Region 4, Central Valley and Pacific Coast Ranges

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Abstract
Region 4 (Fig. 3; Table 2, Heath, this volume) includes two distinctly different geologically and hydrologically separated subregions—the Central Valley and Coast Ranges of California. The hydrogeology of both subregions is discussed together here because they evolved geologically from the same large-scale processes acting on the continental margin of western North America during middle Mesozoic through Cenozoic time. The area of Region 4, as defined in this paper, is bounded on the north by the Klamath Mountains and Cascade Range, on the east by the Sierra Nevada, on the south by the Transverse Ranges, and on the west by the Pacific Ocean (Plate 1). Total land area is about 160,000 km2; of that area the Central Valley, also known as the Great Valley of California, contains about 52,000 km2. The line of demarcation between the Central Valley and the surrounding mountains is the 150-m contour line. Generally, below the 150-m altitude the land surface has very low relief, the result of millions of years of fluvial deposition of sediments derived from the bordering mountain ranges. In contrast to the topographic monotony of the valley, the Coast Ranges rise abruptly from the Pacific Ocean to form a northwest-southeasttrending belt of alternating shallow alluvium-filled valleys and moderately dissected mountain ranges. Altitudes of range crests are moderate, generally less than 1500 m, but relief is great. Peaks as high as 1500 m are within 6 km of the coast line. The northern Coast Ranges are higher than the southern Coast Ranges; maximum altitudes are 2312 m in the north and 2083 m in the south.

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