‘Atmospheric river’ pattern threatens to flood California


Scientists say the state should brace for more devastating storms to come

As the Eastern Sierra braces for a big winter storm this weekend, no one here in snow country is complaining about the forecast.

Enjoy it while you can.

Caused by something meteorologists call an “atmospheric river,” warmer, wet storms are predicted to be come more common in the near future, giving the Sierra comparatively more of its moisture in storms that are dominated by rain, not snow.

That might be bad news for ski resorts, including Mammoth Mountain, but it’s even worse news for the rest of the state.

That’s because the state is past due for such storms—ones that could last weeks instead of days—according to two widely respected scientists who just pre-released an article in Scientific American magazine, (the full article is due out in January) called “The Coming Megafloods.”

“A 43-day storm that began in December 1861 put central and southern California underwater for up to six months, and it could happen again,” wrote authors Michael Dettinger and B. Lynn Ingram.

“Geologic evidence shows that truly massive floods, caused by rainfall alone, have occurred in California every 100 to 200 years. Such floods are likely caused by atmospheric rivers: narrow bands of water vapor (and are) responsible for most of the largest historical floods in many western states.”

The floods have been largely forgotten in the haze of time and poor record keeping, but Ingram’s and Dettinger’s interest was piqued after reading the historical records, so they began to look at the geologic record.

What they found, by studying sediment deposits across the state, is that such epic storms have happened disconcertingly often—every 100 to 200 years.

If another occurs, it will likely be far more devastating to this now highly populated and highly developed state compared to what it was in 1861, they write.

“California bore the brunt of the damage from the only ‘megafloods’ in recorded history,” they wrote. “This disaster turned enormous regions of the state into inland seas for months, and took thousands of human lives. The costs were devastating: one quarter of California’s economy was destroyed, forcing the state into bankruptcy. Today, the same regions that were submerged in 1861-62 are home to California’s fastest-growing cities.”

The article paints a chilling picture of the damage, stating, “sixty-six inches of rain fell in Los Angeles that year, more than four times the normal annual amount, causing rivers to surge over their banks, spreading muddy water for miles across the arid landscape. Large brown lakes formed on the normally dry plains between Los Angeles and the Pacific Ocean, even covering vast areas of the Mojave Desert. In and around Anaheim, flooding of the Santa Ana River created an inland sea four feet deep, stretching up to four miles from the river and lasting four weeks.”

It gets worse.

“Residents in Northern California, where most of the state’s 500,000 people lived, were contending with devastation and suffering of their own,” the article stated. “In early December, the Sierra Nevada experienced a series of cold arctic storms that dumped 10 to 15 feet of snow, and these were soon followed by warm atmospheric rivers storms.

“This enormous pulse of water from the rain flowed down the slopes and across the landscape, overwhelming streams and rivers, creating a huge inland sea in California’s enormous Central Valley—a region at least 300 miles long and 20 miles wide. The water reached depths up to 30 feet, completely submerging telegraph poles that had just been installed between San Francisco and New York, causing transportation and communications to completely break down over much of the state for a month.”

Similar to the storm that flooded the state the first week of December, but lasting weeks and not days, the damage from such a future event would be extreme.

Is this storm a harbinger of much worse storms to come?


One of the most agreed-upon premises for climate scientists is that such extreme weather events will get more frequent over time, the authors wrote.

“Today we have building codes for earthquake safety, but millions of new Westerners are not aware of the region’s calamitous climate history,” they wrote.

“Most have never even heard of the 1861–62 floods, and those may not have been the worst that nature can regularly dish out to the region.”