11.2: Physical Geography
The Central Valley is a vast, flat plain that contrasts with the rugged mountains that surround it. The valley floor is thought to have originated as an offshore area that was depressed by the subduction of the Farallon Plate beneath the North American Plate. The valley has no earthquake faults of its own but is surrounded by faults to the east and west.
The valley was enclosed by the uplift of the Coast Ranges, with its original outlet into Monterey Bay. Faulting subsequently moved the Coast Ranges, and a new outlet developed near what is now San Francisco Bay. Over time, the valley filled with sediment from the Coast Ranges and the Sierra Nevada, creating an extraordinary flatness just barely above sea level. In fact, thousands of years before California's flood control and aqueduct system was built, annual snow melt would turn much of the valley into an inland sea.
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.
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 that extends southwest to northeast across the valley.
Climate
The Central Valley is a large, flat plain in California that is home to a diverse range of ecosystems. The northern Central Valley has a hot Mediterranean climate (Köppen climate classification Csa), while the southern Central Valley is drier and has a Mediterranean steppe climate (BShs) or even low-latitude desert climate (BWh).
The Central Valley is very hot and dry during the summer, with daytime temperatures frequently surpassing 100°F (38°C). Common heat waves can bring temperatures exceeding 115°F (46°C). The rainy season is from mid-autumn to mid-spring, although during the late summer, southeasterly winds can bring tropical thunderstorms, 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 fall months, but snow is extremely rare.
Low Clouds & Fog
Tule fog is a type of low cloud that is characteristically confined to the Central Valley of California. This is because the Central Valley is surrounded by mountains, which trap the cold air in the valley. Tule fog can be very dense and can last for days or even weeks at a time. It can also contain light drizzle or freezing drizzle. Tule fog can be seen from above by driving up into the foothills of the Sierra Nevada or Coast Ranges. Above the cold, foggy layer, the air is typically mild, dry, and clear. Once Tule fog has formed, turbulent air is necessary to break through the temperature inversion layer. Daytime heating can sometimes evaporate the fog in patches, but it reforms soon after sunset. Tule fog usually remains longer in the southern and eastern parts of the Central Valley, because winter storms with strong winds and turbulent air affect the northern Central Valley more often.
Flora & Fauna
The Central Valley is a vast grassland ecosystem that has been significantly altered by human activity. The dominant grass of the valley was Nassella pulchra, but today only 1% of the grassland in the valley is intact. Other grasses that can still be found in the valley include California poppy, lupins, and purple owl's clover. Riverside trees include willows, western sycamore, box elder, Fremont cottonwood, and the endemic valley oak.
The Central Valley was once home to a wide variety of wildlife, including pronghorn antelope, elk, mule deer, California ground squirrels, gophers, mice, rabbits, kangaroo rats, and San Joaquin kit foxes. The valley's wetlands are an important habitat for wintering waterbirds and migrating birds of other kinds. Reptiles and amphibians of the valley include the San Joaquin coachwhip snake, blunt-nosed leopard lizard, Gilbert's skink, and western aquatic garter snake. Endemic invertebrates are also present. The Central Valley is home to endemic fish species, including the Sacramento Pikeminnow, Sacramento Perch, Sacramento Blackfish, and Sacramento Splittail.
Hydrography & The Central Valley Project
Two river systems drain and define the two parts of the Central Valley. The Sacramento River, along with its tributaries the Feather River and American River, flows southwards through the Sacramento Valley for about 447 miles (719 km). In the San Joaquin Valley, the San Joaquin River flows roughly northwest for 365 miles (587 km), picking up tributaries such as the Merced River, Tuolumne River, Stanislaus River and Mokelumne River. The Central Valley watershed encompasses over a third of California at 60,000 square miles (160,000 km2), with 46 percent draining into the Sacramento River, 26 percent into the San Joaquin, and 27 percent into Tulare Lake.
Aerial view of the Delta region, showing the Sacramento River (above) and the San Joaquin River (below).
The Sacramento River carries far more water than the San Joaquin, with an estimated 22 million acre-feet (27 km3) of virgin annual runoff, as compared to the San Joaquin's approximately 6 million acre-feet (7.4 km3). Intensive agricultural and municipal water consumption decreased the rate of outflow to about 17 million acre-feet (21 km3) for the Sacramento and 3 million acre-feet (3.7 km3) for the San Joaquin. These figures vary widely from year to year. Over 25 million people, living in the valley and other regions of the state, rely on the water carried by these rivers (Figure 11.6).
