I still vote for a shanty that came up the river with keelboats, which would also explain DeVoto's mention.

History is never clear and concise, it is written by the winners, the writers and those who take the trouble. I doubt if a keelboatman would have bothered to ask where it came from, or even remember who he learned it from. And if he did know it is unlkely he would or could have written it down.

Richard