California has underestimated the epic potential of future flooding, research shows

For well over a century, the Great Flood of 1862 has remained among California's worst natural disasters — a megastorm that's been used as a benchmark for state emergency planners and officials to better prepare for the future.

A dreaded repeat of the flood — which killed at least 4,000 people and turned the Central Valley into a 300-mile-long sea — would probably eclipse the devastation of a major California earthquake and cause up to $1 trillion in damage, some experts say.

Yet even as California scrambles to cope with the effects of climate whiplash and increasingly extreme weather, new research suggests the potential magnitude of such events could be far greater than that of the 1862 deluge.

After analyzing layers of sediment at Carrizo Plain National Monument, researchers at Cal State Fullerton say they have identified two massive, unrecorded Southern California flood events within the last 600 years.

Shockingly, their analysis suggests the deluges were far larger than the Great Flood, which reshaped much of the Central Valley and Los Angeles Basin.

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Researchers based their conclusions on multiple core samples taken from a "sag pond" along the San Andreas fault in the southeastern corner of San Luis Obispo County. Analysis of the core samples revealed signs of two epic floods — one occurring sometime between 1470 and 1640 and the other between 1740 and 1800.

What they could not find in the core samples, however, was a sign of the Great Flood, suggesting perhaps that it was far less consequential than the other two.

"We're not seeing the geological signature of what's supposed to be the biggest event in historic time, and what we're using as essentially the basis for a lot of models and predictions about future flooding," said Matthew Kirby, a geology professor at Cal State Fullerton and lead author of the study.

"That's a little concerning to us because I think we're probably underestimating the magnitude of naturally occurring flood events, and that's something we need to really understand."

The findings, which were published recently in the Journal of Paleolimnology, add to a growing body of research that suggests Californians may be ignorant of just how devastating future floods could be. If such large floods have always been part of California's natural cycle of drought and downpour, just how much worse could they be in a period of climate change?

"We look back at our history, and these massive events come along, and they're gonna keep coming along," said Josh Willis, a climate scientist at NASA's Jet Propulsion Laboratory in La Cañada Flintridge, who was not involved in the research. "But global warming is almost always gonna make them worse. So, the wild ride is gonna get wilder."

Willis said it was "eye-catching" that the geological record bore no trace of the 1862 flood.

"It begs the question, 'Why wasn't that one in the sediment core?' And if the answer is, well, it wasn't big enough, ... then that's kind of scary for the future," Willis said.

However, he warned against drawing too many conclusions from a single paleoclimate study, saying it "paints one little part of the picture." Willis noted that these two major flood events from ancient times occurred during a period of global cooling known as the Little Ice Age, which spanned roughly the 14th to 19th centuries.

"We're looking [now] at a climate that's not colder, it's going to be warmer," Willis said. "We're heating up the planet, so comparing to the Little Ice Age may not be exactly the best analogue."

But he said it could also indicate that future floods could be worse than in the past, given that in a warmer climate, the atmosphere has the capacity to hold more water. He said these are questions that require more research, and can continue to build on these sag pond findings.

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Tessa Hill, a professor of Earth and planetary sciences at UC Davis and director of the university's Ocean Climate Lab, said the study added to a fuller understanding of past flood events.

"Previous work in this regard has been primarily reliant upon coastal sediment records, which can record very accurate and high resolution climate records but may not capture the complexity of what is happening in different regions of California," said Hill, who also was not involved in the research.

"Understanding the past record of large flood events ... is critical for predicting the consequences of a changing climate for California residents," she said.

Paleolimnology, the study of ancient lakes, is one way researchers are trying to better understand California's past. But there aren't many natural lakes in Southern California, and many of the ones that do exist sit high up in the mountains — not the best location for researchers searching for buried clues about past flood events.

Instead, Kirby and his team turned to sag ponds, or land depressions along active fault lines that often accumulate water.

“Sag ponds may prove a valuable and generally untapped paleo archive," the study authors wrote.

At Carrizo Plain National Monument, the researchers removed five core samples from a now dry sag pond. The core samples, which each measured about 4 to 5 feet long, encapsulated many layers of sediment — earth and biological matter that had been washed into the lake from surrounding hills and shores and settled to the bottom.

Changes in the type and size of the sediment indicated that energy was needed to erode and deposit it in the basin — the larger the grain, the more energy required. Kirby said that helped the team piece together the two discrete flood events — one 380 to 554 years ago, and the other 284 to 224 years ago.

Kirby said the 1862 flood probably left a geological footprint in the core, but it wasn't scientifically significant, especially compared with the two ancient floods.

"It's not showing up in the geological archives like you would expect it should, considering the size," Kirby said. "It's not like [the flood] didn't happen, of course it happened. It was huge. But ... as we dig deeper into the geological record over the past 11,700 years, ... we are able to show, without question, that there's a lot that's happening that we have not seen in a historic time."

The 1862 flood has been used as a key data point in creating the "ARkStorm Scenario," originally projected as California's once-in-a-thousand-years catastrophic flood event, but now some scientists say it may not be extreme enough.

"The potential floods that California may receive in the future could be magnitudes worse than recent floods,” Samuel Hippard, a Cal State Fullerton student and one of the study’s co-authors, said in a statement. “Our research shows the potential risk to millions of Californians.”

Another recent study found that there was much greater atmospheric river activity over the last 3,000 years than in recent history, further indicating that California officials may be underestimating the extent of rainfall and prior floods.

Read more: The 'nightmare' California flood more dangerous than a huge earthquake

Kirby said he hopes to continue focusing his work in this field, looking to document further historic floods from the cores of lakes and ponds.

"It was really exciting to find that we were able to extract paleo storm events from this tiny little lake," Kirby said. "There aren't a lot of lakes in California, especially in Southern California, ... so finding an archive where we can find additional information is a huge boon for us."

Kirby has identified at least three other sag ponds in Southern California for potential research in the future, and several others in the Central Valley and Northern California.

“Scientists know very little about California’s flood history older than the historical record of the past 150 years,” said Kirby, who has been studying Earth’s climate history for years. "If these sag ponds become an archive that we can explore and find these individual events, that's going to really advance our science and understanding of the history, the frequency and the magnitude of past flood events."

This story originally appeared in Los Angeles Times.