In December 1861 and January 1862, rain and snow (delivered via “atmospheric rivers“) inundated California and the Western United States. As nearly every river in the West rose beyond its banks, the deluge swept away buildings, roads and bridges. In those days many cities, industries and transportation were centered along waterways. The damage was immense: 1 in 3 properties were destroyed (including 1 in 8 homes) and the California economy contracted by 25%. In short, the Great Flood of 1862 was the worst disaster in the history of California.

The Central Valley of California was hit particularly hard, due to its low elevation and many rivers going from the Sierra Nevada Mountains towards the ocean. For a time, the valley became an inland sea and the only way to get around safely was via boat. In fact, that is how Governor Leland_Stanford went to his inauguration in Sacramento (when he returned to his mansion, he entered via the second story). Some areas were covered by 30 feet of water, which meant that newly-installed telegraph poles were submerged and non-functional.

In a few places entire towns were wiped out. Buildings in Mokelumne City were either carried away downstream or toppled by landslides. This city, which grew in prominence during the gold rush by offering a year-round deep water port, never recovered and was ultimately abandoned. (1)

It is hard to grasp how quickly the situation changed – within days the entire landscape was transformed. I suppose the same can be said for the incredible dry-lightning-induced wildfires experienced in California in August 2020. Life did not return to normal for months. For people who lost loved ones or saw their homes and/or dreams swept away, normalcy never returned.

These types of weather events can and have occurred every 100 to 200 years, according to geologic evidence. (2) Familiar with this pattern, Native Americans near Marysville, CA reportedly warned white settlers and moved to higher ground in the foothills.

A note on rain in California: in many parts of California we don’t get “real” rain. We get around 300 days of sun per year. When it does rain it is often just a short sprinkle, which would not even register as significant to someone from elsewhere. When it rains for the first time in a while, people here complain and forget how to drive. But occasionally more serious rain does come our way …

An “atmospheric river” is a narrow corridor of moisture in the atmosphere. As the name indicates, it is essentially a river’s worth of water flowing in clouds. So-called “Pineapple Express” atmospheric river storms come from the Hawaiian Tropics. Note that these differ from an El Niño or La Niña type event which can also send large amounts of moisture to the US Pacific Coast. These types of storms originate farther out in the Pacific Ocean and are triggered by changes in water temperature and air pressure on the ocean’s surface. Another difference is that El Niño and La Niña events have a much wider regional impact within the Pacific rim.

References:

1. “I Love You California” podcast episode #29 on Mokelumne City (8 minutes)
2. “California Megaflood: Lessons from a Forgotten Catastrophe” Scientific American (2013)