Need Help! Trying to identify a cappella song from the show

I'm trying to figure out a song I heard on A Prairie Home Companion a year or two ago.

It was an a cappella (sp?) piece, female singers, I think it was a trio. The episode was about the great lakes I think? The song was haunting and beautiful, no instrumentation, just vocals. If anyone can suggest what it might be, I would appreciate it greatly.

Thanks!!!