The river is followed by a variety of other plagues (frogs, locusts, lice, boils to name but 4) and in response, the town starts freaking out and starts to look for someone to blame. They focus on a little girl (AnnaSophia Robb, the bratty Violet Beauregard in the Johnny Depp version of Charlie and the Chocolate Factory) and Swank spends as much time talking people down as she does on the problem.
Swank's usually good value and she was good in this film. She probably helped turn this film from something that was a little derivative (I could see elements of Rosemary's Baby, The Wicker Man and (more recently) Skeleton Key to name but three) into something pretty good.
If you like Swank generally or want to be frightened a little bit on a Friday or Saturday night, you might like this film. It was certainly the sort of film that teenage girls found scary. Three of them were regularly jumping out of their wits in the cinema I was in.