Earthquakes have long been associated with California. And now fire tops our list of natural disasters. But Debbie Duncan says if history is any guide there’s a bigger Big One in store.

Quick! What was the biggest disaster in the history of the state of California? Not the 1906 earthquake and fire that destroyed much of San Francisco, or last fall’s Camp Fire, which devastated the town of Paradise. It was a flood, as in the Great Flood of 1861-62, when it rained for 45 days.

Normal seasonal rainfall in San Francisco is 22 inches; that year 49 inches fell. Leland Stanford traveled from his Sacramento home to his gubernatorial inauguration by rowboat, as the city was 10 feet under water. It remained flooded for three months.

Entire towns in the Sierra foothills were obliterated as one storm after another slammed California. A settlement of Chinese miners drowned when the Yuba River flooded. Residents reported seeing houses, horses, poultry, cattle, barns, bridges, camps, stores, and saloons swept downstream. Hills everywhere became landslides. The Central Valley completely flooded. An inland waterway 300 miles long and 20 miles wide wiped out nearly every house and ranch. No one knows how many thousands of humans died, but at least 200,000 cattle drowned. It took one season for California to switch from a ranching economy to a farming one—when it recovered. The state declared bankruptcy following the Great Flood.

Rains like this will happen again. Geologists have determined that megafloods hit California every one- to 200 years. And that’s without climate change! We have better flood-control infrastructure now than in the nineteenth century, but dams don’t always hold, and there are a lot more people today on those hills, plains and valleys. Yet unlike the big earthquake everyone expects but cannot precisely predict, meteorologists know days in advance about these atmospheric rivers that build in the Pacific.