Latest Study Highlights Likelihood of a California Megaflood

by | Aug 23, 2022 | News & Commentary, Thought Leadership, Uncategorized

A new study from Xingying Huang of UCLA and Daniel Swain of the NOAA has made headlines across the nation by forecasting the chances of an historic ‘Megaflood’ that could devastate California’s environment and economy. This study is a striking call to action for California’s public officials and citizens to protect ourselves from the effects of a storm cycle that, thanks to climate change, we will likely face in this century.

The new study builds on decades of work that has helped us understand California’s history with floods. The largest and most destructive of these was the Great Flood of 1861-2, the product of several storm cycles that dumped enough water to create an inland sea stretching across San Joaquin and Sacramento counties. In fact, storms of this nature have typically occurred every 100 to 200 years and are marked by long, consecutive stretches of days with rain. In 2010 the State of California created models of what a Great Flood would look like in modern times. The results were grim – more than $1 trillion in economic losses, and millions of Californians displaced from their homes.

Californians have long heard forecasts like this about the next large earthquake, but not about floods. After all, California has experienced more than ten years of historic drought, a different but equally threatening trend that has prompted more frequent and devastating deadly wildfires across the state each year. The new study’s authors acknowledge that California is ‘more accustomed to water scarcity than overabundance in the modern era,’ but use historical data to demonstrate that drought is not a permanent condition but that periods of persistent, elevated rainfall are inevitable.

When the rains do come, they will emerge from air that is, on average, warmer than ever before. The warming of our climate impacts both the likelihood of a megaflood and its destructive potential. The authors suggest that our warmer climate has at least doubled the risk of a megaflood, with one expected to occur every fifty years (as opposed to 100 or 200 years in previous studies). The primary danger is posed by atmospheric rivers – long ribbons of moisture that travel from the Pacific Ocean into the state and release water, often at high elevations. Thanks to our warmer climate, these rivers hold more water than ever before; thanks to shorter winters, that water will likely fall to the earth as rain, not snow.

The authors conclude that previous estimates of megaflood rainfall and damage totals are inadequate and should be increased by 200% to 400%. This will place further stress on California’s flood infrastructure, which is already an area of concern. Heavy rainfall in 2017 damaged the Oroville Dam – California’s largest – and prompted the evacuation of nearly 200,000 people. While the Oroville Dam did not fail and has since been repaired and reinforced, a megaflood of the kind identified in this study would surely test it and our state’s 1,500 other dams and spillways.

Californians need only look east to see the human cost of flooding and our inability to contain it. The devastation in Eastern Kentucky, St. Louis and elsewhere is heartbreaking and an important reminder of our need to prepare as best we can for a California megaflood. Stories from the rest of the nation suggest several areas for improvement. First, it is critical that planning officials limit development in areas subject to severe, recurrent flooding; Federal flood reform legislation offered by the Biden Administration takes a step in this direction. Second, we need to work to get relief to displaced persons as quickly as possible, through both NGO and official government channels. Third, we need to get serious about flood insurance.

In the ten counties in eastern Kentucky that suffered the most intense flooding, only 2.3% of property owners have government flood insurance offered through the National Flood Insurance Program. Private flood insurance policies, which can potentially provide broader, more affordable coverage than the NFIP, are another coverage option that not enough consumers purchase or are even aware of. As a result, thousands of Kentucky families will have to rebuild without the financial assistance of a flood policy to repair their homes and replace their possessions.

California can and must do better to educate citizens about their options for protection as they digest this new study and make appropriate preparations and reforms. For more information about flood insurance solutions from Palomar, please visit

Bill Bold

Bill Bold

Bill Bold is the Public Affairs Advisor at Palomar where he is responsible for policy, legal and regulatory affairs. He brings over 30 years of experience in public policy and its relationships with government entities at the international, federal, state and local levels.
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