The River Mother???

[Morgoth the Great]

i have searched for this and nothing came up, so apologies if it has been posted about before. I was reading through the Fellowship of the Ring the other day and i got to Bombadil and Goldberry. It struck me as odd that she was the "River Daughter". I then today asked a friend if he knew who the "River mother" was, for if Tom is the lord of nature in his forest(and perhaps beyond) then surely Goldberry would be the river mother, not river daughter. i was wondering why Tolkien calls her the River Daughter, and if this is the case, who is the River Mother? is it just a way of saying she is directly sent from nature? that was my thoughts, but i thought i would check with the learned on here.