The Lomax Folk Songs of North America is the only place where I've seen the protagonist identified as a cowboy rather than a soldier. I missed it the first time I looked as it's under the title "Wild Rippling Waters," very non-standard it seems to me. (ANd I didn't have a printout of this thread with me when I looked, this is the first time I've managed to access Mudcat today).

I looked in what I thought was Lomax Cowboy Songs but my copy is a different book (John Lomax: Cowboy Songs and Other Frontier Ballads) not even referred to in the op cit Lomax bibliography. I suspect the one referred to was John & Alan's revision of John's earlier book.

Belden has several versions (all with "soldier"), and Sandburg has a couple. I don't know the Judy Collins tune, but if it's roughly the same as the Lomax, I'd bet your riding boots that's the source.