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. In the tradition of grand river adventures, Joseph Helgerson's tale is as twisty and unpredictable as the Mississippi River itself, while an unusual cast of characters adds pepper to the pot. Readers of all ages will enjoy getting in--and out of--trouble with Claire and Duke in this nimble, sharp, and funny fantasy.