The Los Angeles Times reported this past weekend that the California Department of Water Resources is sounding the alarm about the possibility of a catastrophic flood in the Sacramento area:
A recent state report predicts that the right combination of unlucky weather conditions could put some parts of the city under more than 20 feet of water, causing a $25-billion disaster that would cripple state government and ripple through the California economy. ...
Worst-case scenarios project 500 dead, 102 square miles flooded, 300,000 people uprooted, an international airport and state agencies under water, and years of recovery.
This worst-case scenario isn't very likely, but it's well within the realm of possibility. Until the Bureau of Reclamation and the U.S. Army Corps of Engineers got involved about a century ago, the Central Valley ended up being a shallow inland sea for much of the year, and many of the newer suburban developments along the Sacramento River are exceptionally poorly situated.
What's more, as Glen Martin wrote last year in a rather disturbing hypothetical account of a disastrous northern California flood, global warming threatens to make things worse, since in a wet winter rain (as opposed to snow, which would be stored naturally in the Sierras and gradually released) could inundate the Sacramento and American Rivers and overwhelm the dams and levees. In 2006 voters approved a bond measure to substantially upgrade the state's flood control system, but it's still a work in progress.