If you were walking along the bank, the Mississipi would accompany you going North instead of South, if you were following the Drinking Gourd. "Carry" wouldn't have to mean you were floating.
But I reckon there might well be some other meanings around in the background of the song, to do with the spirit world. Was this song collected in many places and many times? Or are we going back to one version? Are there any other songs with similar imagery, either in the Black American traditions, or elsewhere, for example in the Caribbean or West Africa?
I think it's time for some heavy duty folklore imput, like we'd get on the Mudcat if this was a question about an Irish song or a Child ballad.