Pennsylvania has 5,600 miles of dead waterways, polluted by centuries of coal mining. The Little Conemaugh River near Johnstown, Pennsylvania was one of them. Several abandoned mines discharged pollution that turned the river a bright orange, killing aquatic life. A new documentary, “A River Reborn” looks at how the river, which winds through the Allegheny Mountains, was brought back to life through a decade-long effort by a coalition of local groups.