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