The Yuma area has had a long and complex tectonic history. The most southwesterly corner of the area presently comprises a small segment of the Salton Trough, a deep sediment-filled structural depression. Known geothermal anomalies in the Salton Trough make the Yuma area a favorable exploration target even though spreading-center heat sources are not expected to occur there. Geological and geophysical investigations reveal that the area is made up of low, rugged northwest-trending mountains separated by deep sediment-filled basins. Relief is a result of both erosional and structural activity. Northwest-trending en-echelon faults bound the range fronts and the basins, and have created several horst blocks (basement highs) that crop out at or near the surface. The Algodonnes fault is inferred to represent the northeast margin of the Salton Trough and apparently an inactive extension of the San Andreas fault system. Extensive well-pumping and applications of irrigation waters in recent years have created an unnatural state of flux in the hydrologic regime in the Yuma area. Gravity and aeromagnetic anomalies trend strongly northwest through the region as do lineaments derived from Landsat and Skylab photos. Electrical resistivity values in the Bouse Formation are exceptionally low, about 3 ohn-m. Heat flow appears to be normal for the Basin and Range province. Ground-water temperatures indicate zones of rising warm water, with one such warm anomaly confirmed by sparse geothermal-gradient data.