River hears the growl come from a little ways over and she pauses, her feet sliding in the wet grass. She looks over in the direction and notices two young women a few yards away. Werewolves. She recognizes them from one of the packs from around here. She hurries on, not wanting to be involved in a pack war. She hurries down farther a little ways and notices the gleaming silver surface of the lake a few yards away. She smiles and hurries toward it, shedding off her dress as she runs. She sets the dress down by the edge of the lake and walks to the edge of it. She takes a deep breath and runs into the water, as far as she can. 

The transformation is immediately. She has yet to be in the water a full minute when she sees the glittering, silvery-blue tail that is now her own. Her legs melted together to form this thing. She smiles and dives under the water, splashing and kicking around. She practices swimming for a little bit before practicing her magic, making it move the water of the lake.