It's probably tacky to be the best contributor to one's own thread, but...

I'm beginning to believe that Mary Robbins may be a pseudonym. Don't have any verification of that, but it sounds like a good theory.

And, when I actually do the research on the song that I should have done to start with, I find through the Ballad Index that "Wild Rippling Water" is in Alan Lomax's, Folk Songs of North America, and when I look there I find "Mary Robbins" didn't really alter it. (And there Lomax - Cowboy Songs is cited, with the version as a "Lomax adaptation from the Gant Family, Austin, Texas.")