What if the Hoover Dam Failed?

The Hoover Dam is one of those structures that’s tied inextricably to the place where it was built, a feat of engineering as American as Mount Rushmore or the Empire State Building. A hulking, nigh-on-impenetrable arch-gravity dam, the Hoover Dam has stood for nearly a hundred years, holding back the powerful waters of Lake Mead in the Mojave Desert. In total, the water that passes through the dam sustains some eighteen million people downstream, while its hydroelectric power keeps the lights on for millions more. It is indispensable to the southwestern United States, it is a major tourist destination for millions of travelers each year, and despite all of that, it is still a dam—and dams can fail.

A failure at the Hoover Dam would quickly become a catastrophe on multiple levels, for everyone from the communities immediately downstream, to the people who rely on the dam every day to sustain them. But what, exactly, would happen if the dam broke? Just how bad could the situation really be? And in a moment when droughts, heat waves, and wildfires are making this portion of America harder and harder to sustain—could the collapse of the Hoover Dam make this entire expanse uninhabitable?

The Dam

Before we consider a hypothetical collapse of the Hoover Dam, we first have to consider the dam itself, to understand just what it is we’re destroying, and to explain the sheer magnitude of what that sort of destruction could mean. First conceived in the early 1900s, as the potential of hydroelectric power plants was better and better understood, the Hoover Dam was ultimately built during the 1930s, spanning the width of a major geological feature known as Black Canyon. The dam obstructs the Colorado River, a powerful, nearly 1,500-mile-long river that sustains parts of the US states of Nevada, California, Arizona, New Mexico, and others. Despite having been approved by US President Calvin Coolidge and opening for operations under President Franklin Roosevelt, it bears the name of the president in the middle of the two, Herbert Hoover, who led the nation during the early phases of its construction and played a key role in ensuring that it was built. The process to raise the dam into Black Canyon was an arduous one, involving thousands of workers at any given time, forced to endure brutal heat and enduring deaths amongst their number that were a startlingly regular occurrence—112 in total, plus forty-two attributed to pneumonia. It’s a common misconception that some of those workers were entombed within the dam as it was built, although, in reality, that wasn’t possible given how the dam was poured. What is true about the concrete, is that much of it is still curing and hardening to this day, nearly a hundred years after it was first poured.

The dam was formally completed in March of 1936, and ever since then, it’s stood indomitable at the heart of Black Canyon. The Hoover Dam clocks in at a height of 726 feet or 221 meters above the base of the canyon. Its crest, holding back a gargantuan wall of water, is just forty-five feet or fourteen meters wide, while at the base, it’s 660 feet or 200 meters wide.

On average, the wall of water behind it is about 350 feet or 107 meters deep, although it can come up to within twenty-seven feet of the top before it starts to filter through spillways. The dam features two spillways, each capable of releasing 400,000 cubic feet of water per second in case of emergency; that’s 11,000 cubic meters. At the base of the dam is a nineteen-turbine hydroelectric dam, annually generating 3.3 terawatt-hours of power as of the year 2020.

And behind the dam is its reservoir, Lake Mead, with the total capacity to hold back twenty-eight and a half million acre-feet of water or over thirty-five cubic kilometers. At present, it holds closer to fifteen million acre-feet, or about nineteen cubic kilometers. At its deepest, Lake Mead’s waters hit a depth of 590 feet or 180 meters, while its surface area stretches across 247 square miles, or 640 square kilometers.

The sheer volume of water that the Hoover Dam is meant to hold back, is simply extraordinary—and as such, it should be no surprise that resilience against collapse is something that its architects accounted for as they built it. Critical portions of the dam are built to be bomb-proof, at least corresponding to conventional bombs, although nuclear weapons were still more than a decade off from first combat use when the dam was being built. The dam’s spillways have mechanisms to open automatically, with a track record of having only ever been used once to mitigate flooding back in 1983, and they’ve been patched with heavy-duty polished concrete and supplemented with aerators to ensure that they don’t degrade.

