From down at the Arch take the RiverFront trail north.
Cross over the Old Chain of Rocks Road (might want to check to see if it's open) to Illinois.
Go out to Hwy 3 and take the should north until you get to the Lewis and Clark Center near New Poag Rd. (The bike trail on the levee is still messed up from levee repairs AFAIK).
At the Lewis and Clark Center get on the bike trail north.
Cross the bridge in Alton back to Missouri.
Stay on Hwy 367 for a couple of miles until you get to Hwy 94 at West Alton and make a right turn.
Turn left onto Machens Road off of Hwy 94.
That gets you on the very end of the Katy Trail.