BOULDER CITY, Nev. – Hoover Dam towers more than 700 feet above Black Canyon on the Arizona-Nevada state line, holding back the waters of the Colorado River. On top of the dam, where visitors peer down the graceful white arc of its face, one of its art deco-style towers is adorned with a work of art that memorializes the purposes of the dam.

In five relief sculptures by Oskar Hansen, muscular men grip a boat’s wheel, harvest an armful of wheat, stand beside cascading water and lift a heavy weight overhead. Words encapsulate why the dam was built, as laid out in a 1928 law: FLOOD CONTROL, NAVIGATION, IRRIGATION, WATER STORAGE and POWER.

Eighty-six years after its completion in 1935, the infrastructure at Hoover Dam continues doing what it was designed to do: holding water and sending it coursing through intake tunnels, spinning turbines and generating electricity. The rules for managing the river and dividing up its water – which were laid down nearly a century ago in the 1922 Colorado River Compact and repeatedly tweaked – face the greatest strains since the dam was built.

Since 2000, the water level in Lake Mead, which is the reservoir formed by Hoover Dam and holds the title of the largest reservoir in the country, has dropped about 140 feet. It is just 37% full, headed for a first-ever official shortage and sinking toward its lowest levels since it was filled.

One of the West’s driest 22-year periods in centuries is colliding with the river’s chronic overuse. As the reservoir falls toward record lows, its decline threatens the water supplies of cities and farmlands and reveals how the system of managing water in the desert Southwest faces growing risks.

Water levels expected to fall below federal threshold this summer

Mike Bernardo of the federal Bureau of Reclamation leads a team of engineers and hydrologists who plan water releases from Hoover Dam, as well as Davis and Parker dams downstream, sending flows that travel through pipelines and canals to Phoenix, Los Angeles and farmlands in the USA and Mexico that produce crops such as hay, cotton, grapes and lettuce.

Bernardo’s team sets power generation goals and produces a monthly report with the latest projections of how reservoir levels will probably change over the next 24 months.

Lately, each month’s report has brought worsening numbers.

Predicted water-level declines have grown as estimates of inflows into Lake Powell, the upstream reservoir, have shrunk because of extremely parched conditions across the upper watershed in the Rocky Mountains, where much of the river’s flow originates as melting snow.

“Unfortunately, due to how dry things have been,” Bernardo says, “what we’re seeing is Lake Powell’s elevations are dropping.”

That will mean less water flowing into Lake Mead for the rest of the year. The past 12 months have been among the driest on record across the Colorado River Basin. Inflows into Lake Powell from April through July are estimated to be just 26% of the long-term average, and that’s leading to rapid declines in Powell and Mead, the two largest pieces of the river’s water-storage system.

The warm, dry conditions over the past two years have baked the watershed’s soils to such an extent, Bernardo says, that “when the snowmelt starts to run off, it just gets sucked up into the ground like a sponge.”

The demands for water downstream from Hoover Dam continue. The Southwest’s farmlands’ peak irrigation season lasts through June, Bernardo says, so Lake Mead’s surface drops about 1 foot each week.

The reservoir has declined more than 16 feet over the past year and is forecast to fall about 9 feet more by the end of this year.

The latest projections show that by the end of 2021, Lake Mead will decline below an elevation of 1,066 feet, far below the threshold – 1,075 feet – for the federal government to declare a shortage. That’s likely to happen in August, triggering the largest water cuts to date next year for Arizona, Nevada and Mexico.

Even larger cutbacks could come in 2023 if the reservoir declines as projected over the next year into a more severe “Tier 2” shortage.

Lake Mead’s downward spiral is driven largely by the dire situation upstream at Lake Powell, which has declined to 34% of full capacity.

“We need three to four consecutive years of above-average inflow, snowpack runoff and inflow into Lake Powell to refill these reservoirs,” Bernardo says. “So that’s what we’re hoping for.”

