ERinSTL from Saint Louis, MO on 8/17/2010 1:09:17 AM:
I can't give you a definitive answer because I haven't actually tried it, but I'm pretty sure the sanctioned Katy Trail ends at the historic St Charles riverfront.
Another ride you could try, though, is a two-ferries ride. Head for the Golden Eagle Ferry, which crosses the Mississippi River into Calhoun County, IL. Then put together some country roads to either the Grafton Ferry or the Brussels Ferry across the Illinois River. Calhoun County will throw some hills at you -- much more than you'd get on the Katy, of course -- but it's a really nice peaceful ride. And Grafton has some good restaurants if you need a meal stop before heading back. (There's also a restaurant at the Illinois side of the Golden Eagle Ferry, frequented mostly by boaters, but good for a simple meal.)
I haven't done this ride in years (literally), so you might want to get more current info from an area bike shop.