its highly unlikely.
think how long it would take a body to decompose in a river, in my mind, i think it would have to be in there a fucking long time without anyone noticing a body decomposing. then again, there are the factors that maybe someone melted all the flesh off with acid and dumped the bones in the river. OR after floating down a stream for a very long time it could have sunk and been forgotten about. i would love to believe its a skull in your fucking waters but.....i just cant say yeah on this one.