What is a 100-year flood?

The term "100-year flood" is often used as an attempt to simplify the definition of a flood that statistically has a 1-percent chance of occurring in any given year. Likewise, the term "100-year storm" is used to define a rainfall event that statistically has the same 1-percent chance of occurring. In other words, during the course of 1 million years, these events would be expected to occur 10,000 times. Just because it rained 10 inches in one day last year doesn't mean it can’t rain 10 inches in one day again this year. (USGS)

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1. What is a 100-year flood?
2. What is flood stage?
3. What is river stage?
4. What is a flood advisory?
5. What is a flood watch?
6. What is a flood warning?
7. What is a flash flood warning?