Disney Frozen Anna, Elsa, And The Secret River Book

There is a mysterious white river, one that holds the answers to the past. But it's just in a lullaby that Anna and Elsa's mother sings to them. Or is the white river actually real? Anna and Elsa leave the comfort of their bed to look for it in the forest. It could hold the answers to why Elsa has magic--that is, if it exists at all . . .