On a scorching May afternoon, Dell Dickinson was wearing creased jeans, a faded button-down, and a cowboy hat and boots. He kept a nine-millimeter Beretta in a holster on his hip. “I’m not trying to impress anyone with that,” he said, adding that he hasn’t hunted in nearly sixty years. “I don’t get any thrill out of killing something.” But sheep ranchers such as Dickinson sometimes have to deal with predators—black bears, bobcats, and coyotes. Right now he’s more concerned about another kind of interloper: river enthusiasts.Dickinson’s property, Skyline Ranch, sits on the Devils River, a turquoise ribbon of water that begins with a trickle from a spring dozens of miles north, in Sutton County, about 200 miles west of Austin, and flows 94 miles…