The physiography of the region (fig. 8) is a product of the geologic
history of the area. Several coastal mountain ranges underlain by
severely folded, faulted, commonly metamorphosed marine and continental
sediments, form the Pacific Border and the Lower Californian Physiographic
Provinces. In the interior, the granitic rocks that underlie the fault
blocks of the Sierra Nevada and the volcanic rocks of the southern Cascade
Mountains join to form the eastern border of the low-lying California
Trough, which contains the Central Valley. East of the Sierra Nevada,
the land is characterized by a series of low, north-south-trending mountain
ranges and intervening valleys (fig. 9); the ranges and valleys were created
by faulting that resulted in the horst and graben structures that form the
Basin and Range Physiographic Province. In the extreme northeastern part
of Nevada, the southernmost extent of the Columbia Plateaus Physiographic
Province is formed by basalt lava flows.
Land use in Segment 1 is directly related to topography and the availability of water. Major land uses in California and Nevada are shown in figure 10. The flat floor of the Central Valley of California, one of the Nation's most important agricultural areas, is used almost entirely for growing crops. Most of the cropland, however, must be irrigated. The mountains that surround the Central Valley are areas of rugged topography and, accordingly, are used predominantly as forest and woodland, even though they receive large amounts of precipitation. Almost all of Nevada and large parts of southern California receive little precipitation; accordingly, most of the land in these areas is desert shrubland (compare figs. 5 and 10), although sufficient water is available to allow livestock to be grazed in some places.
The major cities in the coastal areas of California appear as large areas of urban sprawl on figure 10. Although coastal California receives moderate to large amounts of precipitation, surface-water and ground-water supplies in those urban areas are not sufficient to provide the water needs of the population. As a result, a huge network of reservoirs, canals, and aqueducts has been constructed in California to transport water to these urban areas and other areas of water deficit.
