The Waterfall
I caught this on HBO tonight and now I know why I never bothered to rent it.
Anyway, "Kate" is trying to recall all these sounds and voices, but doesn't remember jumping into the river at a waterfall? The police hunt up and down a calm and serene stream with riverbanks, and she doesn't think to tell them she made a decision to jump 100 feet off a sheer cliff into a noisy waterfall?
Morgan must have fired that shot through Milk of Amnesia.