The Colorado River naturally cycles through wet and dry periods, but over the past 22 years, the watershed has had 17 dry years, Bernardo says, and only five years with above-average or wet conditions.

Hotter temperatures have evaporated more moisture off the landscape, leaving less flowing in the river and its tributaries. Scientists describe it as a “megadrought” and one that, unlike the long droughts of the past, is amplified by carbon pollution and the heating of the planet.

One of the unknowns facing the officials who manage Colorado River water is how severely the reservoirs could be affected by climate-driven “aridification” in the years to come. Some scientists have estimated the river could lose roughly one-fourth of its flow by 2050 as temperatures rise, and that for each additional 1 degree Centigrade (1.8 degrees Fahrenheit) of warming, the average flow is likely to drop by about 9%.

“With the warmer temperatures,” Bernardo says, “not only do we see things melt off quicker but you have that rising snow line, which creates less inflow.”

The declines in the reservoirs have accelerated over the past two years.

In 2019, representatives of Arizona, Nevada and California agreed under a deal called the Drought Contingency Plan to share in water reductions through 2026 to reduce the risks of Lake Mead falling to critically low levels. The agreement calls for progressively larger cutbacks if Lake Mead drops below lower trigger points in the coming years.

If the reservoir drops below 1,045 feet, California would start to take cuts. Mexico contributes by leaving some water in Lake Mead.

“These mechanisms have been put into place to protect these reservoir elevations,” Bernardo says.

Though the latest agreement is intended as a stopgap measure, officials from the seven states that depend on the river are preparing to negotiate rules for managing shortages after 2026, and those talks promise to be tougher.

Bernardo says the bureau’s responsibilities in managing the dams and water deliveries remain the same, and they include incorporating the latest science and models and providing up-to-date information to representatives of the states, water districts, tribes and other entities along the river “to communicate what’s going on and what we’re seeing, so everyone can act proactively.”

“When you have a river system like this, a complex reservoir and river system especially, that is experiencing the hydrology that we’ve been seeing, and such a quick decline in the Upper Basin over these last two years, transparency and communication is key,” Bernardo says.

Iconic dam holds less and less

Bernardo, 35, has worked for the Bureau of Reclamation for nearly a decade, including the past two years as river operations manager. A mechanical engineer who grew up in New Jersey, he usually works with his staff at the agency’s office in Boulder City, Nevada, but he regularly drives out to visit the dam, sometimes to lead special tours.

Whenever he rounds the curve in the canyon and sees the dam, Bernardo says, he feels awestruck, and “the hair still sticks up on my arms.”

“It never gets old,” he says. “I’m wowed by the engineering marvel.”

Part of that comes from knowing the history of all that went into the dam’s design and construction during the Great Depression, from the hand-drawn blueprints to the blasting with dynamite, the railroad that carried supplies and the massive amounts of concrete poured in, creating a dam that is 660 feet thick at its base – nearly as thick between the reservoir and the downstream side as it is tall. (According to the Bureau of Reclamation, Hoover Dam contains enough concrete to build a sidewalk 4 feet wide around the entire Earth at the equator.)

Bernardo says the dam’s historical significance is inescapable: how it controlled the Colorado’s floods, opened arid lands for farming and fed the rise of cities across the Southwest. As he describes it, the dam “helped nourish our nation” and helped the West thrive.

“We like to show it off,” he says.

Hoover Dam’s normal capacity is 2,074 megawatts, Bernardo says, generating enough power per year to supply approximately 450,000 average households. At today’s lake level, the dam’s capacity has decreased about 25% to 1,567 megawatts, generating enough power for roughly 350,000 homes.

With every foot the lake declines, about 6 megawatts of power-generating capacity is lost.

The lowest level at which Hoover could produce power is about 950 feet, with an expected capacity of 650 megawatts. If the lake fell below that point – a scenario the rules are geared toward avoiding – the dam would no longer be able to generate power.

As the reservoir declines, releasing the same amount of water yields a bigger drop in lake level.

