With river rats young and old haunted by misadventures and illusory romance on the Arkansas side of the Mississippi River, the latest film from Jeff Nichols (Take Shelter) feels a little like a modern-day Huckleberry Finn. A kid named Ellis (Tye Sheridan) and his pal Neckbone (Jacob Lofland) go prowling for a boat wedged in the treetops of a wilderness island and cross paths there with a scraggly fugitive named Mud (Matthew McConaughey). Mud’s obsession with erratic dream-girl Juniper (Reese Witherspoon) brings tattered romance and other troubles into the action. Mud, a battle-scarred neighbor (Sam Shepard), and Ellis’ uncle (Michael Shannon) are all variously compromised alternatives to the kid’s parents who are in the process of breaking up. A vengeful patriarch (Joe Don Baker) from nearby eventually forces a climactic shoot-out, but the movie’s real interest resides in the oddly tender tragicomedy that emerges from the characters’ heedless low-rent dreaming. Pageant Theatre. Rated PG-13.