Missouri was indeed a large producer of cotton in those days, not only in
eastern Missouri, but in the western regions.  Weston, Missouri, for
example, situated on the Missouri River between Kansas City and St. Joseph,
was one of the largest tobacco markets west of the Mississippi, situated
where it was relatively easy to ship cured tobacco north to St. Jo, Omaha
and Sioux Falls or south to Kansas City, Jefferson City and St. Louis.
Eastern Arkansas, mostly flat, alluvial plain, was, then as now, a
cotton-producing region, and Clemens, who obviously was quite familiar with
the topography of the river, and would have undoubtedly picked up cotton to
be delivered to the large, spot cotton market in Memphis, certainly knew
this.  The stone wharfs where the cotton was unloaded are still intact along
the river in downtown Memphis, from which you can see the flat plain, often
flooded, of West Memphis, Arkansas.
Martin Zehr