When early settlers came to the confluence of the Sacramento and American Rivers before the California Gold Rush, Indigenous people warned them that the Sacramento Valley could become an inland sea when great winter rains came. The storytellers described water filling the valley from the Coast Range to the Sierra during these rare events.

And their warnings became realized when Great Flood of 1861-62 hit. A six-week onslaught of at least 10 powerful Pacific storms in December and January carried mighty "atmospheric rivers" of subtropical moisture into California, dumping torrential rains in the valleys and prodigious snows in the mountains. When an unusually warm storm struck in January, heavy rains fell on the enormous Sierra snowpack, melting it.

A cataclysmic flood ensued, inundating the Central Valley and transforming it into a lake 300 miles long and over 20 miles wide; much of the now densely populated coastal plain in present-day Los Angeles and Orange counties was also inundated. As summarized in a 2013 Scientific American overview, the flood killed thousands of people, drowned one-quarter of the state's estimated 800,000 cattle, and submerged downtown Sacramento under more than 10 feet of brown water laden with debris from countless mudslides. With the state's capital city paralyzed, the California legislature was forced to move to San Francisco until the summer of 1862. By that point, the state was bankrupt, as one-third of its taxable properties had been destroyed.