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tlOlS6~ Description of Area The Pacific Southwest (figure I) extends 800 miles north to south and 1,000 miles east to west. It includes all or practically all of California, Nevada, Arizona, and Utah; sections of Wyoming, Colorado, and New Mexico, and fairly small sections of Idaho and Oregon. It represents 18.2 percent of the land area of the coterminous United States. Of its 352 million acres, 57 percent is in Federal ownership, 8 percent is Indian trust land, 5 percent Is owned by the States, and 30 percent is in private individual and corporate ownership. The extensive role of Federal ownership and administration ofland in the four regions of the Pacific Southwest is shown in table I. PHYSIOGRAPHY AND CLIMATE Mountain ranges occur throughout the area, generally trending north~outh. The mountains pro. vide forest, forage, mineral, and recreational re- sources. They are particularly significant for their water runoff which is available for use in the valleys and plains below. The most significant mountains are those along the Pacific coast, the Sierra Nevada, the Wasatch Range and Uinta Mountains of northern Utah, and the various ranges of the Rocky Mountains that form the Continental Divide in western Wyoming, Colorado, and New Mexico. Numerous other ranges in the area between the Sierra Nevada and the Rocky Mountains are less massive and less influential. Most California watersheds drain to the Pacific Ocean. The Klamath, Eel, and Russian Rivers, and numerous smaller streams drain the west slopes of the Klamath Mountains and California Coast Ranges; the Saiinas River drains a valley lying between two segments of the Coast Ranges. The Sacramento-San Joaquin River system drains most of the Central Valley which lies between the California Coast Ranges and the Sierra Nevada, and discharges to the Pacific Ocean through San Francisco Bay. Although Goose Lake, on the California-Oregon border, has overflowed into the Sacramento River Basin in the past, it is now virtually contained in a closed basin. Tulare Basin, in the Central Valley south of the San Joaquin River Basin, is also a closed basin. The Owens River is the only major stream that flows from the eastern slopes of the Sierra Nevada. The Salton Sea, a terminal lake in southwestern' California's desert, is maintained chiefly by drainage from irrigation water diverted initially from the Colorado River. The physiographic Great Basin extends from the Sierra Nevada in California to the Wasatch Range in Utah, and from southeastern Oregon to Clark County in southern Nevada. (In the framework studies the California portion of it is included in the California Region; the Oregon portion is included in the Columbia.North Pacific Region.) The streams of the Great Basin Region end in permanent terminal lakes or playas (intermittent lakes). The largest such terminal lakes are Great Salt Lake in Utah, and Pyramid and Walker Lakes in Nevada. Large nonterminallakes include Lake Tahoe on the California-Nevada border, Utah Lake in Utah, and Bear Lake on the Utah-Idaho border. The principal streams include the Truckee, Carson, and Walker Rivers, all originating in California and drain- ing the eastern slopes of the Sierra Nevada; the Humboldt River in northern Nevada; the Bear, Weber, and Jordan Rivers in the Great Salt Lake drainage basin; and the Sevier River in southwestern Utah. The Upper and Lower Colorado Regions and a small part of the California Region, or nearly half of the Pacific Southwest area, are drained by the Colorado River which discharges into the Gulf of California. Several relatively small closed basins are included within the Upper and Lower Colorado Regions. The Upper Colorado Region also includes the Great Divide Basin in southwestern Wyoming, a closed basin lying between the Colorado River Basin and the Missouri River Basin. Because of the influence of the mountains, sharp contrasts in climate occur - in some places within a distance of a few miles. Latitude and topography are among the principal influences on climate. Along the coast of California, marine and topographic influences dominate. The marine influence, with its narrow range of tempera- ture, decreases rapidly with distance from the ocean. Consequently, most of the Pacific Southwest is characterized by the continental climate with a wide range of temperature. The growing season for culti- vated crops varies from 12 months in the low southern deserts and coastal plains to only 60 to 90 days in high mountain valleys. Most of the area's precipitation comes from frontal storms originating over the Pacific Ocean and 11