Horns and Wrinkles — Joseph Helgerson

How can you tell if a river's under a spell? River trolls, rock trolls, blue-wing fairies -- the usual suspects. The stretch of the Mississippi where Claire lives has rumors of them all, not that she's ever spotted any. But then Claire's cousin Duke takes a swim and sprouts a horn -- a long, pointy, handsome thing. After that, Claire doesn't have much choice but to believe that something rivery is going on, especially since she's the only one who can help Duke lose his new addition.