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A massive amount of rain has fallen in the region surrounding Boulder, Colo., causing widespread flooding that's killed at least four people and taken out roads and houses, according to news reports. The event has sent 20-foot "walls of water" rushing down mountainsides, destroying bridges and isolating entire towns, Boulder County Sheriff Joe Pelle said in a statement.

 The extreme rain and flooding in Colorado was caused when a slow-moving weather system sucked in an unusually large mass of moist air from the Gulf of Mexico, and has been called a "100-year storm." That terminology is a little confusing, though, and requires some explanation.

 100-year flood 

A 100-year flood or storm is an event that has a 1 percent chance of occurring in any given year, said Robert Kimbrough, a hydrologist at the U.S. Geological Survey (USGS) in Colorado. Provisional data from a gauge on Boulder Creek, which runs through the town of Boulder, suggests that the flash flood that occurred there had about a 1 percent chance of occurring — thus, a "100-year-flood."

 But the USGS doesn't use that term anymore. "The reason we got away from '100-year flood' terminology is because people mistakenly thought it's one that only happens every 100 years," Kimbrough told LiveScience. Instead, the new term is a "one percent annual exceedance probability flood," he said.

Though he acknowledged the term doesn't exactly roll off the tongue, it more accurately describes how the term is calculated, he said. [In Images: Extreme Weather Around the World]