I am actually quite convinced that what we are talking here is not a person or a being, and that the "I", "me", etc references in the riddle are a typical example and imitation of Tolkien's tendency to personify such things. In that sense it's almost a riddle within a riddle, but then don't all the best riddles contain misleading things of such nature?
We're looking for a river or other natural feature here, people. I will now guess that it's either one of two things: the
Enchanted River (Gulduin) in Mirkwood, or
The Paths of the Dead. There are parts of the riddle that I can't totally fit to them, however (specifically "you'll beat me" and "your friend is now lost"), but each one we rule out is one less to have to guess.
