There is geologic evidence showing that incredibly massive floods, called megafloods, have occurred in California every 100 to 200 years. These floods are caused by huge amounts of rainfall from “atmospheric rivers,” narrow bands of water vapor found approximately one mile above the surface of the Pacific Ocean, extending for thousands of kilometers.
In the waning days of 1861 and continuing into the early days of 1862, one such massive storm resulted in catastrophic flooding to the Central Valley of California. It rained non-stop for a total of 43 days, creating the only megaflood in the American West in recent history.
In the central part of the state, the storm initially dumped 10 to 15 feet of snow in the Sierra Nevadas, but soon after came 43 days of warm, heavy rain, which melted the gigantic snowpack in the mountains. The resulting slurry gushed and surged through the rivers of the Central Valley, raging torrents that took with them whole communities and many of the mining towns in the state’s Gold Country, leaving death and destruction in their wake.
And, scientists predict that it could happen again.
The Sea of Central California?
The strong atmospheric river that moved into the West at the end of 1861 to 1862 brought with it flooding from one torrential downpour after another, so intense that it transformed the Central Valley of California into a 300 mile long sea, extending from the mountains on one side of the valley to the Coastal Range on the other.
The damage from the massive flooding was catastrophic, destroying approximately one-quarter of the state’s taxable property and decimating its thriving economy. One in eight houses was destroyed or carried away in the flood waters. And, it’s estimated that 200,000 cattle drowned which represented a quarter of all the cattle in what was the “ranching state.” Eventually, the land that was once used to graze cattle would be converted to farmland.
According to a series of studies conducted by a team of researchers at UCLA’s Department of Atmospheric and Oceanic Studies, as the tropical Pacific Ocean and the layer of the atmosphere directly above it warm, it causes more seawater to evaporate, thereby feeding bigger and bigger atmospheric rivers surging toward the coast of California.
Scientists predict that an event like the Great Flood is more likely to happen every 65 years or so vs. every 100 to 200 years.
What if it happens again?
At the time of the Great Flood of 1862, California’s population totaled only 500,000, the Central Valley being sparsely populated. Today, that same valley has a population of about 6.5 million people and is home to the state’s three fastest-growing counties.
Over the past 150 years or so, the Central Valley has become California’s most productive region for agriculture and one of the most productive in the world, providing more than half the fruits, vegetables, and nuts grown in the U.S
Flooding in the Central Valley on the magnitude of that which occurred in 1862 could result in total destruction of a major contributor to both the US and the global food systems. The growing population in the Central Valley makes the potential loss of human life catastrophic.