The dams upstream from the Hoover, including the highly impressive Glen Canyon Dam and its reservoir, Lake Powell, are reliably maintained and ration their water responsibly. Security concerns after the September 11, 2001 attacks in the US led to the construction of a bypass bridge for a prominent state road that had, until the bypass was opened in 2010, allowed traffic to run across the top of the Hoover on its way over Black Canyon. That new bypass means that the Hoover is far more protected from concerns like terrorist attack, while heavy traffic that could cause a dam collapse, like heavy box trucks and busses, have never been allowed to travel over it.

And, to be clear, it is not likely that the Hoover Dam would burst due to overflowing water anytime soon. Lake Mead has been experiencing water shortages for decades, that have grown into a full-blown water crisis over the last several years. Current projections expect that two US states, Nevada and Arizona, will spend the entire coming year of 2025 under water restrictions. Lake Mead and Lake Powell currently combine to store just thirty-seven percent of the water that they could be storing, given their reservoir capacity. According to the Las Vegas Valley Water District, water levels of Lake Mead have dropped more than 150 feet since January of the year 2000. The city of Las Vegas and the surrounding area, for context, get about nine-tenths of their water from the Colorado River that the Hoover Dam intersects. The reasons behind the extreme shortage have to do with many years of below-average snowfall in the Rocky Mountains, where snowmelt feeds into the Colorado from a vast mountain range. Quoting from the Las Vegas Valley Water District here: “It would take many years of above-normal runoff in the Rocky Mountains before Lake Mead’s water level returns to the level before the drought began.” Given the current circumstances, that sort of runoff in the next few decades appears unlikely. Quoting again: “Unfortunately, the best scientific projections available suggest that current Colorado River conditions will continue and worsen.”

Collapse and Ramifications

Now, in the real world, there is a pretty major consolation prize to a long-term lowering of water levels in Lake Mead. That consolation prize is that the precise event we’re discussing, a potential collapse of the Hoover Dam, is thus quite unlikely to happen. For our purposes here, however, it means that we’ve got to create a hypothetical scenario that doesn’t quite match up with the reality on the ground—so let’s construct that hypothetical as we see fit.

Let’s say that this year, completely by surprise, the Rocky Mountains get some incredible snowfall—like, the sort of snowfall that swallows buildings and even trees, on a scale not observed since the start of record-keeping a couple of centuries ago. In springtime, that snow starts to melt, and as it does, the runoff swells the Colorado River to levels that bring the river’s dams and reservoirs close to their breaking point. Now, this does lead to a bit of a tricky situation, when we consider how a collapse of the Hoover Dam would play out, because most of the water that fills Lake Mead gets there after being caught and released from the reservoirs of dams upstream—meaning that those dams would probably have to intentionally or unintentionally release so much water into the Hoover Dam that they’d be responsible for it overflowing. In our hypothetical, mostly because it messes with our math, we won’t suppose that a dam upstream, like the Glen Canyon Dam, breaks first, and sends all the water of Lake Powell rushing into Lake Mead in a cascading effect. That’s an interesting scenario in itself, but we’ll save it for another day. Nor will we suppose that the people who run the Glen Canyon Dam, in the state of Arizona, would be so foolhardy or vindictive as to keep their own dam from collapsing by allowing so much water into Lake Mead that it would shift their problem onto neighboring Nevada. Instead, let’s suppose that conditions are just inches short of panic for both dams, meaning that the Hoover isn’t yet using its spillways at full capacity, and let’s introduce a secondary crisis upstream that would be just enough to push the Hoover over the edge.

To that end, we’ll imagine a break of the Theodore Roosevelt Dam in Arizona, upstream of Lake Mead on a tributary known as the Little Colorado. In flood-control mode, the Roosevelt dam can hold back three and a half cubic kilometers of water, just short of three million acre-feet, or roughly a tenth of the total capacity of Lake Mead. Fill Lake Mead to its total capacity, rupture the Roosevelt Dam via whatever mechanism you’d like to imagine or suppose, and all that water will come crashing down the Little Colorado, through the much smaller Lyman Reservoir and its dam, and to the main flow of the Colorado en route to Lake Mead. Once it arrives, at a dam that we’ll suppose is already stressed to its limits and is unprepared to receive so much water so quickly—the dam collapses.

