Scientists Warn Catastrophic “Megaflood” Could Put California Underwater

Written by Kirstin Harrington
Published: September 28, 2022
© Teerapong Yovaga/
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California is home to towering redwoods, the stunning cliffs of Big Sur, and the entertainment capital of the world. The Golden State is also home to several types of severe weather, including flooding. 

According to a recent study, there will likely be more frequent and intense episodes of record-breaking rainfall in California as a result of climate change. What experts call a “megaflood” is headed to the west coast. The exact timing cannot be predicted, but the research has established that the storm is on its way.

The state of California’s flood control infrastructure, including levees, dams, floodplains, and more, will be put to the ultimate test by this superstorm, also known as atmospheric rivers. Communities in the Central Valley’s floodplains are particularly at risk from unprecedented flash flooding and are therefore more susceptible to them.

How Climate Change Affects Flooding

Climate change makes the atmosphere more capable of storing rain, which results in more moisture falling as rain and causing rapid floods. According to the latest research, the probability of week-long, recurrent strong-to-extreme atmospheric rivers increases quickly throughout the chilly season. 

While one might think California is a beautiful place to visit during the winter, there are dangers possible at this time. An atmospheric river is a long, narrow zone of dense moisture in the atmosphere that acts like a fire hose in the sky, carrying moisture hundreds of miles. They often deliver helpful precipitation to areas like California that are prone to drought, but with rising temperatures, they might soon turn dangerous.

Historically, the Sierra Nevada has received feet of snowfall from these winter atmospheric rivers, but as the climate warms, more snow will turn to rain. It doesn’t melt gradually over time; it all flows off, builds up, and floods at once. California is particularly susceptible to flooding due to its hilly geography and wildfire risk. 

Wildfire burn scars that have persisted can produce a steep, slippery surface that allows water and debris to flow off. Due to climate change, wildfires are growing larger and consuming more land, making more regions vulnerable to catastrophic sediment transport.

Image of a large atmospheric river aimed across California
This image shows a large atmospheric river flowing across California.

©United States Naval Research Laboratory, Monterey, Public domain, via Wikimedia Commons – License

What to Expect

Megafloods have occurred before in California’s history, so they wouldn’t be unheard of. The Sacramento and San Joaquin valleys were transformed into a 300-mile-long “inland sea” due to months of winter storms, and devastating floods normally occur five to seven times each millennium, according to experts. 

The most recent one occurred in 1862. The likelihood of the next catastrophe occurring and being considerably more severe continues to increase. The storm pattern for the megaflood is larger in practically every way in the hypothetical future. 

Such a storm would be significant enough to increase runoff water in the Sierra Nevada Mountains by up to 400% over previous records. Even the most conservative projections of the current climatic scenario show that extensive areas will buckle under about 40 inches of rain in 30 days. Certain locations would see nearly 100 inches of rain in a month.

Longest Lakes in California - Owens River
A megaflood like the one that occurred in 1862 would likely increase runoff water from the Sierra Nevada mountains up to 400% over previous records.


Where Will the Mega Flood Occur?

According to the study’s authors, the Central Valley of California, which includes Sacramento, Fresno, and Bakersfield, will experience the most harm. According to the US Geological Survey, the Central Valley, which is about the same size as Vermont and Massachusetts put together, generates 25% of the country’s food supply. This storm would be devastating to the entire country, not just the area the rain falls.  

UCLA climate scientist and research co-author Daniel Swain had a lot to say in a UCLA press release “There are localized spots that get over 100 liquid-equivalent inches of water (8.3 feet) in the month. On 10,000-foot peaks, which are still somewhat below freezing even with warming, you get 20-foot-plus snow accumulations” he says. “But once you get down to South Lake Tahoe level and lower in elevation, it’s all rain. There would be much more runoff.”

Based on the study, a flood large enough to cover this valley may be the most costly geophysical tragedy ever. This will cause losses of up to $1 trillion and wreaking havoc on California’s lowland regions. This includes the populous Los Angeles and Orange counties. That would cost more than five times what Hurricane Katrina, the most expensive disaster in US history, did.

Beautiful sunset of Los Angeles downtown skyline and palm trees in foreground
The megaflood is likely to cause up to $1 trillion in damage in densely populated areas including Los Angeles.


The Great Flood of 1862

On a map of Northern California, you can see a sizable valley that runs across the center of the region. Many people in this enormous valley call it home, which was sculpted through thousands of years of floods through the state’s center. In truth, Northern California is extremely susceptible to flooding since the majority of the extensive waterways drain into a narrow outlet in the San Francisco Bay.

The Great Flood of 1862 was the biggest flood ever to be documented in the history of California. The state was submerged in water during the flooding that occurred between December 1861 and January 1862, rendering most of the Northern Valley uninhabitable until the summer of 1862. 

A 300-mile-long by 20-mile wide lake that ran through the middle of the state was formed by the flood. Numerous thousands of individuals are thought to have perished in Northern California during the incident.

Beginning in the latter half of 1861, Northern California went through an abnormally rainy winter. This resulted in heavy snowfall in the mountains and rain in the valley. The snowpack that November was unusually thick for the time of year. The valley bottom quickly became soggy from all of the rain. That was only the beginning. 

The lower snowpack was melted and flushed down by the warm, tropical rain. That water then ran through the watershed and reached Sacramento. It was the first of four warm storms that flooded the valley over the course of the following six weeks.

A lithograph of citizens rowing boats through the streets of Sacramento, CA, during the great flood of 1862.
This lithograph shows citizens rowing boats through the streets of Sacramento, CA, during the 1862 flood.

©Public Domain – License

Chances of Another Mega Flood in California

According to the study, if a similar storm occurred today, up to 10 million people would be forced to leave their homes. Major motorways like Interstates 5 and 80 would be closed for several weeks. Places with high populations, like Stockton, Fresno, and some of Los Angeles would be turned to swamps.

The study stated above finds that the likelihood of a flood this extreme increases as the earth’s temperature rises by combining new, high-resolution weather modeling with existing climate models.

The study also discovered that with each extra degree of global warming this century, the likelihood of a megaflood is expected to rise higher. Gov. Gavin Newsom urged state agencies to start planning for a hotter, drier future in early August 2022. Newsom mentioned implementing measures including increasing water conservation and water recycling capacity. 

Unless we are able to stop the consequences that climate change is having on the Golden State, the megaflood appears to be unavoidable. Some of the ways you can help with climate change are:

  • Walking, biking, or using public transportation
  • Switch to LED lightbulbs
  • Reduce, reuse, recycle, and repair
  • Wash your laundry with cold water
  • Hang clothing to dry
  • Eat more vegetables
  • Throw away less food
  • Consider switching to solar panels for your home
  • Drive an electric vehicle
3D illustration of the earth drowning in a sea of water
There are steps that you can take to slow the progression of climate change.

©freie kreation/

Climate change will only be drastically affected in a positive manner if large companies change the way they do business. The everyday person can incorporate some of the suggestions above to do their part and reduce the chances of catastrophic weather conditions such as a megaflood. 

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Cars stuck on flooded road
Cars stuck on a flooded road
© Teerapong Yovaga/

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About the Author

When she's not busy playing with her several guinea pigs or her cat Finlay Kirstin is writing articles to help other pet owners. She's also a REALTOR® in the Twin Cities and is passionate about social justice. There's nothing that beats a rainy day with a warm cup of tea and Frank Sinatra on vinyl for this millennial.

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