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 about a mile above the ocean that extend for thousands of kilometers.
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California Megaflood: Lessons from a Forgotten Catastrophe
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California Megaflood: Lessons from a Forgotten Catastrophe
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
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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 about a mile above the ocean that extend for thousands of kilometers.
The atmospheric river storms featured in a January 2013 article in Scientific Americanthat I co-wrote with Michael Dettinger, The Coming Megafloods, are responsible for most of the largest historical floods in many western states. The only megaflood to strike the American West in recent history occurred during the winter of 1861-62. California bore the brunt of the damage. 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. Although this flood is all but forgotten, important lessons from this catastrophe can be learned. Much of the insight can be gleaned from harrowing accounts in diary entries, letters and newspaper articles, as well as the book Up and Down California in 1860-1864, written by William Brewer, who surveyed the new state’s natural resources with state geologist Josiah Whitney.
In 1861, farmers and ranchers were praying for rain after two exceptionally dry decades. In December their prayers were answered with a vengeance, as a series of monstrous Pacific storms slammed—one after another—into the West coast of North America, from Mexico to Canada. The storms produced the most violent flooding residents had ever seen, before or since.
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.
Residents in northern California, where most of the state’s 500,000 people lived, were contending with devastation and suffering of their own. 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. The series of warm storms swelled the rivers in the Sierra Nevada range so that they became raging torrents, sweeping away entire communities and mining settlements in the foothills—California’s famous “Gold Country.” A January 15, 1862, report from the Nelson Point Correspondence described the scene: “On Friday last, we were visited by the most destructive and devastating flood that has ever been the lot of ‘white’ men to see in this part of the country. Feather River reached the height of 9 feet more than was ever known by the ‘oldest inhabitant,’ carrying away bridges, camps, stores, saloon, restaurant, and much real-estate.” Drowning deaths occurred every day on the Feather, Yuba and American rivers. In one tragic account, an entire settlement of Chinese miners was drowned by floods on the Yuba River.
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. Water covered farmlands and towns, drowning people, horses and cattle, and washing away houses, buildings, barns, fences and bridges. 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. William Brewer wrote a series of letters to his brother on the east coast describing the surreal scenes of tragedy that he witnessed during his travels in the region that winter and spring. In a description dated January 31, 1862, Brewer wrote:
Thousands of farms are entirely under water—cattle starving and drowning. All the roads in the middle of the state are impassable; so all mails are cut off. The telegraph also does not work clear through. In the Sacramento Valley for some distance the tops of the poles are under water. The entire valley was a lake extending from the mountains on one side to the coast range hills on the other. Steamers ran back over the ranches fourteen miles from the river, carrying stock, etc, to the hills. Nearly every house and farm over this immense region is gone. America has never before seen such desolation by flood as this has been, and seldom has the Old World seen the like.
Brewer describes a great sheet of brown rippling water extending from the Coast Range to the Sierra Nevada. One-quarter of the state’s estimated 800,000 cattle drowned in the flood, marking the beginning of the end of the cattle-based ranchero society in California. One-third of the state’s property was destroyed, and one home in eight was destroyed completely or carried away by the floodwaters.
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