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002532 CHAPTER I THE PACIFIC SOUTHWEST Southern California and the Lower Colorado River Basin depend either partially Or totally upon the flows of the Colorado River and its tributaries for their water supplies. This report, therefore, treats these adjoining geographic areas, having common water-supply problems and needs, as a single region termed "the Pacific Southwest." For the purposes of this report, the Pacific Southwest is defined as the Colorado River drainage basin, downstream from Lee Ferry to the Mexican border, plus the southern portion of California, bounded on the north by the Santa Barbara County line and the Tehachapi Mountains~ and on the west by the Sierra Nevada Mountains northward to, and includin~~ Mono County. It includes southern California, most of the State of Arizona, the southern portions of Nevada and Utah, and a portion of western New Mexico. Within this region Bre the metropolitan centers of Los Angeles~ San Diego, Phoenix, Tucson, and Las Vegas, which are among the fastest growing cities of the United States. The area has become one of the most dynamic sectors of the growing national economy. The Pacific Southwest also includes the highly productive agricultural areas of the Imperial, Coachella, Palo Verde, and coastal valleys of southern California, the fertile irrigated desert valleys of central Arizona, and the rich agricul- tural lands along the lower reaches of the Colorado River. All of these Bress are major centers for the production of winter vegetables and specialty crops which are shipped throughout the United States. It is the most critically water-short region of the Nation, and its future growth is dependent on the provision of an adequate water supply. General Description The Pacific Southwest is a land of contrast. It includes Mount Whitney and Death Valley; brown deserts and green forests; and record heat and severe cold. The area, as defined in this report, contains about 190,000 square miles and includes portions of three major physiographic divisions: the Coastal Ranges, the Colorado Plateaus, and the Basin and Range provinces. Southern California--Southern California is a region of extreme topographic and climatic diversity, including areas within both the Coastal Ranges and Basin and Range provinces. Within the Coastal Ranges province, with its equable year-round climate, have developed 1-1