What is a "500 year flood", and how do scientists determine if a flood is considered a "500 year flood"?

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Welcome to CoreFacts, where we're always short on time and big on science. I'm Danielle Eganhouse. Today's question is about flooding. 

What is a "500 year flood", and how do scientists determine if a flood is considered a "500 year flood"?

The term "500-year flood" refers to a potentially imprecise statistical estimate of a flood flow that has a 1 in 500 chance, or in other words a .2 percent chance, of being exceeded in any one year. To come up with these statistics, scientists use streamgages to measure annual peak flow values over many years and feed the annual peak flow data into highly developed probability models.  These streamgages also provide long-term stream flow data that scientists need to better understand floods and define flood-prone areas.

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