Forget eathquakes; we're overdue for a flood

There were terrible floods in 1861 and 1862 in Los Angeles.
Dr. Lucy Jones is quoted in the Los Angeles Times: "Just trying to describe the extent of the damage is overwhelming. Yet 150 years later most Californians are unaware that it ever happened."
That's true. I'm a Californian and I was unaware of it.
I did know about the terrible flooding up in Sacramento in those years. The Gold Rush Era, the theater with its wooden benches awash in water. The new governor (Leland Stanford) being rowed to his inauguration. But I did not know that it hit Los Angeles or Southern California.

"In Los Angeles, the water was described as extending from mountain to mountain, with no dry land between the Palos Verdes Peninsula and the San Gabriel Mountains." That's from Dr. Jones. The article, "California's Flooding 'Nightmare'," appeared Sunday, March 25, 2018.
This picture of Aliso Street east of Los Angeles Street is dated c. 1860. I have not found any pictures of the flooding in Los Angeles in 1861-1862, only the graphic below right.

We've had other floods: 1938 was a terrible year, and 1964 also saw bad flooding. NBC has put up pictures of those floods, and they are fascinating. But not apocalyptic. 1861-1862 was apocalyptic. 66 inches of rain in 45 days.
Los Angeles Magazine reports such monster floods can hit every 100 to 200 years. The phrase "not a questions of if, but when" applies.
If you google with city names, you find tidbits about the flood.
For Long Beach, for example: The floods of 1862 raged through a dense area of willow trees bringing many of them down to the area that would become Long Beach. A new growth of willow trees prompted locals to call the area “Willowville.”
I also found this paragraph in an attachment to a 2013 draft report: "the mouth of the Los Angeles River shifted from Venice to Wilmington. The plains of Los Angeles County were extensively flooded and formed a large lake system where the stronger currents cut new channels to the sea. The Los Angeles, San Gabriel, and Santa Ana rivers converged, forming a solid expanse of water from Signal Hill to Huntington Beach. Runoff transformed much of what is now Orange County into an inland sea that was 4 feet deep in places 4 miles from the Santa Ana River."
Yikes.
How do we anticipate these disasters and what can we do about them? The USGS has a report called ARkStorm. (The first three letters stand for Atmospheric River 1,000.)  Among the possible mega-flood scenarios is this: what could happen if the San Gabriel and Los Angeles rivers filled and spread out, putting areas from West Covina down to Long Beach under water? Or if Orange County got swamped by coastal flooding?
ARkStorm outlines potential scenarios and loss, and is not light reading. I have only glanced at it but do not see much to help me sleep better at night. We are not prepared for such a disaster because, frankly, there probably is no way to be prepared for such a disaster. But as Harvey et al, and before that, Katrina demonstrated, such things do happen.
As for what causes the weather that causes such floods, here's a new vocabulary phrase (to me, anyway): atmospheric rivers. They are what you must be picturing: "narrow bands of water vapor about a mile above the ocean that extend for thousands of kilometers."
Five years ago, Scientific American published an in-depth article describing how atmospheric rivers could produce mega floods, and pointing out that our state is just as susceptible to these as the Midwest. Here's a good summary of it.
The definition of atmospheric rivers is from that summary, by B. Lynn Ingram. It describes the damage done from December 1861 through spring of 1862, not jut in California but throughout the west. Utah, Nevada, Arizona, up to Washington state.
As for Dr. Jones, whose quotes opened this post, she has a new book coming out April 17: The Big Ones: How Natural Disasters Have Shaped Us (And What We Can Do About Them)