Megaflood
The notion of an all-consuming flood is by no means a novel one, as everyone who has ever read the Bible is aware. However, there has never before been a flood of that magnitude, as far as anyone can recollect. However, there is a risky potential in the future due to climate change.
Although earthquakes, droughts, and forest fires are what California is most famous for, floods is also a significant risk there. There is a very real possibility of a megaflood in the state, which could practically shut it down for months. In the past, California has seen devastating floods, although those times were during the Civil War. The state's infrastructure was much less developed at the time, and the destruction was not as severe. Today, transportation, sewage, and communication would all be lost. Flooding of this magnitude was estimated to cost $400 billion and relocate millions of people in 2013.