Californians are always talking about the coming Big One, but what if the big one is a flood, not an earthquake? The ARkStorm would bring with it catastrophic rains, hurricane-force winds and hundreds of landslides. Central Valley flooding alone is projected to span 300 miles.

Is California overdue for biblical, catastrophic flooding? History says yes.

With this recent cavalcade of rainstorms, there’s been renewed interest in a 2011 USGS study on the so-called “ARkStorm.” In it, the USGS lays out a case for a hypothetical “megastorm,” one that could cause up to $725 billion in damage and impact a quarter of California’s homes.

If that sounds far-fetched, there’s historic precedent: Geological evidence indicates that California endures massive flooding caused by atmospheric rivers every 100-200 years. And settlers who moved to California after the Gold Rush soon found what the native population had known for centuries: Northern California is prime flooding territory.

The Great Flood is the largest recorded flood in California’s history. It was estimated that roughly one fourth of the taxable real estate in state was destroyed by the flood, which left California bankrupt. This image from the United States Geological Survey provides a look at how bad the flooding may have been.Picture via USGS

The Great Flood of 1862

The most prominent example is the Great Flood of 1862, a natural disaster that still ranks as the largest flood in the history of the American West. Between Dec. 1861 and Jan. 1862, the West Coast received a near-constant deluge of rain. Sacramento received a stunning 23 inches in that period, turning the city into a watery ghost town.

“The people are leaving the city as rats would a sinking ship” the Red Bluff Independent wrote on Jan. 14.

As flood waters rose, it took entire houses with it. Little two-story wooden houses were carried off whole and eyewitness reports in the local papers said the flowing waters were full of furniture and dead livestock – like in Australia right now. Thirty foot-tall telegraph poles, which had recently been installed between New York and San Francisco, were fully submerged.

The following animation depicts ARkStorm precipitation (rain and snow) in inches. The ARkStorm Scenario combines pre-historical geologic flood history in California with modern flood mapping and climate-change projections to produce a hypothetical, but plausible scenario aimed at preparing the emergency response community for this type of hazard:

That was hardly the worst of it. A Tuolumne County paper reported that 1,400 Chinese migrants died in the flooding state-wide. One-third of the property in the state was destroyed and 800,000 cattle died, a mass die-off that marked “the beginning of the end of the cattle-based ranchero society in California.”

Settlers realized the homes that survived had something in common: They were built in the spots where Native Americans originally put down settlements. Native stories spoke of the Sacramento Valley as an inland sea. For centuries, they’d seen the valley fill with water, and the Nevada City Democrat reported that “Indians living in the vicinity of Marysville left their abodes a week or more ago for the foothills predicting an unprecedented overflow. ”

Vapor transport animation of the USGS ARkStorm scenario, based on actual data from real two California storms from 1969 and 1986:

“Let these facts make a due impression, and serve as a lesson,” the California Farmer and Journal of Useful Sciences wrote on Jan. 17.

When the flood waters finally receded months later, Sacramento began the seven-year process of raising its downtown 10-15 feet. Today, you can peer into the now-underground portions of J Street via skylights.

Other major floods in California

Although the 1862 flood was the biggest, it was hardly the only major flood.

In 1850, the newly built city of Sacramento was nearly wiped out by floods.

In 1907 and 1908, back-to-back floods submerged the entire Sacramento Valley.

By the mid-1910s, Congress had authorized major flood control projects in the valley, the first flood control work authorized outside of the Mississippi Valley.

If you’re thinking of moving to higher ground, you might not be remiss in doing so. The 1862 flooding was due to an atmospheric river, a long, narrow column of vapor that brings massive amounts of rain with it.

The Bay Area was just hit by an atmospheric river that brought mudslides, flooding and high winds.

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[Scientific American, SFGate]