I'm working on a project about folklore/the folk process as a sort of ecology - you know, evolution of songs, instruments diversifying into niches in different environments, that sort of thing - and I've forgotten all identifying details about one of my favorite pieces of evidence.

Once upon a time, I knew a murder ballad which was based, with reasonable certainty, in actual fact: a girl in Ireland, or Scotland, or Britain, drowned (probably by her true love) in a river. The original song specified the river by name (though not, I think, the girl). Later on, the song migrated to the US, where it is sung in various regional versions, most of which use the names of local rivers.

I'm sorry to say that that's all the detail I can dredge up from my memory, but perhaps the same intriguing song lineage has caught someone else's ear as it caught mine...?