Worse Than the Big One: California’s Coming Megaflood

When people think of California and natural disasters, they usually think of earthquakes, wildfires and droughts. But the worst natural disaster to hit the west in the last 160 years was the Great Flood of 1862, a series of storms that inundated much of the land, from Oregon to San Diego. The agriculturally rich Central Valley became a vast inland sea, 300 miles long and 20 miles wide. The state capital in Sacramento was under water for six months, forcing the government to relocate to San Francisco. 33% of California’s state property was destroyed, along with one in every eight private homes. Thousands of people died, possibly up to 1% of California’s entire population. And while floods of this magnitude used to happen every 200 years or so, models generated by Daniel Swain and researchers at UCLA’s Department of Atmospheric and Oceanic Sciences found that they will now happen roughly every 65 years, due to the effects of climate change. Swain also predicts a 20% increase in the intensity of megastorms, meaning the next one could be far more devastating.

Lithograph of K Street in the city of Sacramento, California, during the Great Flood of 1862. By Unknown (published by A. Rosenfield (San Francisco)) – http://content.cdlib.org/ark:/13030/tf2b69p086/?layout=metadata, Public Domain, https://commons.wikimedia.org/w/index.php?curid=24511196

Despite the threat, media coverage has been scant, probably because megaflood events have been so rare in California, compared with the destructive wildfires and droughts that seem to happen every year. Yet the period leading up to the Great Flood of 1862 was eerily similar to today. The state was emerging from two decades of severe drought. Farmers and ranchers had been praying for a wet winter. At first, it looked like they would get what they wanted. The storms began in November, 1861, with weeks of snow at high elevations. But then it started to rain. In California, it rained for 43 straight days, starting on Christmas Eve. Entire communities were swept away. Transportation, mail and communication across the state were disrupted for a month. California’s new governor, Leland Stanford, had to take a rowboat to get to his inauguration. In Nevada, the Carson Valley became a lake. In Idaho, the Boise River swelled to two miles wide, washing away portions of the Oregon Trail. The Colorado and Gila Rivers flooded in New Mexico Territory, turning Fort Yuma into an island.

The flood completely changed the course of California’s economy. Prior to the storm, it had been dominated by cattle ranching in the Central Valley, run predominantly by Mexicans who had gained citizenship and land rights at the end of the Mexican-American War (1846-1842) under the Treaty of Guadalupe Hidalgo. Those who survived the 20-year drought, saw their lands flooded and cattle drowned by the storms. 25% of the state’s 800,000 head of cattle perished in the flood. The ranchers had to sell their land to Anglo settlers at pennies per acre. And the Anglo farmers began growing nuts, cotton and vegetables. Meanwhile, many of the Mexican ranchers were forced to become day laborers just to survive.

The event also influenced the course of wars. In New Mexico Territory, for example, the flooded Rio Grande impeded the California Column as it attempted to cut off the retreating Confederate Army of New Mexico, allowing them to escape into Texas. And in California’s Owens Valley, it brought the Paiutes, who were on the brink of starvation because the storms had decimated the wild game they relied on, into conflict with ranchers, who were trespassing on their lands to graze their herds. Over 200 Native Americans died in the Owens Valley Indian War (1862-1867), along with roughly 60 members of the California Militia.

If the flood sounds cataclysmic, it was. But research indicates that storms of this magnitude have occurred roughly every 200 years in California, which means another is likely, possibly soon. The United States Geological Survey (USGS) examined sediment data from the San Francisco Bay Area, Santa Barbara basin, Sacramento Valley, and Klamath Mountain region and found megafloods occurred in the years 212, 440, 603, 1029, c1300, 1418, 1605, 1750, 1810, as well as 1861-62. Many were worse than the flood of 1862, especially the one in 1605, which they believe was 50% worse than any of the others.

The storms that caused these megafloods were the result of Atmospheric Rivers (AR), thin belts of water vapor that hover about a mile above the Earth’s surface, extending thousands of miles over the sea. They originate in the tropics and carry as much water as ten Mississippi Rivers. Weaker atmospheric rivers hit the California coast yearly, producing 30-50% of the state’s rain and snow in just 10 days each year. However, even these “minor” AR events are significant. They cause more than 80% of the flooding in California’s rivers and 80% of the levee breaks that occur in the Central Valley. 

According to climate models, global warming will increase the number of atmospheric rivers hitting California each year and they will carry more water than previous ones, increasing the intensity of megastorms. Some models predict that the odds of another megastorm like 1862 will at least double by 2100 due to the extra moisture caused by global warming. Swain says that if greenhouse gas emissions continue to increase at today’s rate, we could see a 30-127% increase in the number of people at risk from floods nationwide.

