The Great Flood of 1862 was the largest flood in the recorded history of California, Oregon, and Nevada, inundating the western United States and portions of British Columbia and Mexico. It was preceded by weeks of continuous rains and snows that began in Oregon in November 1861 and continued into January 1862. This was followed by a record amount of rain from January 9–12, and contributed to a flood that extended from the Columbia River southward in western Oregon, and through California to San Diego, and extended as far inland as Idaho in the Washington Territory, Nevada and Utah in the Utah Territory, and Arizona in the western New Mexico Territory. The event dumped an equivalent of of water in California, in the form of rain and snow, over a period of 43 days. Immense snowfalls in the mountains of far western North America caused more flooding in Idaho, Arizona, New Mexico, as well as in Baja California and Sonora, Mexico the following spring and summer, as the snow melted.
The event was capped by a warm intense storm that melted the high snow load. The resulting snow-melt flooded valleys, inundated or swept away towns, mills, dams, flumes, houses, fences, and domestic animals, and ruined fields. It has been described as the worst disaster ever to strike California. The storms caused approximately $100 million (1861 USD) in damage, approximately equal to $3.117 billion (2021 USD). The governor, state legislature, and state employees were not paid for a year and a half. At least 4,000 people were estimated to have been killed in the floods in California, which was roughly 1% of the state population at the time.
Background
The weather pattern that caused this flood was not from an El Niño-type event, and from the existing Army and private weather records, it has been determined that the polar jet stream was to the north, as the Pacific Northwest experienced a mild rainy pattern for the first half of December 1861. In 2012, hydrologists and meteorologists concluded that the precipitation was likely caused by a series of atmospheric rivers that hit the Western United States along the entire West Coast, from Oregon to Southern California.
An atmospheric river is a wind-borne, deep layer of water vapor with origins in the tropics, extending from the surface to high altitudes, often above 10,000 feet, and concentrated into a relatively narrow band, typically about wide, usually running ahead of a frontal boundary, or merging into it. With the right dynamics in place to provide lift, an atmospheric river can produce astonishing amounts of precipitation, especially if it stalls over an area for any length of time.
The floods followed a 20-year-long drought. During November, prior to the flooding, Oregon had steady but heavier-than-normal rainfall, with heavier snow in the mountains. Researchers believe the jet stream had slipped south, accompanied by freezing conditions reported at Oregon stations by December 25. Heavy rainfall began falling in California as the longwave trough moved south over the state, remaining there until the end of January 1862, causing precipitation to fall everywhere in the state for nearly 40 days. Eventually, the trough moved even further south, causing snow to fall in the Central Valley and surrounding mountain ranges (15 feet of snow in the Sierra Nevada).
Impact by region
Oregon
There was an excessive amount of precipitation in November 1861 over most of Oregon, less so in the extreme northwest. It was cold enough at the higher elevations that much snow fell in the Cascade Range, which, when later melted by the warm rains produced a great quantity of water that flooded into the Willamette River and other streams in the Cascades. Tributaries of the Willamette originating in the Oregon Coast Range did not rise as high. A tropical depression that came in at the beginning of December produced strong, warm southerly winds in Oregon, with extremely heavy rain. Flooding was heaviest on rivers with tributaries arising from the snow-covered Cascade Range. The crest of the Willamette flood was reached at Salem on December 3; at Oregon City on the 4th; at Milwaukie, between Oregon City and Portland, on the 5th; at Albany on December 8. The crests at Albany and Salem were the highest ever known at any time. In Oregon, the flood was one of the largest in the recorded history of the Willamette Valley and the rest of Western Oregon.
An article in the December 14, 1861, Oregon City Argus, described the course of the flood at Oregon City: