The resulting snow-melt flooded valleys, inundated or swept away towns, mills, dams, flumes, houses, fences, and domestic animals, and ruined fields.

From the existing Army and private weather records, it has been determined that the polar jet stream was to the north because 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.

[6] 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 400 to 600 kilometres (250 to 370 mi) wide, usually running ahead of a frontal boundary, or merging into it.

[7][8] 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.

Loss of so much wheat flour and the new demand coming since 1860 from the recently opened Idaho gold fields caused a spike in its price from $7 to $12 per barrel.

In Northern California, it snowed heavily during the later part of November and the first few days of December, until the temperature rose unusually high and it began to rain.

[17] The water flooding the Central Valley reached depths up to 30 feet (9.1 m), completely submerging telegraph poles that had just been installed between San Francisco and New York.

[17] The rainy season commenced on the 8th of November, and for four weeks, with scarcely any intermission, the rain continued to fall very gently in San Francisco, but in heavy showers in the interior.

According to the statement of a Grass Valley paper, nine inches of rain fell there in thirty-six hours on the 7th and 8th inst.... the next day the river-beds were full almost to the hilltops.

[10]In Knights Ferry, in the foothills of the Sierra Nevada astride the Stanislaus River, about 40 miles (64 km) east of Modesto, the town's homes, its mill, and most of its businesses were ruined by the flood.

The New York Times reported on January 21, 1862, that a trapper who had spent more than 20 years in California had frequently boated over the city's site, and in 1846, the water at the location was 7 feet (2.1 m) deep for sixty days.

[17] By 1861, the Sacramento flood plain had quickly become inhabited by a growing population during the Gold Rush, and had begun to serve as the central hub for Valley commerce and trade and as the home of the California State Legislature.

Thousands of farms are entirely under water—cattle starving and drowning.From December to January 1862 the series of storms carrying high winds and heavy precipitation left city streets and sidewalks underwater.

In our trip up the river we met property of every description floating down—dead horses and cattle, sheep, hogs, houses, haystacks, household furniture, and everything imaginable was on its way for the ocean.

[10]Dozens of wood houses, some two stories high, were simply lifted up and carried off by the flood, as was "all the firewood, most of the fences and sheds, all the poultry, cats, rats and many of the cows and horses".

[17] A chain gang was sent to break open the levee, which, when it finally broke, allowed the waters to rush out of the city center and lowered the level of the flooding by 5–6 feet (1.5–1.8 m).

Due to the high costs associated with flood recovery, the city of Sacramento reached out to the aid of the Transcontinental Railroad Co., which was a major turning point in levee resilience and reconstruction.

Ruins of the old city remain underneath the streets as tunnels leading nowhere, with hollow sidewalks, filled in entrances, trap doors, and rubble where storefronts and walkways used to be.

Large wooden beams and soil brought in from surrounding areas helped to stabilize and build a foundation on top of the once-flooded city.

[35]The plains of Los Angeles County, at the time a marshy area with many small lakes and several meandering streams from the mountains, were extensively flooded, and much of the agricultural development that lay along the rivers was ruined.

Government surveys at the time indicated that a solid expanse of water covered the area from Signal Hill to Huntington Beach, a distance of approximately 18 miles (29 km).

[38] In San Bernardino County, all the fertile riverside fields and all but the church and one house of the New Mexican colony of Agua Mansa were swept away by the Santa Ana River, which overflowed its banks.

[37] To the north, in the Owens Valley, similar snow and flooding conditions as those to the east in Aurora, Nevada (see below) led to the local Paiute suffering the loss of much of the game they depended on.

The riverside home of steamboat entrepreneur George Alonzo Johnson and the nearby Hooper residence were the only places in the town unharmed because they were built on high ground.

[52] The spring and summer thaw of the immense snowpack during the winter of 1861–1862 in the southern Rocky Mountains and other ranges flooded the Rio Grande, and changed the river's course in the Mesilla Valley.

On July 8, 1862, Lt. Col. Edward E. Eyre, First California Volunteer Cavalry wrote: The Rio Grande has been unusually high this summer, almost the entire bottom between Fort Craig and Mesilla being still overflowed.

[54] This made it an ideal anchorage for maritime craft to load and unload their cargo and passengers from the steamboats that took them up and down river without the danger from the tides that they were having to risk in the estuary at Robinson's Landing.

In the month of March 1865, the schooner Isabel, from San Francisco, commanded by W. H. Pierson, found and entered this slough and discharged her cargo there for the first time.

Subsequently, the steamers, sailing ships and later ocean-going steamships loaded and off-loaded their cargoes there, and the steamboat company established Port Isabel 2.5 miles (4.0 km) above the mouth of the slough.