The only info I know I have learned from "natives" who lived in the area at that time. I live in West County (which is about 20 minutes west of the Arch) and when people I know talk about the flood in '93, I am only familiar with the flooding of the Missouri River, although I believe the Mississippi flooded as well. One of the areas that flooded was the Chesterfield Valley. A lot of this area has been developed within the past few years. The development has been completely commercial -- part of it the Chesterfield Commons, a huge strip of shopping. Anyway -- the following articles talk about that a little bit and how they are sure that the flooding won't be a problem in that area again b/c of what they have done w/ the construction.
http://www.stltoday.com/stltoday/new...C?OpenDocument
http://www.thfrealty.com/community/s_stories/story2.html (broken link)
Hope this helps. If you are concerned about housing in these areas, I don't think you have anything to be worried about. The areas are mainly commercial -- Wal-mart, Target, Best Buy, and much much more.