Even without the effects of climate change, the consequences of a megaflood today are much more serious than they were in 1862, when California had only 500,000 residents. Today there are hundreds of communities and large cities in the Central Valley, with a combined population of 6.5 million people. The Sacramento area, alone, is home to more than one million people, while Fresno has over 500,000 people, and Bakersfield has nearly 400,000 residents. The Central Valley includes the flood plains of two major rivers, the Sacramento and the San Joaquin, as well as many smaller rivers that drain down from the Sierra Nevada mountains.

This is not just a problem for Californians, either. Another flood like the one in 1862 would have a dire effect on the availability and cost of food for everyone in the U.S. The Central Valley comprises less than 1% of all U.S. farmland, yet it produces 25% of the nation’s food supply, including 90% of the broccoli, carrots, garlic, celery, grapes, tangerines, and plums, as well as 40% of the lettuce, cabbage, oranges, peaches and peppers, and over 20% of the milk. That is $46 billion worth of food annually, double the next most agriculturally productive state in the U.S.

A megaflood would also be an ecological nightmare. There are still lots of cows in California, nearly 4 million, to be precise. A massive flood would severely pollute the soil and groundwater with rotting carcasses and concentrated manure. Then there are all the other toxins in the region, like fertilizers and pesticides. In Kern County, alone, farmers use 30 million pounds of pesticides per year, while California, as a whole, uses over 200 million pounds of pesticides. Kern County is also one of the nation’s most prolific oil-producing regions, generating 70% of California’s oil and more than double what the state of Louisiana produces. It also has two large refineries. A major flood would pull much of these toxins into the soil and ground water and quickly spread them throughout the flooded regions, creating by far the biggest Superfund clean-up site in the nation’s history.

Kern River Oil Field; view from Panorama Park in Bakersfield, on the blufftop across the river. This is the most densely developed oilfield in California, and the fifth-largest producer (as of 2006) in the US. By Antandrus at English Wikipedia, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=16373121

In 2005, in the wake of Hurricane Katrina, the U.S. Army Corp of Engineers inspected California’s flood control infrastructure and found that many of its levies were built between 1850 and 1920, with outdated engineering criteria and only sporadic improvements since then. Sacramento, which is situated at the confluence of two major rivers, the Sacramento and the American, had levees designed for an 80-year flood. It was the most at-risk metropolitan area in the country at the time, with less than half the flood protection of New Orleans.

In 2009, the Multi Hazards Demonstration Project (MHDP) of the USGS examined California’s historical record for flooding, as well as its flood control infrastructure. From that data, they made a prediction of the likely effects of a 1-in-1,000-year storm event called ARkStorm (Atmospheric River 1,000 Storm). They found that an atmospheric river lasting just 23 days would cause more than $725 billion in damage to property, business and agriculture, three times the damage from a hypothetical super earthquake (i.e. the Big One). It would affect up to 25% of all California homes and disrupt power, water and sewage for months. 1.5 million people would have to evacuate. And it would likely lead to food shortages. (Keep in mind that our current weather pattern, which started on Dec 31, is projected to continue for at least another ten days).

California has not ignored this report. The state has spent over $2 billion on Central Valley flood control improvements since 2007, much of it on refortifying hundreds of miles of substandard levees. In 2018, Californians passed Proposition 68, which allocated $4 billion for parks and water projects. While the majority of the funding went to parks and recreational projects, $550 million was allocated for flood protection. Proposition 3, which would have allocated another $700 million to Central Valley flood protection and $300 million to dam renovations, was rejected by voters in 2018. Nevertheless, as a result of the investments that have gone toward flood protection, Sacramento is now a Class 2 FEMA community (2nd highest level of premium reductions by flood insurance companies). And Roseville, also in the Central Valley, is the first and only Class 1 FEMA community in the U.S.

Despite these improvements, California still needs an additional $80 billion in investment and three decade’s worth of work to bring the rest of the state up to today’s flood safety standards. A 2017 analysis found that 50% of the urban levees in the Central Valley were not up to current engineering standards. And most of the rural communities in the Central Valley still hadn’t received any funding to shore up their flood protections. The state’s dams are also in deplorable condition. On average, they are 70 years old. The ARkStorm report warned that spillways could fail in a megastorm. We saw a harbinger of this during the 2017 AR event, when the Oroville Dam’s spillway failed and 188,000 people had to be evacuated, some on only a few minutes notice. 2017 was Northern California’s wettest winter in 100 years and the flooding resulted in $1.5 billion in damage. Yet, it was still a “mild” winter compared with 1862.

Oroville Dam Spillway Failure. By Dale Kolke / California Department of Water Resources – California Department of Water Resources, Public Domain, https://commons.wikimedia.org/w/index.php?curid=56725440

There are twenty-six dams in California with extremely high downstream risk, meaning hundreds of thousands, or millions, of people could be affected by a failure of any one of them. History has shown us the consequences of not shoring up dams for severe storm events and maintaining them to modern engineering standards. In 1975, Typhoon Nina produced so much rain that the Banquio and Shimantan Dams failed, killing 240,000 people in China and making 11 million homeless. A similar event in India killed 5,000 people in 1979. And in 1928, the St. Francis Dam, just 40 miles north of downtown Los Angeles, failed because of engineering flaws, killing at least 431 people. (In 1928, the population of L.A. County was only 10-20% of what it is today).

