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the expanding metropolitan centers of Los Angeles and San Diego. The climate of the coastal valleys, moderated by ocean breezes, has average monthly temperatures ranging between 50 and 75 degrees Fahrenheit. Annual precipitation in the area varies from about 14 to 17 inches. The growing season in this area is year-long. The Basin and Range province includes the great Mohave and Colorado Deserts which have hot, dry climates, with average monthly temperatures ranging from 30 to 85 degrees. Precipitation occurs mostly during the winter months, and varies from an annual average rainfall of about 20 inches at Mono Lake to about 21/2 inches at Brawley, near the Salton Sea. The frost-free period ranges from about 140 days at Bishop, in the Owens River Valley, to about 286 days at Indio, in the Coachella Valley. Lower Colorado River Basin--The Lower Colorado River Basin is by far the largest of the geographic areas in the Pacific Southwest, containing some 132,000 square miles of drainage area. The major tributaries of the Colorado River are the Gila, Bill Williams, Virgin, and the Little Colorado Rivers. The basin contains portions of two major physiographic divisions: the Basin and Range, and the Colorado Plateau provinces. Generally, southwestern Arizona, southern Nevada, and a portion of southwestern New Mexico lie within the Basin and Range province and have elevations which vary from 141 feet at Yuma to about 4,000 feet in Sulphur Springs Valley. Mountain peaks within the province extend to higher elevations. When irrigated, valley soils are highly productive. The major agricultural developments and the metropolitan centers of Phoenix and Tucson lie within this area. The scant annual rainfall varies from about 2.5 inches at Yuma to 7 inches at Phoenix and 10 inches at Tucson. Average monthly temperatures in this desert area vary from 50 to 90 degrees. Growing seasons of 300 days are not uncommon. The Colorado Plateau province covers the northeast portion of Arizona, southern Utah, and extends into western New Mexico. The plateau elevations range between 3,000 and 8,000 feet, and the surface is basically composed of volcanic and sedimentary rocks with only a thin soil mantle. This area is rather sparsely settled. Rainfall ranges from about 9 to over 30 inches per year. The temperature of the plateau area is relatively severe, with average monthly temperatures ranging from 30 to 70 degrees. The growing season varies from about 130 days to about 180 days, depending upon the location. Natural Resources The Pacific Southwest is rich in natural resources which have contributed importantly toward its development. In southern California, crude petroleum production is of major national importance. Arizona is I-2Epson Perfection 4870 Photo, 400 dpi, 8 bit, 14,157,824 byte