California residents love to complain about the weather. When my daughter went to college in Santa Cruz, she found that the least bit of drizzle would inspire a chorus of whining about the weather. Growing up in New England, she just didn’t get it.

Northern California residents may be more justified in their complaints this time. Over the past five days, two major storm systems have dumped more than 12 inches of rain in the Shasta Lake area and west slope of the Sierra Nevada mountains, and a third storm is on the way. There has been flooding of the Napa and Russian Rivers, and several thousand customers have lost power in Sacramento and San Francisco.

A report just published in Scientific American, though, suggests that there might be a whole lot more to worry about.

Historical flooding

Months of rain in California and the Pacific Northwest a century-and-a-half ago caused the worst flooding in U.S. history. Northern California rains began on Christmas Eve 1861 (they began earlier in Oregon and Washington) and didn’t let up for 43 days.

In San Francisco, 24 inches of rain fell in January, 1862 alone, following 8.6 inches in December, and followed by 7.5 inches in February. Sacramento was ten feet underwater, forcing the state legislature to move to San Francisco for six months.

The San Joaquin and Sacramento River valleys converged into a vast inland lake stretching up to 300 miles in length and averaging 20 miles wide in the Central Valley. In Los Angeles, 35 inches fell during this period.

California was a young state back then, but its fledgling economy was pummeled. The Sacramento Daily Union reported at the time that a third of all property in the state was lost, one-quarter of all cattle drowned, one out of every eight houses destroyed. Thousands of people are believed to have perished (though there are no reliable estimates of the exact number), and the state was bankrupted.

Given all this, I am astounded that I had never even heard of this flooding. It was only brought to my attention by a January 2013 Scientific American feature article, “Megastorms Could Drown Massive Portions of California,” that was pre-published online November 30th.

Atmospheric rivers

What causes such intense rainfall events, it now appears, are “atmospheric rivers” that carry vast amount of water vapor for thousands of miles. These mile-high rivers of moisture are described as being fairly narrow—just 250 miles wide—but may contain as much water as ten to fifteen Mississippi Rivers. When conditions are just right, these conveyor belts of moisture travel eastward until they hit the Sierras and drop their moisture as rainfall or snow.

Apparently, these atmospheric rivers were discovered only recently. In 1998, NOAA scientists outfitted a specialized aircraft to study an approaching winter storm in the North Pacific. They found that this single storm was carrying about 20% of the moisture moving poleward from middle latitudes. By being so high, its water vapor content had been hard to measure.

The same year, researchers at MIT determined that outside of the Tropics as much as 95% of all atmospheric moisture being transported poleward occurred in just five or six narrow bands of atmospheric flow, and they coined the term “atmospheric rivers.” Significant research has been conducted on this phenomenon since.

These atmospheric rivers are not all bad. In fact, they deliver much of California’s critical water resource today. Between 1950 and 2010, smaller atmospheric rivers delivered 30% to 50% of California’s water. They do this in very short spans of time—just ten days of rainfall events, o average, each year. But these events also account for 80% of flooding in California.

A history of megastorms

To some, the 1861-62 California flood may seem like a freak event, but the Scientific American article suggests that it is not. By analyzing sediment deposits off the California Coast, scientists have been able to determine that such megastorms occur about every 200 years.

While very thin layers of sediment are typically deposited from spring runoff each year, sediments as thick as two inches indicate massive floods. Such sediment cores collected off the coast of Santa Barbara indicate megafloods in A.D. 212, 440, 603, 1029, 1418, and 1605. Sediment evidence from the 1861-62 flood is difficult to discern because of the extensive sedimentation that resulted from the California Gold Rush around that time—when placer mining washed millions of tons of sediment into the Pacific Ocean.

Will global warming affect the atmospheric rivers?

Nobody really knows what effect climate change will have on atmospheric rivers. On the one hand, according to the article, warmer temperatures will result in more atmospheric moisture. On the other hand, rising temperatures, particularly in polar regions, may alter the global convection currents, affecting these atmospheric rivers. Winds over the Pacific are expected to lessen somewhat as a result of warming.

According to the Scientific American article, of seven leading climate models, six project that the moister air will win out over weaker winds, and the amount of precipitation delivered to California by atmospheric rivers will increase with climate change. A 10% increase in this precipitation is forecast by 2100.

If such flooding occurred today

When the 1861-62 megastorm hit California, the state population was only a half-million. Today, there are 6 million residents just in the Central Valley. A storm of the magnitude of the one 150 years ago would do vastly more damage.

To get a sense of the extent of damage, the U.S. Geological Survey (USGS) recently used a storm damage simulation program to predict damage from a storm like the one in 1861-62—only they modeled a storm lasting 23 days instead of 43 days. The results showed that 1.4 million residents would have to be evacuated and $400 billion in property damage and agricultural loses would result, with long-term business and employment interruptions eventually reaching $700 billion. These losses are roughly three times as high as would result, according to USGS simulations, from a magnitude 7.8 earthquake striking Southern California (the “Big One” that everyone has been talking about).

What this means for resilience

A comprehensive agenda of resilient design for buildings and communities in California will not protect us from a megaflood of such proportions. But it will help to protect people outside of the most severe flood zone who would be affected by the region-wide power outages, destruction of natural gas pipelines, loss of the Central Valley’s agricultural output, and breakdown of highway networks that carry everything from gasoline to the goods and services we depend on.

Let’s hope the rains in California taper off next week without serious damage. But at the same time, let’s do more to prepare for the massive flooding that could arrive next year or in the coming decades.

Along with founding the Resilient Design Institute in 2012, Alex is founder of BuildingGreen, Inc. and executive editor of Environmental Building News. To keep up with his latest articles and musings, you can sign up for his Twitter feed.