Steve, you might want to look at Harriet Frazier's book: Runaway and Freed
Missouri Slaves and Those Who Helped Them, 1763 - 1865 and my own book:
Searching for Jim, Slavery in Sam Clemens's World.  They will give you some
notion of what slaves who ran away in Missouri actually did.
    Southern Illinois was a hotbed of pro-slavery sentiment as evidenced by
the county results on the black code issues that came before the voters and
the thicket of slave-catchers.  You might want to review Illinois history as
well. (I can't put my hands on the book I want to refer you to on slavery
and Illinois. I'll contact you when it comes to me.) Cairo would never have
been a destination for a slave from Northern Missouri unless he was hired
onto on a steamboat and Cairo was his northern-most and only opportunity.
The Ohio and the Mississippi were heavily patrolled. To use a cold-war
analogy, escaping by going south on the Mississippi and going all the way
down to Cairo and then up the Ohio would have been the same as escaping from
the northern part of East Germany by getting right next to the fence and
walking to Czechoslovakia before attempting to cross to the west.  You would
have to go by several hundred guard towers instead of just one.  Sheer
folly.  There were some slaves who floated by raft down the Missouri and to
Illinois, but it happened late during the Civil War when the slave patrol
system had gone completely to pot and eastern troops frequently aided
runaways.
    I have been studying original slave material on Missouri now for seven
years and have not encountered a single case where anyone ever escaped by
such a method.  It is also inconsistent with the methods that we do know
about.  Could someone do it?  Of course.  Would someone do it? Not in the
real world.
    How is that a criticism of Huck Finn?  It is a novel.
    Terrell Dempsey