Worse Than the Big One—California’s Looming Megaflood

Sacramento, January 1862, USGS

Intense rainstorms began to pound central California on Christmas Eve, 1861, and continued nonstop for 43 days, converting the rivers that flow from the Sierra Nevada into torrents that flooded the Central Valley. Entire communities were swept away. Sacramento was under water for the next six months, forcing the state government to be relocated to San Francisco. The Central Valley became an inland sea that was 300 miles long and 20 miles wide. Thousands of people and hundreds of thousands of cows were killed. The state was bankrupted by the disaster. (From Scientific American, “Megastorms Could Drown Portions of California.”)

Sounds cataclysmic and it was, but new evidence indicates that storms like this have occurred roughly every 200 years in California, many far more intense than the one in 1861. A megaflood in 1605 is estimated to have been 50% worse than any of the others, including the one in 1861. These megastorms are caused by Atmospheric Rivers, 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 10 Mississippi Rivers. While the megastorms occur relatively infrequently, weaker atmospheric rivers hit the California coast yearly and, for the past 50 years, have produced 30-50% of the state’s rain and snow in just 10 days each year.

Hypothetical Flooding From Megastorm, USGS

According to Scientific American, climate models suggest that global warming will increase the number of atmospheric rivers hitting California each year and they will carry more water than previous ones, increasing the frequency and intensity of megastorms. At the same time, the state’s population is far higher than it was in 1861. Hundreds of communities and large cities now exist in the Central Valley, with a combined population of 6 million people. The Sacramento area alone is home to more than 1 million people, while Fresno has over 500,000 people. The Central Valley is also one of the most agriculturally productive regions in the world, producing about $20 billion in crops annually. A model projecting the effects of an atmospheric river lasting only 23 days found there would be more than $700 billion in damage to property, business and agriculture. It would also likely lead to food shortages.

Atmospheric rivers also bring devastation to other west coasts and even some inland regions. Nashville’s flooding in 2010 (30 deaths, $2 billion in damage) was due to an atmospheric river. There was substantial damage to England and Spain from atmospheric rivers in 2009. Last month, Wales and England experienced their worst flooding in 50 years. Other regions susceptible to megaflooding from atmospheric rivers include Chile, Namibia and Western Australia.

Ironically, while Californians are hypersensitive to the risks of earthquakes and the state regularly prepares contingencies and practices emergency procedures for earthquakes, virtually nothing is being done to protect the state from megafloods, which would be roughly three times more costly than a 7.8 magnitude earthquake hitting the state’s populous south. The state’s levees, for example, are in deplorable condition and could be damaged by much milder storms, risking the inundation of croplands with sea water. At the same time, while earthquakes cannot be predicted, an atmospheric river can be seen coming days in advance and NOAA is now monitoring them very closely.