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The Central Valley

The Central Valley

What is the Central Valley in California?

The Central Valley (also known as “The Valley”) is a large, flat valley that dominates the central portion of the U.S. state of California. It is home to many of California’s most productive agricultural efforts. The valley stretches approximately 400 miles (640 km) from north to south. Its northern half is referred to as the Sacramento Valley, and its southern half as the San Joaquin Valley. The Sacramento valley receives about 20 inches of rain annually, but the San Joaquin is very dry, often semi-arid desert in many places. The two halves meet at the shared Sacramento-San Joaquin River Delta of the Sacramento and San Joaquin Rivers, a large expanse of interconnected canals, stream-beds, sloughs, marshes and peat islands. The Central Valley is around 42,000 square miles (110,000 km2), making it roughly the same size as the state of Tennessee.

Boundaries and population

Bounded by the Cascade Range, Trinity Alps and Klamath Mountains to the north, the Sierra Nevada to the east, the Tehachapi Mountains to the south, and the Coast Ranges and San Francisco Bay to the west, the valley is a vast agricultural region drained by the Sacramento and San Joaquin rivers.

Counties commonly associated with the valley:

  • North Sacramento Valley (Shasta, Tehama, Glenn, Butte, Colusa)
  • Sacramento Metro (Sacramento, El Dorado, Sutter,Yuba, Yolo, Placer)
  • North San Joaquin (San Joaquin, Stanislaus, Merced)
  • South San Joaquin (Madera, Fresno, Kings, Tulare, Kern)

About 6.5 million people live in the Central Valley today, and it is the fastest growing region in California.[citation needed] There are 10 Metropolitan Statistical Areas (MSA) in the Central Valley. Below, they are listed by (MSA) population. The largest city is Fresno, followed by the state capital Sacramento.

  • Sacramento Metropolitan Area (2,042,283)
  • Fresno Metropolitan Area (1,002,284)
  • Bakersfield Metropolitan Area (827,173)
  • Stockton Metropolitan Area (664,116)
  • Modesto Metropolitan (505,505)
  • Visalia Metropolitan Area (410,874)
  • Merced Metropolitan Area (241,706)
  • Chico Metropolitan Area (214,185)
  • Redding Metropolitan Area (179,904)
  • Yuba City Metropolitan Area (165,080)


The flatness of the valley floor contrasts with the rugged hills or gentle mountains that are typical of most of California’s terrain. The valley is thought to have originated below sea level as an offshore area depressed by subduction of the Farallon Plate into a trench further offshore. The San Joaquin Fault is a notable seismic feature of the Central Valley.

The valley was later enclosed by the uplift of the Coast Ranges, with its original outlet into Monterey Bay. Faulting moved the Coast Ranges, and a new outlet developed near what is now San Francisco Bay. Over the millennia, the valley was filled by the sediments of these same ranges, as well as the rising Sierra Nevada to the east; that filling eventually created an extraordinary flatness just barely above sea level; before California’s massive flood control and aqueduct system was built, the annual snow melt turned much of the valley into an inland lake.

The one notable exception to the flat valley floor is Sutter Buttes, the remnants of an extinct volcano just to the northwest of Yuba City which is 44 miles north of Sacramento.

Another significant geologic feature of the Central Valley lies hidden beneath the delta. The Stockton Arch is an upwarping of the crust beneath the valley sediments which extends southwest to northeast across the valley.

Physiographically, the Central Valley lies within the California Trough physiographic section, which is part of the larger Pacific Border province, which in turn is part of the Pacific Mountain System.


The northern Central Valley has a hot Mediterranean climate (Koppen climate classification Csa); the more southerly parts in rainshadow zones are dry enough to be Mediterranean steppe (BShs, as around Fresno) or even low-latitude desert (BWh, as in areas southeast of Bakersfield). It is hot and dry during the summer and cool and damp in winter, when frequent ground fog known regionally as “tule fog” can obscure vision. Summer daytime temperatures reach 90 °F (32 °C), and occasional heat waves might bring temperatures exceeding 115 °F (46 °C). Mid Autumn to mid spring comprises the rainy season — although during the late summer, southeasterly winds aloft can bring thunderstorms of tropical origin, mainly in the southern half of the San Joaquin Valley but occasionally to the Sacramento Valley. The northern half of the Central Valley receives greater precipitation than the semidesert southern half. Frost occurs at times in the winter months, but snow is extremely rare.

Tule fog

Tule fog (pronounced /ˈtuːliː/) is a thick ground fog that settles in the San Joaquin Valley and Sacramento Valley areas of California’s Great Central Valley. Tule fog forms during the late fall and winter (California’s rainy season) after the first significant rainfall. The official time frame for tule fog to form is from November 1 to March 31. This phenomenon is named after the tule grass wetlands (tulares) of the Central Valley. Accidents caused by the tule fog are the leading cause of weather-related casualties in California.

Sacramento Valley

The 375-mile (604 km) Sacramento River drains the northern third of the Central Valley, an area referred to as the Sacramento Valley. While the Sacramento portion of the valley is smaller than the San Joaquin Valley, it carries twice as much water due to its greater rainfall. After exiting Shasta Lake, a large reservoir at the northern extreme of the valley formed by Shasta Dam, the Sacramento River flows south, receiving water from the Feather and American rivers. Tributaries above Shasta Lake include the Pit and McCloud rivers. (If the Sacramento were combined with the Pit, the resulting length would be 690 miles (1,110 km), the longest river in California.) The Yuba River is a major tributary of the Feather. Oroville Dam, on the Feather River, forms Lake Oroville, and Folsom Dam, on the American River, forms Folsom Lake. The Sacramento eventually feeds the Sacramento-San Joaquin Delta.


The Sacramento-San Joaquin Delta forms the outflow for all the runoff from the Central Valley, eventually spilling into San Francisco Bay. The Sacramento flows in from the north, the Mokelumne and its tributary, the Cosumnes from the east, and the San Joaquin River from the south. The delta is an inverted river delta, meaning that it is formed by many branches that converge into a single outflow. The delta’s many islands, separated by sloughs and marshes, are vastly fertile, and were originally a tidal freshwater marsh; now the region is predominantly agricultural. The waters of the Sacramento and San Joaquin rivers that come together in the delta are the water source of about 25 million California citizens, over two-thirds of the state’s population.


Agriculture is the primary industry in most of the Central Valley. A notable exception to the predominance of agriculture has been the Sacramento area, where the large and stable workforce of government employees helped steer the economy away from agriculture. Despite state hiring cutbacks and the closure of several military bases, Sacramento’s economy has continued to expand and diversify and now more closely resembles that of the nearby San Francisco Bay Area. Primary sources of population growth are people migrating from the San Francisco Bay Area seeking lower housing costs, as well as immigration from Asia, Central America, Mexico, Ukraine and the rest of the former Soviet Union.

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