While talking about the flooding in the Midwest today, Matt Lauer brought up the point that the flooding in 1993 was a "100-year flood event" and then expressed surprise that floods of that magnitude were happening a mere 15 years later. What he's missing is the fact that a 100-year flood is not necessarily a flood that occurs every 100 years, but a flood that has a 1% chance of occuring in any given year.