The title, "The Willow Tree," causes confusion with the sad ballad of parting and a reunion that came too late which is found in Sam Henry.

Probably covered in another thread, but among the variants in Child, "The English and Scottish Popular Ballads," variant G from the UK, wherein the rogue had drowned a knight's six daughters, covers about the same story as "The King's Seven Daughters." The ballad came to North America without much change.
Most American collections have one to several variants. J. H. Cox, Folk-Songs of the South," has nine variants collected in West Virginia.