Here's a peice of documentary info that's interesting if not explanatory on this topic. I didn't notice it while scanning previous posts but may have just missed earlier references, because it would be surprising not to have this on here yet. So maybe this is old news, but there is a very famous (at one time, anyway) Pulitzer prize winning nonfiction book about the western fur trade in the late 18th and early 19th century entitled Across the Wide Missouri by Bernard De Voto, first published 1949. De Voto was a highly noted scholar and author of many books on American History. The frontispiece of the book presents a three verse version of the song he apparently thought was itself entitled Across the Wide Missouri. So he actually named his most famous book after the song, and the song is posted without any attribution or explanation about authorship or history,etc., traditional or otherwise. Considering that this guy was a famous historian, writer and educator and unquestionably wanted to be careful with information, documenting stuff assiduously, it seems that 1n 1949 he certainly thought that these were the lyrics of a very old traditional song indigenous to his subject. He didn't say that though, and of course he could have been wrong about it anyway. Here are the first lines of each verse as he chronicled them:

Oh, Shennydore, I long to hear you;

'Tis seven long years since I first seed 'ee;

Oh, Shennydore, I love your daughter;