Real estate agents, inspectors and even local appraisers that have been around the area for sometime are also going to (personally) know which areas are prone to flooding- which there are small pockets that have in the past, and may be vulnerable in the future. I think they are the best place to research! =) FYI- Past flooding can (and most likely will) be made evident during the disclosure, and later the professional inspection processes in a real estate transaction. Reading the local paper in the days after the flood(s) could also give you some ideas of where there were problems, too.
Generally speaking most St. Helens homes are in the clear. It is notable that the "in-town" areas of St. Helens are well known to be home to a significant layer of rock. I have been advised it is specifically of the blue basalt-type, extremely dense and some of the hardest rock there is in the world! So it goes without saying that because it doesn't absorb anything, flooding can and has occured in lower isolated areas. Currently, the city of St. Helens is actively working to upgrade the sewer system, in part to help avoid these problems in the future.