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PART II DESCRIPTION OF BASIN e Bill Williams River, are used to control floods originating below Davis Dam and above Parker Dam. Headgate Rock Dam, Palo Verde Diversion Dam, and Imperial Dam all serve as diversion structures with practically no storage. Imperial Dam, located some 150 miles downstream from Parker Dam, is the major diversion structure to irrigation projects in the Imperial Valley and Yuma areas. It diverts water on the right bank to the All American Canal, which delivers water to the Yuma Project in Arizona and California and Imperial and Coachella Valleys in California. It diverts on the left bank to the Gila Gravity Main Canal. The Senator Wash Dam, an offstream storage facility, also affords regulation in the vicinity of Imperial Dam and assists in the delivery of water to Mexico. This facility is used for pumpback storage, power generation, and recreation. The Morelos Dam is located just below the Northern International Boundary with Mexico and is the last dam on the Colorado River. This small diversion dam diverts water into the Alamo Canal which delivers water to northern Mexico. e D. Geology and Soils The geology of the Colorado River Basin is highly varied. Igneous, metamorphic, and sedimentary rock types are present and range in age from approximately 625 million years old to recent alluvial deposits. Structural features, including anticlines, domes, and faults contribute to both the topographic relief and the geohydrology of .the region. Several of the sedimentary formations in the basin were deposited in marine or brackish water environments. Occurrences of bedded and disseminated sodium chloride (halite) and calcium sulfate (gypsum) are observed, as are c lays wi th high contents of exchangeable sodium and magnesium. The soils of the Colorado River Basin closely resemble the geologic formations from which they were derived. Residual soils derived from shale or sandstone are generally shallow. These soils can contain appr~ciable soluble mineral content due to residuum and secondary mineral formation from the parent material. Upon weathering or irriga- tion, salts may accumulate on or near the surface due to evaporation or consumptive use by plants. e Soils derived from alluvial materials vary in composition and thickness. The deposits vary in origin and range from alluvial fans and terraces to outwash plains and lake sediments. Some soils are com- posed of material transported short distances. Soils that have been transported longer distances are well mixed with respect to texture and composition. 10