. What if this isn't a temporary blip? What happens if these water shortages become more frequent and persistent in the Southwest's future?

As Holthaus notes, the 1922 agreement that divvied up water from the Colorado River was forged during one of the wettest periods in the last millennia. At the time, people assumed droughts would occur, but they didn't assume droughts would get off-the-charts terrible.

Water users are, essentially, hoping for wet years and future surpluses

That's increasingly looking like a bad bet. Scientists have uncovered evidence that decades- or even centuries-long "megadroughts" have occurred in the distant past, and could well occur again in the Southwest going forward. And even if that doomsday scenario doesn't come to pass, climate models still expect droughts to get more frequent and severe in the American Southwest if global warming continues apace. There could very well be less water to share in the future.

Despite all this, Lake Mead's users have been overdrawing water in recent years — essentially assuming that there will be wet years in the future to provide surplus water and recharge the system.

In an earlier post, Fleck pointed to an eye-opening presentation by the Central Arizona Project that asks what would happen if water levels at Lake Mead keep dropping — down to 1,000 feet, say. In that case, Las Vegas would face even bigger challenges drawing water from the reservoir. The Hoover Dam would struggle to generate electricity. Arizona, Nevada, and California would all see steeper cuts in water deliveries. Unless, of course, states figure out how to better manage and share their water supplies.

Up until now, there's always been plenty of water from the Colorado River to go around. It's increasingly hard to take that for granted.

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* California, Nevada, and Arizona make up the "Lower Basin" of the Colorado River. There's also the Upper Basin (Colorado, Utah, Wyoming, New Mexico, and a bit of Arizona), which divvies up a separate share of Colorado River water, according to a 1922 interstate compact. These latter states aren't affected by the low levels of Lake Mead.

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