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A2 REGIONAL AQUIFER-SYSTEM ANALYSIS-CENTRAL VALLEY, CALIFORNIA tion of preliminary reports on water quality by Fogelman (1982a, 1982b, 1983); water use by Williamson (1981) and Diamond and Williamson (1983); hydrogeology by Page (1981, 1983), French, Page, and Bertoldi (1982, 1983), Page and Bertoldi (1983), French, Page, Bertoldi, and Fogelman (1983), and Berkstresser and others (1985); and ground-water hydraulics, regional flow, and aquifer mechanics by Williamson and Prudic (1986) and Prudic and Williamson (1986). In addition, Nady and Larragueta (1983a, 1983b) and Mullen and Nady (1985) described streamflow and irrigation development. PURPOSE AND SCOPE The purpose of Professional Paper 1401 is to describe major aspects of the geology, hydrology, and geochem- istry of the Central Valley aquifer system. These descrip- tions are derived largely from the study results and preliminary reports of the 5-year study; however, they also utilize the extensive hydrologic literature on the California Central Valley (see references cited in chap- ters A-D). Professional Paper 1401 consists of the following chap- ters: Chapter A (this report) summarizes the important aspects of the geologic framework, regional ground- water flow, effects of development, and ground-water quality in the Central Valley. Chapter B (Hull, 1984) describes the geochemistry of ground water in the Sacramento Valley. Chapter C (Page, 1986) describes the geologic frame- work of the Central Valley, with emphasis on textural changes in the alluvial deposits that constitute the aquifer system. Chapter D (Williamson and others, 1989) discusses ground-water hydraulics, with emphasis on an analysis of regional ground-water flow prior to and after extensive ground-water withdrawals. This regional analysis is based on computer simulation and presents a new, somewhat different concept of the aquifer system. BASIN ENVIRONMENT The Central Valley of California, viewed from the air or on a shaded relief map (fig. 1A), stands out as a notable topographic basin. It is about 400 mi long and averages about 50 mi in width. Surrounded on all sides by mountain ranges, the valley has only one natural outlet through which surface water drains. That outlet, the Carquinez Strait, cuts through the central Coast Ranges (fig. 1A) on the west boundary of the valley. In this study, the boundary of the Central Valley represents the areal extent of the valley's aquifer system rather than a physiographic boundary. The aquifer system's boundary is defined as coincident with the topographically highest occurrence of alluvial fan or alluvial plain deposits (allu- vial boundary) of Pleistocene or Holocene age. In the northern part of the valley, discordant with the uniform flatness of the landscape, is the only notable topographic feature, Sutter Buttes (figs. 1A and 2). There, north and south Buttes, remnants of an ancient volcanic plug, rise to altitudes of 1,860 and 2,130 ft above sea level, respectively. The Central Valley is composed of parts of four hydrographic subregions or drainage basins named for the major natural surface-water feature in each subre- gion (fig. I A). Sacramento Valley, the northernmost third of the Central Valley, has an area of about 4,400 mi2 and is drained by its namesake, the Sacramento River. Of the four hydrographic subregions, the Sacramento Valley is the least intensively developed. San Joaquin Valley, the southern two-thirds of the Central Valley, is made up of parts of two subregions: the San Joaquin Basin and, at the southern end, a basin of interior drainage called the Tulare Basin after a Pleistocene lake that occupied most of the area. The fourth hydrographic subregion is the Delta, a low-lying area that drains directly to the Sacramento-San Joaquin Delta rather than to either river (fig. 1A). The lower part of the Delta subregion consists of wetlands interspersed with hundreds of miles of channels and numerous islands. Climate in the Central Valley is the Mediterranean type (Blair and Fite, 1957, p. 323). Average annual precipitation ranges from 13 to 26 in. in the Sacramento Valley and from 5 to 16 in. in the San Joaquin Valley. About 85 percent of the annual precipitation occurs from November to April. Summers are hot; winters are mild, allowing a long growing season. In contrast to the low precipitation in the valley, mean annual precipitation in the adjacent Sierra Nevada increases with altitude and ranges from 40 to more than 90 in. (Rantz, 1969). Much of the precipitation in the mountains is snow, especially in the higher southern Sierra Nevada. Variations in the volume of snowpack and delays in its melting produce differences in the timing of runoff in the two valleys. Peak runoff into the Sacramento Valley generally lags peak precipitation in the surrounding mountains by 1 to 2 months whereas peak runoff in the San Joaquin Valley generally lags peak precipitation by 5 to 6 months (fig. 3). Streamflow, a very important factor in the water supply of the Central Valley, is almost entirely depend- ent on precipitation in the Sierra Nevada and part of the Klamath Mountains in the north (fig. 1A). No perennial streams of any significant size enter the valley from the west, except for Stony Creek, Cache Creek, and Putah Creek at the northwest end of the valley (fig. IB). Mean annual streamflow entering the Central Valley around its perimeter is 31.7 million acre-ft.