Data Series 285
U.S. GEOLOGICAL SURVEY
Data Series 285 (ver 1.1, August 2018)
The Southern Sacramento Valley GAMA study unit lies within the Central Valley and partly within the North Coast Ranges hydrogeologic provinces described by Belitz and others (2003) (fig. 1). The Southern Sacramento Valley GAMA study unit covers an area of approximately 2,100 mi2 and includes parts of Placer, Sacramento, Solano, Sutter, and Yolo counties (fig. 2). The study unit is bounded to the west by the Northern Coast Ranges, to the east by the Sierra Nevada, to the north by the central Sacramento Valley, and to the south by the Sacramento–San Joaquin Delta and the San Joaquin Valley. For the purposes of this report, the Southern Sacramento Valley GAMA study unit has been divided into six study areas: North American (NAM), South American (SAM), Solano (SOL), Yolo (YOL), Suisun–Fairfield (SUI), and the Uplands (or QPC for “Quaternary Pleistocene Semiconsolidated Deposits), which includes portions of both the NAM and SAM subbasins as defined by the California Department of Water Resources (DWR; fig. 2). Four subbasins within the Sacramento Valley ground-water basin, as defined by DWR, were used to locate four of the study areas: the North American subbasin, the South American subbasin, the Solano subbasin, and the Yolo subbasin. A fifth study area includes the uplands area on the eastern sides of the North and South American subbasins. The sixth study area is the DWR-defined Suisun–Fairfield ground-water basin.
The Southern Sacramento Valley GAMA study unit has a Mediterranean climate with hot, dry summers and wet, mild winters (Blair and Fite, 1957, p. 323). Average annual rainfall across the study unit ranges from 17 to 23 in. (California Department of Water Resources, 2005a, 2005b, 2005c), with higher amounts on the western and eastern sides of the valley and lower amounts in the central region of the valley. The Southern Sacramento Valley GAMA study unit is drained by several creeks and rivers. The Bear, American, and Cosumnes Rivers, as well as their tributaries, drain the eastern portions of the study unit. These rivers drain directly into the Sacramento River, which flows south and empties into the Sacramento–San Joaquin Delta, or directly into the Delta itself, which discharges its waters to the San Francisco Bay. The west side of the study unit is drained by Putah and Cache Creeks and other smaller tributaries, which also drain into the Sacramento River or the Sacramento–San Joaquin Delta.
The GAMA boundaries of the NAM study area (fig. 2) are based on the DWR-defined “North American Subbasin,” which is described as the aerial extent of unconsolidated to semiconsolidated sedimentary deposits north of the American River, east of the Sacramento River, south of the Bear River, and west of the consolidated bedrock of the Sierra Nevada (California Department of Water Resources, 2005a). For the purposes of this report, the GAMA NAM study area is defined as the aerial extent of the unconsolidated Quaternary age deposits north of the American River and west of the older exposed Pliocene and Pleistocene semiconsolidated deposits that border the consolidated bedrock of the Sierra Nevada. This study area includes parts of Placer, Sacramento, and Sutter counties. The study area is drained by the American, Bear, and Sacramento Rivers. The main water-bearing aquifer units within the NAM study area include the Older Alluvial deposits of the Modesto, Riverbank, and Turlock Lake Formations, flood basin deposits, and the deeper Laguna, Mehrten, and Valley Springs Formations (California Department of Water Resources, 2005a). The Mehrten Formation is a Miocene–Pliocene age volcanic deposit considered to be the oldest fresh water-bearing deposit on the east side of the basin because the older underlying Valley Springs Formation produces only minor amounts of water (California Department of Water Resources, 1967).
