From its ancient headwaters on the semiarid plains of eastern New Mexico to its mouth at the Gulf of Mexico, the Brazos River carves a huge and paradoxical crescent through Texas geography and history. Its average flow is the largest of Texas rivers, but its floods, low flows, silt, and natural salt have often frustrated human desires. It is one of the most dammed of Texas rivers, but its lower four hundred miles constitute one of the longest undammed stretches of river in North America. In "Exploring the Brazos River," Jim Kimmel follows this long, changeable river from its rocky "arms" in...