Central Valley (California)
The Central Valley is a flat valley that dominates the geographical center of the U.S. state of California. It is 40 to 60 miles (60 to 100 km) wide and stretches approximately 450 miles (720 km) from north-northwest to south-southeast, inland from and parallel to the Pacific Ocean coast. It covers approximately 18,000 square miles (47,000 km2), about 11% of California's total land area (or about the size of the Dominican Republic). The valley is bounded by the Sierra Nevada to the east and the Coast Ranges to the west.
The valley encompasses all or parts of 19 California Counties: Butte, Colusa, Glenn, Fresno, Kern, Kings, Madera, Merced, Placer, San Joaquin, Sacramento, Shasta, Solano, Stanislaus, Sutter, Tehama, Tulare, Yuba, and Yolo.
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 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 (71 km) north of Sacramento.
The "Central Valley grassland" is the Nearctic temperate and subtropical grasslands, savannas, and shrub lands ecoregion that was once a diverse grassland containing areas of desert grassland (at the southern end), prairie, savanna, riverside woodland, marsh, several types of seasonal vernal pools, and large lakes such as now-dry Tulare Lake (the largest lake in the United States west of the Mississippi), Buena Vista Lake and Kern Lake. However, much of the Central Valley environment has been removed or altered by human activity including the introduction of exotic plants, especially grasses. The oak woodlands and chaparral that fringe the valley have been categorized as the California interior chaparral and woodlands ecoregion.
The Great Valley Grasslands State Park preserves an example of the native grass habitat in the valley, while oak savanna habitats remain near Visalia. There are areas of wetland and riverside woodland in the north especially on the Sacramento River system including the Nature Conservancy's Cosumnes River Preserve just south of Sacramento, Gray Lodge Wildlife Area, Butte Sink Wildlife Management Area, and other patches in the delta area. Remaining vernal pools include Pixley National Wildlife Refuge between Tulare, California, and Bakersfield and Jepson Prairie Preserve in the delta. There are large blocks of desert scrubland in the southern San Joaquin Valley and in the Carrizo Plain, which is just outside the valley but has a similar landscape.
The valley gives its name to Valley fever, which is primarily a disease of the lungs that is common in the southwestern United States and northwestern Mexico. It is caused by the fungus Coccidioides immitis, which grows in soils in areas of low rainfall, high summer temperatures, and moderate winter temperatures. These fungal spores become airborne when the soil is disturbed by winds, construction, or farming. This illness frequently takes many weeks or months to resolve. Occasionally Valley Fever is life-threatening and even fatal.
In the south part of the San Joaquin Valley, the alluvial fan of the Kings River and another one from Coast Ranges streams have created a divide and resultantly the currently dry Tulare basin of the Central Valley, into which flow four major Sierra Nevada rivers, the Kings, Kaweah, Tule and Kern. This basin, usually endorheic, formerly filled during heavy snowmelt and spilled out into the San Joaquin River. Called Tulare Lake, it is usually dry nowadays because the rivers feeding it have been diverted for agricultural purposes.
Runoff from the Sierra Nevada flows into the Central Valley and provides one of the largest water resources of California. The Sacramento River is the second largest river to empty into the Pacific from the Continental United States, behind only the Columbia River and greater than the Colorado River. Combined with the fertile and expansive area of the Central Valley's floor, the Central Valley is ideal for agriculture.
Most lowlands of the Central Valley are prone to flooding, especially in the old Tulare Lake, Buena Vista Lake, and Kern Lake rivers. The Kings, Kaweah, Tule and Kern rivers originally flowed into these seasonal lakes, which would expand each spring to flood large parts of the southern San Joaquin Valley. Due to the construction of farms, towns and infrastructure in these lakebeds while preventing them from flooding with levee systems, the risk of floods damaging properties increased greatly.