When that happens, it’ll immediately have released the water of what would be the largest man-made reservoir in the United States. The figure on just how much water we’re talking about, again, would come out to thirty-seven square kilometers in total, although it would only be able to drain as fast as the walls of Black Canyon would allow. With its spillways overwhelmed, a swell of water from the Roosevelt Dam, cresting over the top of the Hoover Dam in our hypothetical universe, would likely be enough to begin a collapse—potentially worsened by the effects of water flow that have been observed in past instances where the dam’s spillways have been used. The Hoover Dam is shaped like a gigantic wedge, with a far thicker base than the wall that’s visible on top, and it’s unlikely that the far thicker base of that wedge would collapse right away. As such, the visual phenomenon isn’t likely to be the entire, gigantic wall of the Hoover Dam simply bursting as one—but instead, a quick and catastrophic erosion of the upper portion of the dam, exposing dozens or potentially hundreds of feet of spillover from Lake Mead, cascading down to a Colorado River that is utterly unequipped to handle it. Let’s even be optimistic, in saying that a fair portion of those thirty-seven cubic kilometers of water would remain in Lake Mead, held back by the portion of the dam that still remained. But the flood that would result from a catastrophic overflow of its upper portions, whether it releases ten, or fifteen, or twenty cubic kilometers of water, would be disastrous nonetheless.

In the immediate vicinity of the dam break, the Hoover’s hydroelectric component would likely be overwhelmed and destroyed, while whatever unfortunate sightseers happened to be on top of the dam at the time, if it was for some reason not blocked off from public access, would be all but certain to perish. The walls of Black Canyon, hit with a force of water more devastating than what they’ve experienced in many human generations, would likely erode rapidly in the immediate vicinity of the dam, and walls downstream would almost certainly be changed forever by the fast passage of floodwaters. Any bridges built over the canyon, including the bypass built very near to the Hoover Dam itself, would be in imminent danger of a collapse of their own, at the mercy of the rocky foundation underneath.

Luckily, communities in the path of the floodwaters would have at least a little bit of time to evacuate, if alerts were to be sent out at the moment of rupture. At the very least, however, those communities would be rendered uninhabitable by the coming flood, even if residents were able to evacuate in time, and the vast majority of their worldly possessions would be lost in the process. More likely, depending on the time of day when a collapse happens, the slowness that an alert will take in getting to everyone in these communities, and the logistical and human difficulties of fast evacuation on limited roads, is that not everyone will be able to make it out in time.

Eighty minutes south of the Hoover Dam by car is the city of Laughlin, Nevada, a resort town with a static population of about 8,600, but a much higher population when tourist traffic is factored in. Across the river is Bullhead City, Arizona, population 41,000, also at risk of being largely destroyed. In California, the city of Needles, population five thousand, would be overrun, as would the much larger urban center of Lake Havasu City in Arizona, population roughly sixty thousand.

The floodwaters would devastate Yuma, also in Arizona, estimated population about 100,000, and after the Colorado River flows into northern Mexico, it would wipe out an even larger city, San Luis Rio Colorado, population 176,000. Those same waters would flow across and devastate multiple Native American reservations, and flow through historic flood zones of the Colorado River, perhaps as far away as an inland saltwater lake in California known as the Salton Sea. Eventually, overflowing with debris, bodies, and materials that once made up the cities that it crashed through, the overflowing Colorado River would spill out into the Gulf of Mexico.

In time, the flow would stop, and the now un-dammed lower portion of the Colorado River would wind its way through a fundamentally changed landscape.

All across the southwestern United States, the lights of 1.3 million people would go dark, now deprived of the hydroelectric power that the dam had sustained them with for many decades. Cities like Las Vegas would, at the very least, weather frequent blackouts, if the electrical grid can avoid a wider collapse. Farmland in the path of the floodwaters, of course, would be rendered useless, but the impact would extend far beyond that. Critical irrigation canals, including to the agriculturally indispensable Imperial Valley, would run dry, collapsing a very fragile farmland ecosystem that rests on land that should be open desert—and that the desert will almost certainly take back. Cities like Las Vegas, Los Angeles, Phoenix, and San Diego would be at least temporarily deprived of a water source, and could very well lose that water for the long run, depending on the circumstances behind the Hoover Dam’s collapse. Immediate casualties, depending on those same circumstances, could run into the tens or even hundreds of thousands, with millions displaced from their homes. The long-term economic and infrastructural damages could run into the hundred-billions or even trillions of dollars, and an internal refugee crisis, a potential breakdown of social order, and the long-term health effects stemming from the destruction of so much human infrastructure, could keep the indirect death toll climbing for years to come.

