Research shows California has underestimated the epic potential of future floods

For more than a century, the Great Flood of 1862 has ranked among California’s worst natural disasters: a megastorm used as a benchmark for state emergency planners and officials to better prepare for the future.

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

But even as California scrambles to deal with the effects of climate whiplash and increasingly extreme weather, new research suggests that the potential magnitude of such events could far exceed that of the 1862 Flood.

After analyzing sediment layers at Carrizo Plain National Monument, researchers at Cal State Fullerton say they have identified two massive, unrecorded floods in Southern California in the past 600 years.

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

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Researchers based their conclusions on multiple core samples taken from a so-called “subsided 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 occurred sometime between 1470 and 1640 and the other between 1740 and 1800.

What they couldn’t find in the core samples, however, was any sign of the Great Flood, perhaps indicating that it was far less consequential than the other two.

“We’re not seeing the geological signature of what should be the largest event in historical time, and what we use as the basis for many models and predictions about future flooding,” said Matthew Kirby, professor of geology. at Cal State Fullerton and lead author of the study.

“That worries us a little bit because I think we’re probably underestimating the extent of naturally occurring flooding, and that’s something that we really need to understand.”

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

“We’re looking back at our history and these massive events are coming, and they’re going to keep coming,” said Josh Willis, a climate scientist at NASA’s Jet Propulsion Laboratory in La Cañada Flintridge, who was not involved in the study. “But global warming will almost always make them worse. So the wild ride is going to get even wilder.”

Willis said it was “striking” that the geological record showed no trace of the 1862 flood.

“It begs the question, ‘Why wasn’t that in the sediment core?’ And if the answer is, it wasn’t big enough, then that’s kind of scary for the future,” Willis said.

However, he cautioned against drawing too many conclusions from a single paleoclimate study, saying it “paints a small part of the picture.” Willis noted that these two great ancient floods occurred during a period of global cooling known as the Little Ice Age, which stretched roughly from the 14th to the 19th centuries.

“Was looking 1717371247 in a climate that is not colder, it will become warmer,” Willis said. “We’re warming the planet, so comparing it to the Little Ice Age might not exactly be the best analogy.”

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

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Tessa Hill, professor of earth and planetary sciences at UC Davis and director of the university’s Ocean Climate Lab, said the study contributed to a better understanding of past floods.

“Previous work in this regard has relied primarily on coastal sediment records, which can capture very accurate, high-resolution climate records but may not capture the complexity of what is happening in different regions of California,” said Hill, who also did not was involved. in the investigation.

“Understanding the history of major flooding… is critical to predicting the impacts of a changing climate on 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 those that do exist are high in the mountains—not the best location for researchers looking for hidden clues about past floods.

Instead, Kirby and his team focused on subsided ponds, or land depressions along active fault lines that often collect water.

“Sagging ponds may prove to be a valuable and generally unused paleoarchive,” the study authors wrote.

At Carrizo Plain National Monument, the researchers removed five core samples from a now-dry subsided pond. The core samples, each about four to six feet long, encapsulated many layers of sediment: soil and biological matter that had washed into the lake from surrounding hills and shores and settled at the bottom.

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

Kirby said the 1862 flood likely left a geological footprint in the core, but it is not scientifically significant, especially compared to the two ancient floods.

“It doesn’t show up in the geological record as you would expect, given its size,” Kirby said. “It’s not like that [the flood] It didn’t happen, of course it happened. It was huge. But if we dig deeper into the geological record over the past 11,700 years, we can show beyond a shadow of a doubt that there is a lot happening that we haven’t seen in any historical 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-year catastrophic flood, but now some scientists say it may not be extreme enough.

“The potential flooding that California faces in the future could be greater than recent flooding,” 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 past 3,000 years than in recent history, further indicating that California officials may be underestimating the magnitude of rainfall and past flooding.

Read more: California’s ‘nightmare’ flood is more dangerous than a massive earthquake

Kirby said he hopes to continue to focus his work in this area, documenting further historical flooding from lake and pond cores.

“It was really exciting to find that we could get paleostorm events from this little lake,” Kirby said. “There aren’t many lakes in California, especially in Southern California, … so finding an archive where we can find additional information is a huge boon to us.”

Kirby has identified at least three other subsidence ponds in Southern California for possible future investigation, and several others in the Central Valley and Northern California.

“Scientists know very little about the history of California flooding, which is older than the historical record of the past 150 years,” said Kirby, who has studied Earth’s climate history for years. “If these subsided ponds become an archive that we can examine and find these individual events, that will really advance our science and our understanding of the history, frequency and magnitude of past flood events.”

This story originally appeared in the Los Angeles Times.

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