Great Flood of 1862

The Great Flood of 1862 was the largest flood in the recorded history of Oregon, Nevada, and California, occurring from December 1861 to January 1862. It was preceded by weeks of continuous rains and snows in the very high elevations 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. Immense snowfalls in the mountains of the far western United States caused more flooding in Idaho, Arizona, New Mexico, 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.

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 weather experts 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 narrow band of water vapor about a mile above sea level and about 400 to 600 kilometres (250 to 370 mi) wide.

...
Wikipedia