A More Realistic Crisis

Considering the sheer magnitude of the crisis that would result from a collapse of the Hoover Dam, you’d be well within your right to count humanity lucky that such a scenario is a long way off. For the Hoover Dam to overflow in that way, it would first have to be full to capacity, something that probably won’t happen for decades—if it ever happens again in our lifetimes. The prospect of a nigh-on-biblical flood, passing over cities like Yuma or San Luis Rio, is not likely to become real, and in a way, that’s good news. The problem, however, is that even if the Hoover Dam and Lake Mead aren’t headed toward structural failure, they’re still headed toward a different sort of crisis: Dead pool.

No, we’re not trying to tell you that Ryan Reynolds is about to rock up on Lake Mead in a red body-suit and start causing chaos. Instead, ‘dead pool’ in a water-management sense is a real and serious phenomenon, and one that isn’t nearly so simple to recover from as a devastating, but still geographically contained flood. Dead pool refers to the situation where, in relation to the Hoover Dam specifically, the water levels of Lake Mead would drop so low that the Colorado River would no longer flow through the dam at all. For Lake Mead, dead pool will happen when the water levels drop below 895 feet in elevation above sea level—owing to the fact that the mouth of Lake Mead, where the Hoover Dam is, isn’t nearly the deepest part of the lake overall. In some parts, the lake would still be more than a hundred feet deep if water levels dropped that low. But at the Hoover Dam, they’d be too low for any water to flow through the hydroelectric dam that’s located there, and thus, the entire riverbed of the Colorado past Lake Mead would be rendered bone-dry—permanently.

If the river were to reach dead pool, there would be no catastrophic flood to mark the beginning of the end of the American Southwest—but all the other results wouldn’t just stay the same; they’d get worse. In our hypothetical scenario, the Colorado River would at least keep flowing; the water could be harnessed in some ways, diverted back into the remnants of whatever irrigation system was left over. But if Lake Mead reaches dead pool, it’s over; no water is coming, probably not even in a good year for snowmelt in the Rockies, as any subsequent water flow would first have to refill Lake Mead and compensate for additional water loss past dead pool—evaporation, and things like that.

At dead pool, the tens of millions of people who rely on the Hoover Dam in some way—to grow their food, to provide their drinking water, or to power their homes—would immediately be out of luck. Once the last drips of water stop flowing, there will be no replacement from sources south of Lake Mead, while the water remaining in the lake itself would have to be brought up somehow and pushed into the Colorado River bed via means that don’t exist today. Building those mechanisms would be a very difficult process, not least because the Hoover Dam’s long-time ability to pay for itself, means that nobody’s been setting aside money in case it needs major repairs. Nor can you simply blast away and create new outflows for the remaining water, without compromising the dam entirely.

In the absence of relief, a tremendously agriculturally productive landscape in the areas past the Hoover Dam will wither and die, as irrigation networks become useless and the deserts swell to take back farmland that the United States was lucky to create in the first place. A near-complete lack of water in many major cities will force civil authorities to create other solutions: maybe desalination plants in coastal areas, maybe new canals from other water sources, or maybe fundamental changes to water-rights law in the region, with states or cities hoarding what water they can, instead of allowing it to flow. In many places, there simply may not be a solution, other than importing water in bottles and trucks at skyrocketing, unsustainable prices. Add in some rolling blackouts and a complete electrical cutoff to many areas, and you get mass displacement of what could be millions upon millions of people in the US and Mexico combined. At that scale, we’re straying into the realm of societal breakdown—and while we won’t be presenting that extensive a work of fiction in this episode, we can certainly recommend a couple. For younger viewers, or those who’d like to keep at least some of their sanity, we recommend Dry by Neal and Jarrod Shusterman. For those who’d prefer something that will not protect their sanity—try The Water Knife. Good luck.