“That’s one of the concerning pieces,” Bernardo says. “The reservoir is shaped, we call it a teacup, but more like a martini glass. And the lower the elevation goes, the faster the rate of decline.”

That dynamic affects how much the planned water cuts could help Mead’s level. Under a first-tier shortage next year, for example, Arizona, Nevada and Mexico are preparing for cuts totaling 613,000 acre-feet, which Bernardo says is equivalent to 7-8 feet of elevation in Lake Mead.

If the reservoir dropped below 1,025 feet, the total cuts among the three states and Mexico would add up to more than 1.3 million acre-feet. That amount, Bernardo says, would equal nearly 20 feet conserved in Lake Mead at those low levels.

When representatives of California, Arizona and Nevada negotiated the deal, they decided on 1,025 feet as a threshold to avoid, one they thought the lake would be unlikely to reach. The agreement includes a backup provision. If the two-year projections show Mead is likely to decline below 1,030 feet, the agreement says, the states and the interior secretary “shall consult and determine what additional measures will be taken.”

The government’s latest five-year projections, using an approach that considers the river’s lower flows over the past three decades, estimates a 25% chance of Lake Mead declining below 1,025 feet in 2025.

Much could change if there’s a snowy winter in the mountains.

“We hope and we feel very strongly that the measures that have been put into place should slow down the decline,” Bernardo says. “Now, if it’s enough to make it recover, your guess is as good as mine, because the hydrology has been so bad.”

If the river basin gets a wet year with average flows, Bernardo says, the cutbacks in the plan “will buy us time to get to the next year, in hopes to get a better water year.”

“And I think that’s what the system is designed to do,” he says.

An ‘Era of Limits’

The outlook for the Colorado River has grown increasingly dire over the past several years. In one study, scientists found that about half the trend of decreasing runoff in the Upper Colorado River Basin since 2000 was due to unprecedented warming.

Other researchers warned in a report this year that an “incremental approach to adaptation” is unlikely to be enough. They pointed out that flows from 2000 through 2018 were about 18% less than the 20th century average and said the downward trend will probably continue as temperatures rise with climate change.

Worries about overusing the Colorado river predate the current dry spell. Some early warnings came before the legal framework that divided the Colorado among the seven states and Mexico.

John Wesley Powell voiced concerns in 1893, 24 years after his expedition down the river in the Grand Canyon, when he told attendees at the International Irrigation Congress in Los Angeles, “I tell you, gentlemen, you are piling up a heritage of conflict and litigation over water rights, for there is not sufficient water to supply these lands.”

Under the 1922 Colorado River Compact and subsequent agreements, the river has long been severely overallocated. As University of Arizona law professor Robert Glennon put it, “there are more water rights than there is water.”

So much has been diverted that most of the river’s delta in Mexico was transformed decades ago into stretches of dry riverbed that wind through farmlands and desert in the Mexicali Valley. Only a smattering of natural wetlands remain.

In his 1986 book “Cadillac Desert,” Marc Reisner wrote that Hoover Dam “rose up at the depths of the Depression and carried America’s spirits with it. Its electricity helped produce the ships and planes that won the Second World War, and its water helped grow the food.”

Reisner wrote that from these hopeful beginnings, “the tale of human intervention in the Colorado River degenerates into a chronicle of hubris and obtuseness” and that people in the river basin – at that time 20 million – “will probably find themselves facing chronic shortages, if not some kind of catastrophe.”

“One could say that the age of great expectations was inaugurated at Hoover Dam,” Reisner wrote. “And one could say that, amid the salt-encrusted sands of the river’s dried-up delta, we began to founder on the Era of Limits.”

Authors Eric Kuhn and John Fleck wrote in their 2019 book “Science Be Dammed: How Ignoring Inconvenient Science Drained the Colorado River” that “even absent climate change, we would be in trouble” and that the current problems surrounding the river “are the inevitable result of critical decisions made by water managers and politicians who ignored the science” as early as the 1920s.

Scientific analyses in the 1920s found the Colorado River would be in deficit if dams and canals were built to meet the anticipated demand, Kuhn and Fleck wrote. The scientists’ warnings were ignored, and that “set in motion decades of decisions that would end in the overuse seen today.”

They suggested that addressing the river’s deficit will require recognizing that the “over-allocation became embedded in basin rules in very specific ways that remain unresolved” and should be fixed.

Negotiating the post-2026 rules will be challenging for everyone involved, Kuhn and Fleck wrote, and some of the fundamental issues facing negotiators are similar to those a century ago, including questions of how much water the river will provide in the years ahead and how the system should be governed amid uncertainty.

The Colorado River Basin needs “a stable and effective governance of the use of the river’s waters under conditions where current demands already exceed the exiting supplies,” Kuhn and Fleck wrote. “Like one hundred years ago, the river’s future is not all dark. Innovation, cooperation, and an expanded reliance on science are now the foundation for basin-wide solutions.”

One effort to restore some of the wetlands and ecosystems in Mexico began last month, as water began flowing into the delta under an agreement between the U.S. and Mexican governments. The water releases in the delta, which will total 35,000 acre-feet from May to October, are intended to nourish vegetation and wildlife at habitat restoration sites where conservation groups planted cottonwoods and willows.

The influx of water is supposed to mimic a small portion of the floods that once swept across the delta toward the Sea of Cortez. This year’s releases amount to a smaller version of a planned flood that coursed through the delta in 2014. In that “pulse flow,” 105,000 acre-feet of water brought back a flowing river in areas that had been dry since floods in the late 1990s.

The releases in the delta this year, using water previously stored in Lake Mead, amount to just 5 inches of water in the reservoir. Much more of the water that passes through Hoover Dam is pumped to Phoenix, Tucson and Los Angeles and flows through canals to irrigate farmlands along the river from Parker to Yuma, and across the Coachella, Imperial and Mexicali valleys.

Low water levels bring risks

If the water declined about 125 feet from where it stands, below the elevation of 950 feet, Bernardo says, Hoover Dam would lose the ability to generate power.

“That’s what we call minimum power pool,” Bernardo says.

If Mead falls further, the dam could still release water down to a level of 895 feet.

“At 895 and below, Hoover Dam is unable to pass water by any conventional means. So you would essentially have to pump it out of Lake Mead. That’s what we call dead pool,” Bernardo says. “And at dead pool, Lake Mead still has 2.5 million acre-feet in storage, but there’s just no way to get it out.”

If the lake declines that much, only the Southern Nevada Water Authority, which supplies Las Vegas, has an intake deep enough to continue pumping water.

The risks of Mead falling to such lows gave impetus to the last round of negotiations, which led to the signing of the Drought Contingency Plan in 2019 at Hoover Dam.

The river would have been in a shortage years ago if the states and Mexico hadn’t made concerted efforts to prop up Lake Mead’s levels, Bernardo says, and those steps included various conservation programs that have yielded 4 million acre-feet over the past 15 years, about 50 feet of water in the lake.

During the unrelenting dry years, he says, “we knew that we couldn’t postpone a shortage forever.”

He reiterates that the shortage measures, including the mandatory cutbacks, were adopted to reduce risks.

“And although it’s scary that this will be the first time we’re using them, they were designed by very smart people throughout the Colorado River Basin,” Bernardo says. “And let’s hope that they work the way that they were designed to work.”

If the situation worsens, he says, everyone involved in managing the river’s water will get together again, as stipulated in the 2019 agreements, to take steps to protect the reservoirs. About 40 million people rely on water from the Colorado and its tributaries, he says, and “all of us as water managers have a responsibility to all of those that are in the basin.”

By mid-June, Lake Mead is set to decline to its lowest levels on record. Hoover Dam will soon hold the smallest amount of water since it was filled in the 1930s. The next few years may show how much water use needs to decrease to rebalance the river and reduce the risk that Hoover Dam might one day fall silent.