River's Edge - The River Immortals, #1

Eighteen-year-old Kasia Maier planned to spend her summer studying turtles in the sand prairie, not trudging through fearsome backwaters with her new coworker, Blayne. And when a monster drags Blayne into the Mississippi River, Kasia never expected herself to jump in after him. She saves him, but soon she's discovering the monstrous in other places .