The GAMA-defined boundaries of the SAM study area (fig. 2) closely match those defined by DWR for the “South American Subbasin,” which are described as the aerial extent of unconsolidated to semiconsolidated sedimentary deposits south of the American River, east of the Sacramento River, north of the Cosumnes River, and west of the consolidated bedrock of the Sierra Nevada (California Department of Water Resources, 2005a). For the purposes of this report, the GAMA SAM study area is defined as the aerial extent of the unconsolidated Quaternary age deposits north of the American River and west of the older exposed Pliocene and Pleistocene semiconsolidated deposits that border the consolidated bedrock of the Sierra Nevada. The study area is located in Sacramento County. The study area is drained by the American, Cosumnes, and Sacramento Rivers. The main water-bearing aquifer units within the SAM study area include the Older Alluvial deposits of the Modesto and Riverbank Formations, flood basin deposits, and the deeper Laguna, Mehrten, and Valley Springs Formations (California Department of Water Resources, 2005a). The Mehrten Formation is a Miocene–Pliocene age volcanic deposit considered to be the oldest fresh-water-bearing deposit on the east side of the basin (California Department of Water Resources, 1967).
The boundaries of the GAMA YOL study area (fig. 2) match those defined by the DWR for the “Yolo Subbasin” and are described as the aerial extent of unconsolidated to semiconsolidated sedimentary deposits that are bounded by the Coast Ranges to the west, Cache Creek to the north, the Sacramento River to the east, and Putah Creek to the south (California Department of Water Resources, 2005b). This study area lies within Yolo County. The main water-bearing units within the YOL study area are the Red Bluff and Tehama Formations, Older Alluvium, flood basin deposits, and Younger Alluvium, which, when combined, range from a few hundred feet thick near the Coast Ranges to more than 3,000-ft thick near the eastern edge of the YOL study area (Bertoldi and others, 1991).
The boundaries of the GAMA SOL study area (fig. 2) match those defined by DWR for the “Solano Subbasin” and are described as the aerial extent of unconsolidated to semiconsolidated sedimentary deposits that are bounded by the Coast Ranges to the west, Putah Creek to the north, the Sacramento River to the east, and the Sacramento–San Joaquin Delta to the south (California Department of Water Resources, 2005b). This study area lies within Solano County. It is drained by Putah Creek and the Sacramento River in addition to smaller creeks and sloughs, which all eventually flow into and become part of the Sacramento–San Joaquin Delta (California Department of Water Resources, 2005b). The main water-bearing units within the SOL study area are the Red Bluff and Tehama Formations, Older Alluvium, flood basin deposits, and Younger Alluvium, which, when combined, range from a few hundred feet thick near the Coast Ranges to more than 3,000-ft thick near the eastern edge of the SOL study area (Bertoldi and others, 1991)
The boundaries of the GAMA SUI study area (fig. 2) match those defined by the DWR for the “Suisun–Fairfield Ground-Water Basin” and are described as the aerial extent of unconsolidated to semiconsolidated sedimentary deposits that are bounded by the Coast Ranges to the west and north, the Sacramento ground-water basin to the east, and the Sacramento–San Joaquin Delta and Suisun Bay to the south (California Department of Water Resources, 2005b). This study area lies within Solano County. Surface drainage flows southward to the Sacramento–San Joaquin Delta and Suisun Bay (California Department of Water Resources, 2005b). The main water-bearing units within the SUI study area are the Sonoma Volcanics, Older Alluvium, flood basin and marsh deposits, and Younger Alluvium, which, when combined, are as much as 1,500-ft thick near the Sacramento–San Joaquin Delta (Thomasson and others, 1960).
The GAMA QPC study area (fig. 2) is defined as those portions of the DWR-defined North and South American ground-water subbasins, which include the surficial extent of the semiconsolidated Pliocene and Pleistocene age deposits west of the consolidated bedrock of the Sierra Nevada. The geologic formations identified within this study area include the Modesto, Riverbank, Victor, Laguna, Mehrten, and Valley Springs (Piper and others, 1939; Olmstead and Davis, 1961; California Department of Water Resources, 1974; and Helley and Harwood, 1985).
U.S. Department of the Interior | U.S. Geological Survey
URL: https://pubs.usgs.gov/ds/285
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Page Last Modified: Monday, 06-Aug-2018 17:55:41 EDT
