Sorcha--thanks for both of those links, one a fascinating essay on witches in Scotland and the other a great Steve Miller tune I had forgotten about!

I was wondering if the original poster had perhaps been referring to "She is like the Swallow" which contains the lyric "She is like the river that never runs dry..."

?? Not sure since the original poster did not come back to confirm that your links fit the bill...but I wanted to thank you for them cuz they were fascinating!