USGS Multimedia Gallery: (Audio)--We had a "100-year flood" two years in a row. How can that be?

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Welcome to CoreFacts, where we're always short on time, but big on science. I'm Steve Sobieszczyk. Let's get right to it, today's question is:

We had a "100-year flood" two years in a row. How can that be?

The term "100-year flood," is used to describe the recurrence interval of floods. The "100-year recurrence interval" means that a flood of that magnitude has a one percent chance of occurring in any given year. In other words, the chances that a river will flow as high as the 100-year flood stage this year is 1 in 100. Statistically, each year begins with the same 1-percent chance that a 100-year event will occur. But, just because a 100-year flood happened last year doesn't mean that it won't happen this year, too. In other words, future rainfall and floods don't depend on the rainfall and floods that happened in the past year. The past records are mainly used to show what kind of river flows can be expected. So, when you hear about a 100-year flood, at least you have a general idea that it does mean a BIG flood, and if you hear of a 200-year flood, you know that it means one even BIGGER!

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