It's a good time to revisit the slow-motion water crunch in the American Southwest. Last week, Lake Mead — a key reservoir that helps supply water for 25 million people in Nevada, Arizona, and California — shrunk to its lowest level ever. And the question of how to grapple with water scarcity is making headlines yet again.

Back in the 20th century, the United States built an army of dams across the West to tame rivers, generate electricity, and store water in reservoirs for cities and farms. This intricate system is why metropolises like Los Angeles, Las Vegas, and Phoenix have been able to survive in what's basically a desert. Large-scale farming is really only possible in California's Imperial Valley or central Arizona because of these dams.

But rising demand and 16 years of drought have put a severe strain on this system. Dean Farrell has created a terrific interactive map showing how key reservoirs in the West have seen their water levels drop dramatically of late:

Check out California first. Many of the state's reservoirs fell below 50 percent during its recent (and brutal) drought. Last winter's El Niño brought heavy rains to the northern part of the state, refilling reservoirs there. But reservoirs in the south remain depleted — and the state's water woes, while partly alleviated, haven't gone away.

Now let's focus on those two huge red circles near Arizona: Lake Mead and Lake Powell. These gigantic reservoirs, which help store and supply water from the Colorado River to farmland and cities throughout the Southwest, are in rough shape. On Thursday, Lake Mead officially hit its lowest level ever, just 1,074.6 feet above sea level. (The last record low came a year ago, in May 2015.)

That's a big deal. If water levels at Lake Mead continue to plummet, the federal government could declare an official water shortage and force (potentially) painful cutbacks. The good news is that policymakers are aware of this and discussing ways to stave off crisis — though it won't be easy. That vast system that once allowed vegetables, lawns, and golf courses to flourish in the desert no longer works the way it used to.

Lake Mead, a crucial reservoir, is at record lows

In 1936, the federal government completed the Hoover Dam, which blocked the flow of the unruly 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.

Water from these two reservoirs gets divvied up between California, Nevada, Arizona, and Mexico according to a longstanding set of agreements . California has built aqueducts to carry the lake's 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 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. The state as we know it today simply wouldn't exist without this project.

But as John Fleck, a terrific writer on water issues who has been covering the Colorado River for many years, explained in 2015, there was a catch. In order to win support from California politicians, the Arizona 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 move to the back of the line.

Now, for the first time, a shortage looks plausible. 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 strain. 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 37.5 percent capacity:

When Lake Mead was at its fullest, the water reached an elevation of 1,220 feet above sea level. In May 2016, it was down to just 1,074.6 feet — the lowest level since the dam was built.

Federal officials say they expect water levels to keep dropping this summer but then rise above 1,075 feet before the start of 2017. If levels remain below 1,075 feet by the start of the year, however, the US government will declare an official shortage. That increasingly seems likely — if not in 2017, then in 2018. And once that happens, the real fun begins.

What a Lake Mead water shortage would mean for Arizona, Nevada, and California

So what would an official "shortage" mean in practice? On this, everyone should read this post by Fleck. As he notes, a shortage at Lake Mead under current rules would mainly affect Arizona. However, there's also a major effort underway to rethink those rules.

Under the 1968 agreement, Arizona has to cut back on water use before anyone else. And, Fleck explains, the state does have a plan for this. The Central Arizona Project would continue to keep water flowing to cities like Phoenix and Tucson, Indian tribes, and high-priority agriculture. But it would cut back on "low-priority" agriculture and delay refills to groundwater storage:

In 2015, Summer Pauli of the Tucson Sentinel wrote a great piece on what those cuts might look like. Some Arizona farmers will no doubt conserve water by switching from flood irrigation to more efficient drip irrigation. (Many have already been doing this, though installing these systems can cost more upfront.) Others may try to pump more water from underground aquifers (though Arizona has stricter controls on groundwater pumping than, say, California). Others may just cut back on growing crops like alfalfa.

These cuts won't be apocalyptic, but they won't be minor either. "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.

Next comes Nevada. Las Vegas — which gets 90 percent of its water from Lake Mead — is likely to be okay 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 California. Under the original 1968 agreement, California wouldn't see any cutbacks from the Colorado River until the Central Arizona Project went dry. In the event of an official shortage, Los Angeles, San Diego, and the Imperial Valley would continue to get their full share of Colorado River water for years to come.

But those are the current rules. More recently, officials in Arizona, Nevada, and California have been discussing a brand new agreement that, they hope, would divvy up the Colorado River's increasingly scarce water in a fairer and more sensible way. Under a renegotiated deal, Arizona and Nevada would cut back even more sharply in the event of a shortage — but California would also have to start sharing the burden and make some sacrifices of its own.

As Tony Davis reported for the Arizona Daily Star, the current proposed agreement would see the Central Arizona Project reduce water use up to 15 percent once a shortage is declared at Lake Mead (as opposed to the current 11.5 percent). Arizona officials have also been talking about water cuts to all sectors of the state's economy rather than agriculture alone. Meanwhile, California would need to start reducing water deliveries if levels at Lake Mead dropped about 30 feet lower than they are today, rather than waiting for Arizona to go dry. The bulk of these cuts would likely be borne by farmers in the Imperial Valley.

This is far from a done deal, and negotiating water agreements, both between states and among individual state water districts, is notoriously tricky. (Fleck has another good post about this.) But it's a sign of growing awareness that the current arrangement is no longer viable. Lake Mead is unlikely to survive the status quo.

The looming question: What happens if water shortages get worse in the future?

One big reason these agreements are being rethought is that as the Southwest enters its 16th year of drought, these shrinking reservoirs are becoming harder to dismiss as a temporary blip.

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

That's increasingly looking like a bad bet. Scientists have uncovered evidence that decades-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 is very likely to be less water to share in the future.

Yet 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 a separate 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 difficult to take that for granted.

Further reading: