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requirements with Mexico, and for developing power head for the production of electrical energy at Davis Powerplant. The river flows through a natural channel for about 10 miles below Davis Dam at which point the river enters the broad Mohave Valley 33 miles above the upper end of Lake Havasu. N Ull en ro Lake Havasu backs up behind Parker Dam for about 45 miles and serves as a forebay from which the Metropolitan Water District of Southern California pumps water into the Colorado River Aqueduct. Lake Havasu also serves as a forebay for the Central Arizona Project Havasu Pumping Plant and pumps water into the Hayden Rhodes Aqueduct. Alamo Dam and Reservoir, on the Bill Williams River, is used to control floods originating above and below Alamo Dam. Headgate Rock Dam, Palo Verde Diversion Dam, and Imperial Dam all serve as diversion structures with very little storage. Imperial Dam, located some 150 miles downstream from Parker Dam, is the major diversion structure for irrigation projects in the Imperial Valley and Yuma areas. It diverts water on the west 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 east 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 and recreation. The Morelos Dam, located just below the Northern International Boundary with Mexico, is the last dam on the Colorado River, This small diversion dam diverts water into the Alamo Canal, which delivers water to northern Mexico. 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 Colorado River 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 clays with 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 appreciable soluble mineral content due to residual and secondary mineral formation from the parent material. Upon weathering or irrigation, salts may accumulate on or near the surface due to evaporation or consumptive use by plants. 8