After five years I thought I'd refresh this thread.
The origin of "Baby Did You Hear" is still a mystery. As Peace notes above, it's credited on Burl Ives' records as "traditional." That's not a safe assumption to make, though, for Ives himself was a songwriter in a small way and seems occasionally to have slipped one of his compositions in under a "traditional" credit. (A likely example is "Pueblo Girl.")
There's also the question where this river is. Somewhere, I no longer remember where, I got the impression the locale of the song was the St. Johns River, longest river in the state of Florida. That's certainly the one that comes up when you wiki it. There is, though, the St. John River that flows 410 miles from Maine into the Bay of Fundy. It's easy enough for a singer to tack an S onto the end of it.
So do any of you know which St. John's River is meant? I still think Florida but have no very good evidence for thinking so.