Well, are आप surprised about that? I must say that I thought that River was an old Jenny, या a regenerated Susan या simply, the Doctor's wife, but, when I noticed that River is Amy's baby it was something like "WTF?"
Exactly! If आप think about it, 'River' is a body of water like a POND. And 'Song' is also a MELODY. Melody Pond is her name. :) I learned so much from a TARDIS Database website that I found.
