California Megafloods

California is a land of weather extremes.  Most of the California's precipitation falls from December through March, during the so-called "wet-season."  However, the precipitation that falls each wet season varies greatly, leading to precipitation extremes that include seasonal or multi-year droughts and pluvials (periods of increased rainfall). I hesitate to use the word "cycles" to describe the fluctuations between these precipitation extremes because they are not regular.  Instead, it is probably best to say that the precipitation climate of California is highly variable.

Only last year California was mired in extreme drought.  That has certainly changed this year with heavy precipitation and remarkable mountain snowpack.  However, as impressive as this past winter has been so far, the potential exists for even greater precipitation.  Geologic evidence suggests that massive flooding strikes California every 100-200 years (some studies suggest a longer return interval).  A nice article by Dr. Lynn Ingram discussing these Megafloods appeared in the Scientific American four years ago (available here).

Source: Scientific American

In the article, Dr. Ingram writes,

"The only megaflood to strike the American West in recent history occurred during the winter of 1861-62. California bore the brunt of the damage. This disaster turned enormous regions of the state into inland seas for months, and took thousands of human lives. The costs were devastating: one quarter of California’s economy was destroyed, forcing the state into bankruptcy. Today, the same regions that were submerged in 1861-62 are home to California’s fastest-growing cities. "

During that season, Los Angeles observed 66 inches of rain.  Much of the Central Valley was under water.  

Although one might hope that dams and levees would help today, recent studies suggest that such infrastructure would be overwhelmed by a several-week sequence of storms of the type observed during the 1861-62 winter.  A recent USGS report examining such a scenario pegs the damage estimates for California in the neighborhood of $0.5 to $1 trillion (see Overview of the ARkStorm Scenario).

For more on this, as well as extreme drought, see Dr. Ingram's excellent book, "The West Without Water," which we've reviewed in the past and I consider essential reading for anyone interested in the weather and climate of western North America.