The term "500-year flood" has been used to describe the recent
flooding in the Midwest. Midwesterners who experienced the Great
Flood of 1993 – said to be a 500-year flood at the time – can
hardly be faulted for thinking they were off the hook for seeing
that designation again for, say, a few hundred years. "The term
‘500-year flood' can be a little misleading," said Robert
Holmes, USGS National Flood Specialist. "We hydrologists realize
the term has instant public recognition and we use it to point
to the extraordinary nature of such floods. However, the
occurrence of a 500-year flood doesn't depend on what happened
last year or fifteen years ago or 100 years ago. It's based on
the annual likelihood of the degree of flooding - in other
words, the odds. A 500-year flood magnitude has a 0.2 percent -
1 in 500 - of being exceeded in any one year. A 100-year flood
magnitude has a 1 percent chance - 1 in 100 - of being exceeded
in the same place, and so on. To come up with these statistics,
we take our annual pea