There's a fantastic map on the Des Moines Register's website. It's an aerial image (captured pre-flood) of the city overlayed with polygons for both the 100yr and 500yr flood plains.
It's an outstanding graphic and I'm a little jealous of it. SO cool.
The flooding, however, is so NOT cool.
Most scientists seem to agree that the damage done by these infrequent, periodic, massive floods is less than if there were more moderate and more frequent flooding.
I don't quite get it (not my area of study) but it kind of makes sense.