I can't attest to its origins (though WE certainly didn't write it), but my band sings an additional verse to Dillon Bustin's Shawneetown that fits the information Sandy provided above about keelboats vs. flatboats. This verse should come just before the reprise of the first one:

Keelboat boys call loud and long
They're round as a barrel but twice as strong
Hard on the beech oar, she moves too slow,
Way down to Shawneetown on the Ohio.