The Pecos River Basin alluvial aquifer is an aquifer in western Texas. "Thick and extensive alluvial deposits of Cenozoic age compose the Pecos River Basin alluvial aquifer in western Texas. The aquifer is in the Great Plains Physiographic Province and underlies approximately 5,000 square miles in parts of Andrews, Crane, Ector, Loving, Pecos, Reeves, Ward, and Winkler Counties. The topography in the area consists mostly of flat to rolling plains that slope gently toward the Pecos River. Ground water in the Cenozoic alluvium is of major importance in this area where average annual rainfall is less than 12 inches."