The Yuma basin is located in the southwestern Basin and Range Province of Arizona adjacent to the San Andreas Transform boundary and the Gulf of California. The Colorado River marks the western boundary of the Yuma basin for the purposes of this study, although it has no subsurface significance (Plate 1). Similarly, the Arizona-Sonora border marks the southern boundary of the Yuma basin for the purposes of this study, but also has no subsurface significance. The southwestern part of the Yuma basin is, geologically, part of the Salton Trough (Plate 1). Along the flanks of the basin are the northwest-trending Chocolate Mountains in California to the northwest, Laguna and Gila Mountains to the north and northeast, and Tinajas Atlas Mountains t...