(Thanks to Jim and Ruth Stretch for sending this along.)
Geologic evidence shows that truly massive floods, caused by rainfall alone, have occurred in California every 100 to 200 years. Such floods are likely caused by atmospheric rivers: narrow bands of water vapor about a mile above the ocean that extend for thousands of kilometers.
The atmospheric river storms, featured in a January 2013 article in Scientific American… The Coming Megafloods, are responsible for most of the largest historical floods in many western states. The only megaflood to strike the American West in recent history occurred during the winter of 1861-62. California bore the brunt of the damage. This disaster turned enormous regions of the state into inland seas for months, and took thousands of human lives. The costs were devastating: one quarter of California’s economy was destroyed, forcing the state into bankruptcy…
Today, the same regions that were submerged in 1861-62 are home to California’s fastest-growing cities. Although this flood is all but forgotten, important lessons from this catastrophe can be learned.