It’s in these closing moments of this episode, that we imagine you might be looking for a source of hope—or some indication that this future, for America’s Southwest, can still be diverted. And it’s in these closing moments, where we here at Megaprojects have to be the bearer of bad news: Relief is not coming. The process of reaching dead pool will be a slow and drawn-out affair, as early instances of dead pool are offset for a while in the wet season upriver, but the average water level of the dam continues to drop lower and lower, until even that runs past the dead pool threshold. That death knell for the region is harder to estimate. But according to some projections, Lake Mead’s first instance of dead pool is no longer years away, but just months. There is a strong chance that 2025 will see Lake Mead reach dead pool for the first time ever, and even if the reservoir doesn’t hit that benchmark this year, it’ll be the next year, or the year after that. If the Colorado River can make it to 2030 without reaching dead pool, it’ll be a miracle.

We titled this episode, “What Would Happen if the Hoover Dam Failed”, with the sneaking suspicion that most of you are here to learn about the Hoover Dam in a state of collapse. But the far more dire predicament facing the Hoover Dam, and the communities that rely on it, is the one that comes after the far more likely state of dam failure that appears imminent today. It’s a slower crisis, one with all the sadness and human suffering but little of the accompanying cinema—and it’s the crisis that the United States and Mexico are about to enter.

What would happen if the Hoover Dam failed? We’re about to find out.

Key Takeaways

• The Hoover Dam is crucial for water and power to 18 million people.
• A dam failure would cause catastrophic flooding and power outages.
• Lake Mead’s water levels are critically low, risking a ‘dead pool’ scenario.
• Dead pool would stop water flow through the dam, causing long-term crises.
• The southwestern U.S. faces severe water shortages and potential uninhabitability.

Sources

• Original MegaProjects video: What if the Hoover Dam Failed?
• https://www.grunge.com/226074/the-truth-about-the-nazi-plot-to-blow-up-the-hoover-dam/
• https://www.blazinpaddles.com/what-would-happen-if-the-hoover-dam-stopped/
• https://science.howstuffworks.com/engineering/structural/hoover-dam-broke.htm
• https://www.thoughtco.com/geography-of-hoover-dam-1435729
• https://hoover.archives.gov/hoovers/hoover-dam#:~:text=The%20dam%20was%20dedicated%20in,%241%20billion%20adjusted%20for%20inflation
• https://www.nps.gov/articles/-the-greatest-dam-in-the-world-building-hoover-dam-teaching-with-historic-places.htm
• https://waterandpower.org/museum/Construction_of_Hoover_Dam.html
• https://time.com/4045046/hoover-dam-name-history/
• https://www.maxtour.co/how-deep-is-the-water-in-front-of-the-hoover-dam/
• https://www.lvvwd.com/conservation/measures/index.html
• https://www.lasvegasnevada.gov/News/Blog/Detail/lake-mead-water-shortage
• https://www.brookings.edu/articles/5-facts-you-need-to-know-about-lake-meads-water-crisis/
• https://www.usbr.gov/newsroom/news-release/4934
• https://www.nbcnews.com/science/environment/lake-mead-nearing-dead-pool-status-engineer-was-named-horrified-rcna35030
• https://www.newsweek.com/lake-mead-dead-pool-hoover-dam-ramifications-1781704#:~:text=Dead%20pool%20is%20when%20the,no%20water%20passes%20through%20it.%22&text=If%20dead%20pool%20were%20reached,all%20its%20operations%20to%20cease
• https://www.sierraclub.org/sierra/what-does-dead-pool-mean-american-west
• https://lasvegassun.com/news/2022/may/26/understanding-dead-pool-and-how-water-officials-ar/
• https://yellowscene.com/2024/05/10/the-colorado-river-water-emergency/
• https://www.latimes.com/environment/story/2022-12-16/risk-of-dead-pool-looms-at-colorado-river-meeting
• Hero image source by RuggyBearLA / openverse, by.

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