Two 500-Year Floods Within 15 Years: What are the Odds?

Heavy rainfall across the Midwest has caused major flooding, which has been described as a “500-year flood.” So what does that mean? This term is based on the annual likelihood of the degree of flooding. A 500-year flood magnitude has a 0.2 percent chance — or 1 in 500 — of being exceeded in any one year. A 100-year flood magnitude has a 1 percent chance — or 1 in 100 — of being exceeded in the same place, and so on. These statistics are not dependent on what happened last year, 15 years ago or even 100 years ago. Instead, they are calculated by taking annual peak flow values from the network of USGS streamgages and feeding that data into highly developed probability models. Streamgages provide long-term stream flow data that scientists need to better understand floods and define flood-prone areas. For more information about Midwest flooding, visit http://www.usgs.gov/homepage/science_features/flooding_june08.asp. To listen to an interview with USGS scientist Robert Holmes on this topic, visit http://www.usgs.gov/corecast/details.asp?ID=81. For more information, contact Heidi Koontz at 303-202-4763 or hkoontz@usgs.gov.