A disastrous 'megaflood' flood in sunny and dry California? It's happened before

Mike Snider
Grayscale lithograph of K Street in the city of Sacramento, California — during the Great Flood of 1862. The flood affected the Western United States, from Oregon through California, and Idaho through New Mexico.
  • New research suggests climate change increases the likelihood of a massive California "megaflood," akin to the Great Flood of 1862.
  • That disaster, brought on by more than 40 days of constant rain, led to the death of 4,000.
  • Flood waters in that disaster created an "inland sea" 300 miles long and 60 miles wide in some places.

A new study raises concerns about climate change-fueled floods dropping massive amounts of water on drought-plagued California – an unlikely sounding scenario that has actually happened before.

While intense droughts, wildfires and earthquakes are typically the main concern across the West, the study released Friday warned of another crisis looming in California: "Megafloods." It notes climate change is increasing the risk of floods that could submerge cities and displace millions of people across the state. It says an extreme monthlong storm could bring feet of rain – in some places, more than 100 inches – to hundreds of miles of California.

While the scenario might sound like something out of a movie, it's happened before.

California has experienced severe floods throughout the 20th Century, including in 1969, 1986, and 1997. But a flood from farther in the past – the Great Flood of 1862 – is being eyed by researchers as the threat to California grows by the day.

Though it occurred 160 years ago, the flood – deemed a "megastorm" for its historical rainfall covering huge swaths of the state – illustrates that the threat is not merely theoretical.

In fact, the UCLA researchers studying "megafloods" say such storms typically happen every 100-200 years.

Researchers are sounding the alarm because flood of that scale today would have far more devastating impacts in a state that is now the nation's most populous. 

And the Great Flood of 1862 was also preceded by drought.

How bad was the Great Flood of 1862?

Intense rainstorms pummeled central California "virtually unabated" from Christmas Eve 1861 until January 1862, Scientific American chronicled in a 2013 story on "The Coming Megastorms."

The flow of water created "a huge inland sea …  a region at least 300 miles long," leaving Central and southern California underwater for up to six months, the magazine said. Floodwaters stretched as wide as 60 miles across, wrote UCLA researchers in their recent flood risk study.

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"Thousands of farms are entirely under water – cattle starving and drowning," wrote scientist William Brewer (author of "Up and Down California in 1860-1864") in a letter to his brother, cited by Scientific American. "All the roads in the middle of the state are impassable; so all mails are cut off. The telegraph also does not work clear through. In the Sacramento Valley for some distance the tops of the poles are under water." 

An estimated 4,000 people died and one-third of all property in the state was destroyed, including one-fourth of its 800,000 cattle, which either drowned or starved, wrote the SFGate news site in a retrospective earlier this year.

The Great Flood of 1862 would be much worse if it happened today

The region that was underwater in 1862 is now home to many more people than it was then — it's home to some of California’s fastest-growing cities including Bakersfield and Sacramento.

Back then, the state’s population was about 500,000, but today it's nearly 40 million. "Were a similar event to happen again, parts of cities such as Sacramento, Stockton, Fresno and Los Angeles would be under water even with today’s extensive collection of reservoirs, levees and bypasses," researchers who worked on the flood-risk study released Friday said in a press release.

The resulting disaster would cause an estimated $1 trillion in damage, the biggest disaster in world history, they say.

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And the effects would go beyond central and southern California, said Daniel Swain, a UCLA climate scientist and the study's co-author. “Every major population center in California would get hit at once – probably parts of Nevada and other adjacent states, too,” he said.

Major highways such as Interstate 5, which runs along the Pacific coast from Canada to Mexico, and I-80, which dissects California through San Francisco and Sacramento, would likely be shut down for weeks or months, he said.

The ripple effects would impact global economics and supply chains.

Lightning strikes east of the eastern front of the McKinney Fire, in the Klamath National Forest near Yreka, Calif. on Aug, 2, 2022.

What causes megafloods?

Atmospheric rivers are long water vapor streams formed about a mile above Earth. They can "carry as much water as 10 to 15 Mississippi Rivers from the tropics and across the middle latitudes," wrote Michael Dettinger, research hydrologist for the U.S. Geological Survey, and Lynn Ingram, a University of California, Berkeley, professor of earth and planetary science, in Scientific American.

When one comes across the Pacific Ocean and hits the Sierra Nevada, "it is forced up, cools off and condenses into vast quantities of precipitation," they wrote.

Warming temperatures are making extreme storms more likely – with more runoff, researchers say. In a 2018 study, Swain estimated there was a 50-50 chance of a megaflood the size of the Great Flood of 1862 happening again by 2060, Popular Science reported. “It would essentially inundate land that is now home to millions of people,” he said then.

The new research suggests climate change has already doubled the likelihood of extreme storms and each additional degree of global warming increases the likelihood of a megaflood.

Research is continuing on potential flood effects and how to prepare for the them. Keeping the issue alive in the mind of Californians is important because drought, wildfires and earthquakes get all the attention, Swain said.

"There is potential for bad wildfires every year in California, but a lot of years go by when there’s no major flood news," he said. "People forget about it.”

Follow Mike Snider on Twitter: @mikesnider.