Central Valley rivers converge in the Sacramento-San Joaquin Delta, a network of marshy channels, distributaries, and sloughs that wind around islands mainly used for agriculture. There the rivers merge with tidewater, and eventually reach the ocean after passing through Suisun Bay, San Pablo Bay, upper San Francisco Bay, and finally the Golden Gate. Many of the islands lie below sea level because of intensive agriculture, and face a high risk of flooding, which would allow salt water to rush back into the delta, especially when too little fresh water is flowing in from the Valley (Figure 11.7).
In the south part of the San Joaquin Valley, the alluvial fans of the Kings River and Coast Ranges streams have created a natural divide. The dry Tulare basin of the Central Valley receives flow from four major Sierra Nevada rivers, the Kings, Kaweah, Tule, and Kern. This basin, usually endorheic, formerly filled during snowmelt and spilled out into the San Joaquin River. Called Tulare Lake, it is usually dry because the rivers feeding it have been diverted for agricultural purposes (Figure 11.8).
The 1933 Central Valley Project (CVP)
The Central Valley Project was the world's largest water and power project when undertaken during Franklin D. Roosevelt's New Deal public works agenda, which was designed for infrastructure development, economic stimulus and to ultimately employ millions of out-of-work Americans, due to the Great Depression. The Central Valley Project was the culmination of eighty years of political fighting over the state's most important natural resource - Water. The Central Valley of California lies to the west of the Sierra Nevada Mountains with its annual run-off draining into the Pacific Ocean through the Sacramento–San Joaquin River Delta. It is a large geological floodplain moderated by its Mediterranean climate of dry summers and wet winters that includes regular major drought cycles. At the time of its construction, the project was at the center of a political and cultural battle over the state's future. It intersected with the state's ongoing war over land use, access to water rights, impacts on indigenous communities, large vs. small farmers, the state's irrigation districts and public vs. private power. Its proponents ignored environmental concerns over its impacts, as long as the outcome did not damage the major stakeholders at that time.
The Central Valley has gone through two distinct culturally driven land use eras. The first was the indigenous tribal period that lasted for thousands of years. Then came the arrival of Europeans, first by the Spanish colonial model of Catholic missions and ranchos (1772–1846). It was then followed by the current United States era. Due to its Mediterranean climate, the first cultural period was hunter-gatherer based. The Spanish missions' ranching and tanning business was built on the forced labor of California’s tribes. Spain's model of land use with the grazing of livestock for meat, wool and leather started along Alta California's coast, eventually spreading inland. The U.S. era evolved from primarily ranching to large-scale plantations or more commonly known today as corporate farming that turned the Central Valley into the breadbasket of the U.S.
Following the 1848 California Gold Rush, large numbers of U.S. citizens came into the region and made attempts to practice rainfed agriculture, but most of the Central Valley land was taken up by large cattle ranchers like Henry Miller who eventually controlled 22,000 square miles of land. The large-scale levee construction by Chinese workers along the Delta was where limited irrigation for orchards first started.
Following the arrival of the Transcontinental railroad, immigration from Asia and the rest of the U.S. led to growing numbers of settlers in the region. Despite the rich soils and favorable weather of the 42,000-square-mile (110,000 km2) Central Valley, immigrants to the valley who were unfamiliar with its seasonal patterns of rainfall and flooding began to take up irrigation practices. Farmers soon found themselves troubled by frequent floods in the Sacramento Valley and a general lack of water in the San Joaquin Valley. The Sacramento River, which drains the northern part, receives between 60 and 75% of the precipitation in the Valley, despite the Sacramento Valley covering less area than the much larger San Joaquin Valley, drained by the San Joaquin River, which receives only about 25% of the rainfall. Furthermore, cities drawing water from the Sacramento-San Joaquin Delta faced problems in dry summer and autumn months when the inflowing water was low. To continue to sustain the valley's economy, there needed to be systems to regulate flows in the rivers and equally distribute water among the north and south parts of the valley.
In addition to water storage and regulation, the system has a hydroelectric capacity of over 2,000 megawatts and provides recreation and flood control with its twenty dams and reservoirs. It has allowed major cities to grow along Valley rivers which previously would flood each spring and transformed the semi-arid desert environment of the San Joaquin Valley into productive farmland. Freshwater stored in Sacramento River reservoirs and released downriver during dry periods prevents salt water from intruding into the Sacramento-San Joaquin Delta during high tide. There are eight divisions of the project and ten corresponding units, many of which operate in conjunction, while others are independent of the rest of the network. California agriculture and related industries now directly account for 7% of the gross state product for which the CVP supplied water for about half.