California Water Science Center
Study Unit Description
The San Joaquin-Tulare Basins NAWQA Study Unit is located in central California and includes the San Joaquin Valley, the eastern slope of the Coast Ranges and the western slope of the Sierra Nevada. The Sierra Nevada is predominantly forested land, and the Coast Ranges and the foothills of the Sierra Nevada are predominantly rangeland. Most of the valley floor is used for agriculture. Changes to water quality are primarily due to the large amount of irrigated agriculture, which affects the quality of both surface and ground water in the valley. The total population in the study unit is approximately 3.5 million. Major population centers include Fresno (441,200), Bakersfield (254,400), Stockton (251,100 in the city), and Modesto (194,4000).
The study unit can be separated into the San Joaquin Basin to the north and the Tulare Basin to the south. Altitude ranges from near sea level in the San Joaquin Valley to a maximum altitude of 14,195 feet above sea level at Mount Whitney, the highest point in the conterminous United States. The San Joaquin Valley is a monotonously flat structural basin bounded by the Sierra Nevada to the east, the Coast Ranges to the west, the Tehachapi Mountains to the south, and the Sacramento-San Joaquin Delta to the north. The crests of the Coast Ranges are about 3,000 to 5,000 feet, the Tehachapi Mountains, about 5,000 to 8,00 feet, and the Sierra Nevada, about 8,000 to 14,000 feet. Land-surface altitude of the valley rises from near sea level to about 1,000 feet at the top of the dissected older alluvium in the southeast.
The San Joaquin and Tulare Basins constitute the southern two-thirds of the Central Valley of California, which is part of a large, northwest-trending, asymmetric structural trough, filled with marine and continental sediments up to 6 miles thick. The bedrock geology of the areas adjacent to the east and west sides of the San Joaquin Valley contrasts sharply. To the east of the valley, the Sierra Nevada is composed primarily of pre-Tertiary granitic rocks and is separated from the valley by a foothill belt of Mesozoic and Paleozoic marine rocks and Mesozoic metavolcanic rocks along the northern one-third of the boundary. The Coast Ranges west of the valley have a core of Franciscan assemblage of late Jurassic to late Cretaceous or Paleocene age and Mesozoic ultramafic rocks. These rocks are overlain by marine and continental sediments of Cretaceous to Quaternary age and some Tertiary volcanic rocks.
The San Joaquin Valley has an arid-to-semiarid climate that is characterized by hot summers and mild winters. The eastern slopes of the Coast Ranges and the valley are in the rain shadow of the Coast Ranges. Warm, moist air masses from the Pacific Ocean are forced aloft by the Sierra Nevada. The air masses cool and the moisture condenses, resulting in heavy precipitation on the western slopes. This precipitation, occurring both as rainfall and snow, is the major source of water entering the basin. Mean annual precipitation on the valley floor ranges from less than 5 inches in the south to 15 inches in the north. Average annual precipitation in the Sierra Nevada, mostly in the form of snow, ranges from about 20 inches in the lower foothills to more than 80 inches at some higher altitude sites. Precipitation in the Coast Ranges varies from less than 10 inches to more than 20 inches. As in the valley, precipitation in the Sierra Nevada and Coast Ranges increases from south to north.
The study unit includes parts of five U.S. Environmental Protection Agency ecoregions, which are based on perceived patterns of combined causal and integrative factors including land-surface forms, potential natural vegetation, land use, and soils. The major parts of the study unit are in the Sierra Nevada, Southern and Central California Plains and Hills, and Central California Valley ecoregions. The Sierra Nevada ecoregion is characterized by high mountains of mixed conifer forest, red fir forest, and lodgepole pine/subalpine forest. Land use is forest (logging) and woodland grazing, and soils are classified as Ultisols. The Southern and Central California Plains and Hills ecoregion is characterized by low mountains, tablelands of moderate to considerable relief, and irregular plains. Natural vegetation is oak woodlands, chaparral, and California steppe. Land use is open woodland grazing, and the soils are light-colored of subhumid regions. Finally, the Central Valley ecoregion of California is characterized by flat plains. The natural vegetation of California steppe and tule marshes mostly has been replaced by irrigated agriculture, other crop-land, and grazing land. Soils are recent alluvial soils, light-colored soils of the wet and dry subhumid regions.
The San Joaquin River receives water from tributaries draining the Sierra Nevada and Coast Ranges, and except for streams discharging directly to the Sacramento-San Joaquin Delta is the only surface-water outlet from the study unit. The water quality of the San Joaquin River is of critical interest because it flows to the delta. Both the Delta-Mendota Canal, which supplies irrigation water to farms in the western San Joaquin Valley, and the California Aqueduct, which supplies part of the drinking water for 15 million people in southern California, originate in the delta.
The aquifer system of the San Joaquin-Tulare Basins study unit is contained within the southern two-thirds of the Central Valley aquifer system. The aquifer system is made up of Post-Eocene continental rocks and deposits, which contain most of the fresh water in the valley. Pre-Tertiary and Tertiary marine sediments underlying the continental deposits contain mostly saline water, except where fresh water may have flushed out some of the saline water. The continental deposits consist primarily of lenses of gravel, sand, silt, and clay, derived from fluvial and alluvial fan sediments and interbedded with lesser amounts of lacustrine deposits. The major aquifer in the San Joaquin Valley consists of continental deposits. Abundant lenses of fine-grained deposits make up more than 50 percent of the aquifer thickness. The thickness of the aquifer system, based largely on the generalized thickness of continental deposits, averages 2,400 feet and increases from north to south to a maximum thickness of more than 9,000 feet near Bakersfield. In the southeastern San Joaquin Valley, sandy marine beds underlying the continental deposits contain freshwater and are hydrologically part of the aquifer system.