Horrible thought! But an interesting one.
No one can know for sure the result of a disaster of that magnitude. But if the roughly 25 million acre feet (eight trillion gallons) of water in Lake Mead were suddenly unleashed, they'd cover 25 million acres to a depth of one foot. How many acres is 25 million? It corresponds to around 40,000 square miles, slightly less than the size of the state of Ohio.
Certainly, the water would overflow the banks of the original Colorado River and inundate downriver communities such as Laughlin and Boulder City, Needles, Lake Havasu, Parker, Yuma, and San Luis Rio Colorado before emptying into the Gulf of California on the east side of the Baja Peninsula.
How far the water would spread from the riverbanks is unclear, but it would probably wipe out a good section of southeastern California, all the way to the Salton Sea, which the Colorado regularly flooded before Hoover Dam was built (and was a large part of the reason it was erected).
What would happen to Las Vegas is a matter of conjecture. It wouldn't be flooded, since the water would start draining southeast of the city and rush south down the Colorado canyon. However, Las Vegas gets all of its water from Lake Mead, so the city would quickly dry up. The water system has a few days worth of reserves, but after that? No more Bellagio fountain show. No more Adventuredome flume ride. No more MGM pool or Tropicana swim-up blackjack. Heck, no more water when you turn on the tap or try to flush the toilet (and remember, there are more than 150,000 toilets in hotel and motel rooms alone). We wouldn't want to be around when it happens.
And it's certainly not just Las Vegans who'd suffer. Lake Mead supplies water to nearly 15 million people and irrigates more than two million acres of crop land. Southern California gets 4.4 million acre feet (compared to southern Nevada's 300,000).
How likely is it ever to happen? Impossible to say. The dam is pretty solid: seven million tons of concrete, 660 feet thick at the bottom. It was built to last "indefinitely," though it's already almost 70 years old. However, it would take a suitcase nuke to knock it down, and the odds against that are pretty high. All we can do is hope and pray that this is one disaster that never befalls us (those toilets...).
