Rivers are built from just a few parts. While the sand and soil of a streambed is fluid, the framework — the shape of a river — is directed by roots and rocks. Time and the tenacity of flowing water changes the shape of the hardest rocks, eventually carving granite into a new form, eroding and molding a riverbank toward a new course. And while nothing is eternal in a river or its floodplain, there’s enough permanent structure in a stream — the immovable objects — that good trout take notice. So does the big fish hunter . . .