The headwaters of this river come from forks leaking westward out of the Appalachians. They join in the foothills like estranged brothers and are then fed by countless and nameless tributaries dribbling from porous slabs of Kentucky limestone to form the Cumberland, which widens and deepens and continues snaking westward, twisting and falling through the rolling region and finally dropping down into Tennessee, where it spills through the concrete walls of the Cordell Hull Dam, the tailwaters bending around and meeting up with the Caney Fork, a cold, north-flowing river, in the town of Carthage, the county seat of Smith County, near where the father now waits for what’s his.  “Ain’t much opportunity in this world for boys like you,” he says. His youngest son is lying on the passenger-side bucket seat beside him, wearing a grocery bag for a diaper, listening.  They all slept here in the van last night beneath a line of hackberry trees. Now it’s near dawn and the sky has the color of a …