Looking downstream in San Francisquito canyon after the St. Francis Dam collapsed. By SkyG702 – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=92935257

Though flood experts have known for decades about the threat of another megaflood, many regional flood managers, particularly in Los Angeles and Orange Counties, have argued that resources would be better spent on educating the public about the risks of living in a floodplain than on shoring up the state’s outdated dams and levees. But this isn’t like Covid, where you can educate people about the risks and they can easily go out and get free vaccinations and cheap masks if they want them. Many people who live in floodplains cannot afford to move or put their homes on stilts. Many are living there precisely because of the skyrocketing rents and mortgages in the less flood-prone areas of San Francisco, San Jose, San Diego or Los Angeles, or because their jobs are there. And educating them does nothing to protect the rest of us from the decimation of our food supply when the Central Valley becomes inundated, or from the ecological destruction that a megaflood would cause.

Southern California is particularly at risk. During the 1862 flood, the Santa Ana, Los Angeles and San Gabriel rivers merged, creating an inland sea in Los Angeles and Orange Counties. In his book, Ecology of Fear, Mike Davis talks about people being able to row from where the Los Angeles Civic Center is today to Newport Beach. Like the Central Valley, Los Angeles and Orange Counties were sparsely populated then. Now there are millions of people living in the floodplains of those two counties, all at significant risk in the next megaflood. Yet, for a moment in history, it looked like development might not go this way. According to Davis, there were several developers, like the Olmsted Brothers, who, in the 1920s and 1930s, wanted to turn the Los Angeles River and adjacent plains into a green beltway, specifically for flood control and public recreation. They even called for the parklands to extend into low-income neighborhoods to benefit working class families. They envisioned retaining the broad natural channels of the alluvial plain as a nature preserve and as a sponge for the next flood. Instead, Southern Pacific Railroad, which owned much of the land, began the process of developing the area into a private subdivision, to the cheers of the LA Times and large speculators.

The Flood of 1938 put a temporary damper on this speculative exuberance. Southern California got a year’s worth of rain in about two days, likely from an atmospheric river. Again, the Los Angeles, San Gabriel and Santa Ana Rivers merged, creating another inland sea. Up to 115 people died in this flood, while 5,600 homes were destroyed. In response, the U.S. Army Corps of Engineers and other agencies began converting the Los Angeles River into a paved channel that would flush future storms out to sea. Reassured, the speculators returned and the housing boom resumed.

Today, the Los Angeles River seems like a joke. Usually there is far more concrete visible than water, with trees growing up from the cracks in the center of it. Hollywood has cultivated its image as a useless piece of concrete that is only good for drag racing, gang fights and murders. Yet the next big flood will be no joke. The river was only designed for a 50-year flood. The Army Corps of Engineers found that a 100-year storm would inundate 1,000 acres of adjacent lands, up to 18-feet deep in some places, affecting 3,300 parcels of land, including parts of Griffith Park, Glendale and Burbank. And this is nothing compared with what an ARkStorm would do.

Like with pandemics, scientists have known for decades about the risks of the next megaflood and government agencies have had ample opportunities to prepare. Yet with pandemics, those agencies did very little. They let the public health infrastructure deteriorate for decades. They failed to maintain a sufficient stockpile of medical supplies and PPE. Hospitals didn’t have enough beds and staffing. When scientists were close to developing a coronavirus vaccine in 2016, they let the funding dry up. And we continue to destroy habitats at an alarming rate, increasing the risk of more zoonotic events and new pandemics.

While California has been doing something to prepare for the next megaflood, it has been far too little. Likewise with the federal government, which one would think might have an interest in protecting the nation’s food supply. Meanwhile, private insurers have stopped offering flood insurance to people living in many of the nation’s floodplains. And though the federal government has stepped in with the National Flood Insurance Program (NFIP), that program owes over $20 billion to the U.S. Treasury and is less than $10 billion away from its borrowing limit of $30 billion, which wouldn’t come close to covering the projected $725 billion in damages of an ARkStorm anyway.

We may not have thirty years before the next ARkStorm hits, particularly with climate change exacerbating the threat. But we could allocate the $80 billion California needs to bring its levees and dams up to today’s flood safety standards and start the work now. Maybe even make it part of a Green New Deal, like the original New Deal, which put 3 million people to work, over nine years, just in the Civilian Conservation Corps, with jobs in flood and erosion control, restoration of bridges and dams, fire prevention and forest restoration, as well as insect and disease control.

$80 billion to save lives and provide green jobs would be chump change compared with the $725 billion cost of staying the course.

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