After Anna destroys the dam with the help of Lt. Mattias and the rock giants, she is saddened because she believes that Elsa and Olaf are dead and that Arendelle is utterly destroyed. Can you imagine her surprise when Elsa appears riding out from the river on a water horse? This reminds me of that scene in The Lord of the Rings: The Two Towers when Aragorn, Legolas, and Gimli confron who they think is Saruman, only to find out that it is Gandalf who has come back as Gandalf the White at the turning of the tide.