According to Scientific American
“The intense rainstorms sweeping in from the Pacific Ocean began to pound central California on Christmas Eve in 1861 and continued virtually unabated for 43 days. The deluges quickly transformed rivers running down from the Sierra Nevada mountains along the state’s eastern border into raging torrents that swept away entire communities and mining settlements. The rivers and rains poured into the state’s vast Central Valley, turning it into an inland sea 300 miles long and 20 miles wide. Thousands of people died, and one quarter of the state’s estimated 800,000 cattle drowned. Downtown Sacramento was submerged under 10 feet of brown water filled with debris from countless mudslides on the region’s steep slopes. California’s legislature, unable to function, moved to San Francisco until Sacramento dried out—six months later. By then, the state was bankrupt.”
This article from Berkley said
Atmospheric rivers occur when corridors of water vapor come up from the tropics, traveling across the Pacific Ocean for thousands of miles to the West Coast. An atmospheric river storm brings really heavy rain and it’s warm because it’s coming from the tropics, so that means it falls more as rain in the mountains (as opposed to snow), so you actually get more runoff filling the Central Valley. That’s why they cause more flooding than regular, colder storms. Most of our major floods in California correspond to these atmospheric river storms. But they’re really important because, on average, they provide 30 to 50 percent of our water resources in just 10 days a year.
Many of the articles blame climate change although this happened before most think anthropogenic emissions ramped up.