For more than a century, the Great Flood of 1862 has remained among California's worst natural disasters — a megastorm that has been used as a benchmark for state planners and emergency 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 dwarf the devastation of a major California earthquake and cause up to $1 trillion in damage, some say. experts.
However, even as California struggles to cope with the effects of climate whiplash and increasingly extreme weather, new research suggests that the potential magnitude of such events could be far greater than that of the 1862 flood.
After analyzing sediment layers in the Carrizo Plain National Monument, Cal State Fullerton researchers say they have identified two massive, unrecorded floods in Southern California in the last 600 years.
Surprisingly, 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.
The researchers based their conclusions on multiple core samples taken from the so-called “sunken pond” along the San Andreas Fault in the southeast corner of San Luis Obispo County. Analysis of core samples revealed signs of two epic floods: one that occurred sometime between 1470 and 1640 and the other between 1740 and 1800.
However, what they couldn't find in the core samples was a sign of the Great Flood, perhaps suggesting that it was far less consequential than the other two.
“We're not seeing the geologic signature of what is supposed to be the largest event in historical time, and what we're essentially using as the basis for many models and predictions about future floods,” said Matthew Kirby, professor of geology. at Cal State Fullerton and lead author of the study.
“That's a little worrying for us because I think we're probably underestimating the magnitude of naturally occurring flooding, and that's something we really need to understand.”
A member of the Cal State Fullerton research team samples a “sunken pond” at Carrizo Plain National Monument in San Luis Obispo County to investigate ancient flooding.
(Matthew Kirby)
The findings, which were recently published in the Journal of Paleolimnology, add to a growing body of research suggesting that Californians may be unaware of how devastating future floods could be. If flooding this large has always been part of California's natural cycle of drought and downpours, how much worse could it be in a period of climate change?
“We look back in our history and these massive events appear and will continue to occur,” 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 will almost always make them worse. Then the wild ride will get even wilder.”
Willis said it was “striking” that the geological record had no trace of the 1862 flood.
“The question arises: 'Why wasn't that in the core of the sediment?' And if the answer is, well, it wasn't big enough… then that's a little 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 spanned roughly the 14th to 19th centuries.
“They were looking 1717341852 In a climate that is not colder, it will be warmer,” Willis said. “We are warming the planet, so comparing it to the Little Ice Age may not exactly be the best analogy.”
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 ability to hold more water. He said these are questions that require more research and can continue based on these sunken pond findings.
Tessa Hill, a professor of earth and planetary sciences at UC Davis and director of the university's Ocean Climate Laboratory, said the study contributed to a more complete understanding of past flooding.
“Previous work in this regard has relied primarily on coastal sediment records, which can record very accurate, high-resolution climate records, but may not capture the complexity of what is happening in different regions of California,” Hill said, who did not participate either. on the research.
“Understanding the past history of major flooding… is critical to predicting the consequences of a changing climate for California residents,” he 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, which isn't the best place for researchers looking for buried clues to past floods.
Instead, Kirby and his team turned to sunken ponds, or depressions of land along active faults that often accumulate water.
“Sunken ponds may prove to be a valuable and generally untapped paleo archive,” the study authors wrote.
At Carrizo Plain National Monument, researchers removed five samples from a now-dry pond. The core samples, each measuring 4 to 5 feet long, encapsulated many layers of sediment—soil and biological matter that had been washed into the lake from the surrounding hills and shorelines and settled on the bottom.
Changes in sediment type and size indicated that energy was needed to erode it and deposit it in the basin: the larger the grain, the more energy was needed. Kirby said that helped the team reconstruct the two discrete floods: one 380 to 554 years ago and the other 284 to 224 years ago.
Kirby said the 1862 flood likely left a geological imprint on the core, but it was not scientifically significant, especially compared to the two ancient floods.
“It doesn't appear in the geological archives as you would expect, considering the size,” Kirby said. “It's not like [the flood] It didn't happen, of course it did. It was huge. But… as we delve deeper into the geological record of the last 11,700 years,… we can show, without a doubt, that there is a lot going on that we haven't seen in historical time.”
The 1862 flood has been used as a key data point to create the “ARkStorm Scenario,” originally projected as a once-in-a-thousand-year catastrophic flood in California, but now some scientists say it may not be extreme enough.
“The potential flooding that California may receive in the future could be magnitudes worse than recent flooding,” Samuel Hippard, a student at Cal State Fullerton 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 more atmospheric river activity in the last 3,000 years than in recent history, further indicating that California officials may be underestimating the magnitude of past rainfall and flooding.
Kirby said he hopes to continue focusing his work in this field, seeking to document more historic floods in the center of lakes and ponds.
A sediment core from the Carrizo Plain National Monument that the Cal State Fullerton team analyzed.
(Matthew Kirby)
“It was really exciting to discover that we were able to extract paleostorm events from this small 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 big help to us.”
Kirby has identified at least three other sunken ponds in Southern California for possible future research, and several others in the Central Valley and Northern California.
“Scientists know very little about the history of California flooding beyond the historical record of the last 150 years,” said Kirby, who has been studying Earth's climate history for years. “If these sunken ponds become an archive that we can explore and find these individual events, that will really advance our science and understanding of the history, frequency and magnitude of past floods.”
