1862
It happened.
Sacramento goes under. The new governor travels to his own inauguration by rowboat. The legislature moves to San Francisco.
The 1862 flood43 days
of storms in the winter of 1861–62.
Scientific American
300 mi
of the Central Valley under water in the USGS scenario, 20 miles or more across.
USGS, 2011
$725B
USGS’s estimate of the total cost: property damage plus lost business.
USGS, 2011
2×
Climate change has already doubled the odds of a storm sequence this big.
Huang and Swain, 2022
Start here
Watch the film USGS made in 2011.
The U.S. Geological Survey produced this short film alongside its first ARkStorm study. It covers California only and it shows its age. It’s still the best short introduction there is.
Video: U.S. Geological Survey with Designmatters at ArtCenter College of Design, played from the USGS YouTube channel. We skip the closing card that sends viewers to Arkstorm.com, because that site is gone. The credits still play.
The short version
The story in three dates.
2011
USGS models it.
A team of 117 experts works out what the same kind of storm would do to modern California. They call it ARkStorm.
The 2011 study2022
The update.
Climate scientists re-run the storm for a warmer world. The update puts it 200% to 400% more severe, with over $1 trillion in damage in California.
The 2022 updateThe long record
Ten megastorms in about 1,650 years.
Layers of silt show floods this size in 212, 440, 603, 1029, about 1300, 1418, 1605, 1750, 1810 and 1861–62. The last one was more than 160 years ago.
Beyond California
It did not stop at the state line.
The storm of 1861–62 hit what are now seven western states, plus Mexico and Canada. The ARkStorm studies cover California only. We start with Arizona.
Research library
The latest work on ARkStorm.
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Apr. 6, 2026 · Reporting
A biblical megaflood could hit the US at any moment. And that’s only the beginning
BBC Science Focus on the megaflood risk and how ready California is.
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June 2023 · Data
ARkStorm 2.0 storm simulations released as an open dataset
The simulations behind the 2022 study were opened to other researchers and won DesignSafe’s 2023 Dataset Award.
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Aug. 12, 2022 · Study
Climate change is increasing the risk of a California megaflood
Huang and Swain, in Science Advances. The first part of ARkStorm 2.0.
Check your address
Is your home in the old floodplain?
FEMA’s flood maps only go back to the late 1960s. See whether an address sits where the water went in 1862.
Prepare
What you can do now.
Official flood maps, planning guides and alerts from FEMA and the National Weather Service, in one short list.
Analysis · Overlooked dangers
Where does all that water go?
In Southern California, the Los Angeles and San Gabriel rivers carry it to San Pedro Bay, at the foot of the ports of Los Angeles and Long Beach. We estimate it would cost more than $2 trillion just to repair the destruction there. It’s one of more than 100 places our founder is mapping.
40%
of all container shipments into the United States come through the two ports on San Pedro Bay.
San Pedro Bay port complex
The warning was written for planners. This site is for everyone else.
USGS wrote ARkStorm for emergency planners, utility operators and policymakers. Its 2011 film points the public to Arkstorm.com. That site is no longer maintained. USGS told us the reason was funding.
ARkStorm.org is an independent, non-political effort to keep that information public: everything known about ARkStorm in one place, explained plainly and updated as new work is published.