It has already happened once.
No one alive remembers it. In the winter of 1861–62, a 43-day run of storms filled California’s Central Valley with water and left parts of the state flooded for up to six months.
How it unfolded.
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December 1861
The storms begin.
A series of storms starts hitting California. It doesn’t let up for 43 days.
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January 10, 1862
Sacramento goes under.
Floodwater sweeps through the capital on the day Leland Stanford is to be inaugurated as governor. The ceremony goes ahead. He gets there by rowboat.
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January 22, 1862
The government leaves.
Unable to work in a flooded city, the legislature gives up and moves to San Francisco to wait out the water.
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The months after
An inland sea.
Water covers the Central Valley across an area at least 300 miles long and 20 miles wide. In the south, the Santa Ana River spreads into a sheet of water four feet deep around Anaheim. It stays for four weeks.
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The seven years after
Sacramento rebuilds higher.
The city raises its downtown district by 10 to 15 feet.
“The entire valley was a lake extending from the mountains on one side to the coast range hills on the other.”