Though Northern California saw some intense rain over the last couple of weeks, bringing rainfall totals for the month of January well above average for many cities and towns, these numbers are still nowhere near the torrents of rain that fell on the West Coast between December 1861 and January 1862, creating what was called The Great Flood of 1862. And much like those that have been hitting California this winter, the cause of the Great Flood was a relentless series of atmospheric rivers that dumped 24.63 inches of rain in San Francisco in the month of January and 66 inches in Los Angeles that year (four times the average), and flooded Sacramento so severely that it was widely called "Lake Sacramento" leading to a raising of the entire city eight to ten feet so that it could remain the state capital.
The storms of December 1861 and January 1862 caused widespread flooding and damage from Canada down to Sonora, Mexico, and this was capped off with early snowmelt triggered by warmer temperatures in the Sierra and elsewhere.
The rain started in Washington and Oregon earlier, in November, with warm temperatures triggering quick melting of snowpack in early December leading to Oregon City, south of Portland on the Willamette River, flooding beyond recognition, and several nearby towns Linn City, Orleans, and Champoeg, the site of the first provisional government in the state being completely destroyed and never rebuilt.
For California, much like we've seen in recent weeks, it was not so much constant rainfall as high accumulation from several brief, distinct periods of heavy rain and this was not, in fact, an El Nino season either followed by the fast melting of accumulated snow. Most of the state's 500,000 inhabitants at that time lived in Northern California, which is where we have most of the records of the floods though one can imagine that the economic and human impact of such a flood would be exponentially greater today. At the time, one home in every eight was destroyed statewide.
Bridges were washed away in Gold Country in the Sierra foothills, and drowning deaths were recorded daily along the Yuba and Feather rivers, with "an entire settlement of Chinese miners" drowned along the Yuba River according to Scientific American in a 2013 piece about the floods.
And unlike the recent spate of rains, these atmospheric rivers hit the entire coast with equal vengeance over the course of 43 days. In less populated Anaheim, an inland sea was created by a flooded Santa Ana River that spread up to four miles wide in parts, was four feet deep, and lasted about a month before dissipating.
Another inland sea that was as deep as 30 feet in some places was created in the Central Valley according to Scientific American, submerging newly installed telegraph polls that connected San Francisco to New York, temporarily disrupting all transcontinental communication. The water submerged farmlands, drowned people and livestock, and washed away homes and whole towns.
In a letter dated January 31, 1862, William Brewer wrote to his brother on the East Coast:
Thousands of farms are entirely under water cattle starving and drowning. All the roads in the middle of the state are impassable... 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.
As the American River Watershed Project explained, Native Americans who'd lived for centuries in the region "knew the Sacramento Valley as an inland sea when the rains came," and their "storytellers told of water filling the valley from the Coast Range to the Sierra." But newer settlers in the decade after the Gold Rush had mostly only known drought conditions, and as Scientific American explains it, "In 1861, farmers and ranchers were praying for rain after two exceptionally dry decades." Unfortunately, "their prayers were answered with a vengeance, [with] a series of monstrous Pacific storms," and the volume of rainfall is the biggest recorded before or since.
The problem, though, is that such recordings weren't kept for the western United States much before the 1840s, which has led hydrology researchers like Dave Reynolds to have to argue the case for the Great Flood of 1862 being a 200-year flood event, since we really only have about 150 years of good records for California so far. At an "Extreme Precipitation Symposium" in 2012, Reynolds was recognized for his work in pushing for new structural standards for urban areas in the west based on such 200-year catastrophes, including stronger levees and the raising of dam heights.
Sacramento's levees had the effect of trapping floodwater in the city in January 1862, leading to the levees needing to be busted open. The capital of California had to be temporarily relocated to San Francisco on January 23, 1862 as a result of the flood.
Though nothing so severe has hit California in the 155 years since the Great Flood, there have periodically been similar atmospheric river events in December and January. In December 1955, a northern California storm brought huge amounts of rain in a 24-hour period, with Shasta County recording a record 15.34 inches in just one day, on December 20. On December 23, 1955, the Russian River reached a crest of 49.7 feet in Guerneville, the highest ever recorded there, and a broken levee along the Feather River on Christmas Eve flooded Yuba City, drowning 37 people.
Meanwhile, a bit more rain is headed to NorCal the middle of this week, but it won't be of the same severity as what we saw the last two weeks.
The point here, though, is that such events can always happen again, and too many years of drought conditions can leave us surprised when the big rains return. As Monterey-based National Weather Service forecaster Bob Benjamin tells the Chronicle, "I think people haven’t experienced this type of weather in so long that it’s appearing like a total anomaly I’m not sure it’s quite the case. What we are seeing is closer to normal than we’ve experienced the last five to six years."