CVP Geography and Climate


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The Central Valley Project

The Central Valley
Geography and Climate
by
William Joe Simonds

The Central Valley Basin is comprised of two river basins, the Sacramento and San Joaquin River Basins. The combined basin, over 500 miles long and averaging 120 miles wide, covers more than one-third of the State of California. Except for a small opening on the western edge, the basin is entirely surrounded by mountains. The Cascade range to the north, and the Sierra Nevada to the east, rise more than 14,000 feet from the valley floor, while the coastal range to the west rises some 6,000 feet, separating the valley from the Pacific Ocean. The valley floor, covering one-third of the Central Valley Basin, is about 400 miles long and averages 45 miles in width, and covers 15,000,000 acres, an area about the size of England.

The Central Valley Basin has two major river systems, the Sacramento, and the San Joaquin. The Sacramento River, which drains the northern part of the basin, has its source near Mount Shasta at the northern end of the basin, and flows southerly through the entire length of the Sacramento Valley. The northeastern portion of the basin is drained by the Pit River, which joins the Sacramento River near Redding. The Feather, Bear, Yuba, and American Rivers drain the Sierra Nevada to the east before joining the Sacramento. The Coastal Range to the west is drained by Stony, Cache and Putah Creeks. In addition, several other smaller tributaries combine to add significantly to the flow of the Sacramento River.

The San Joaquin Valley is divided into two sub-basins. The northern portion of the valley is drained by the San Joaquin River that flows north until it meets with the Sacramento River at the Sacramento-San Joaquin Delta. A number of major tributaries drain the Sierra Nevada to the east and flow into the San Joaquin River. These tributaries include the Fresno, Chowchilla, Stanislaus, Merced, and Tuolumne Rivers. The Calaveras, Mokelumne, and Cosumnes Rivers drain directly into the Delta. The waters of the Sacramento and San Joaquin Rivers and their tributaries combine at the Sacramento-San Joaquin Delta, southwest of Sacramento, before flowing out to the Pacific Ocean through San Francisco Bay.

In most years there is no surface water outlet from the southern portion of the San Joaquin Valley. Over period of several centuries, eroded material from the Sierra Nevada has been carried into the valley by the Kings River and deposited in the valley, forming a low barrier between the two halves of the valley. Prior to the development of the area, there was a large lake in the southern portion of the valley that was maintained by the flows of the Kings, Kern, Kaweah, and Tule Rivers. This lake, known as Tulare Lake, would occasionally overflow into the San Joaquin River. The last such overflow occurred in 1878. The flows of the rivers that fed the lake have been appropriated to such an extent that very little water ever reaches the lake bed.

A major feature effecting the Central Valley Basin is the climate. The valley floor has warm, dry summers characterized by an almost complete absence of rain, and mild winters with relatively light rains. The mountains that surround the valley are also generally warm and dry in the summer, but in the winter months, temperatures will frequently fall below freezing. Precipitation in the mountains is significantly greater than that of the valley floor, with the majority falling in the form of snow.

The dry summers of the valley are caused by a belt of subtropical high pressure located off the California coast that prevents summer rainfall. In the winter, the high pressure area moves to the south, allowing Pacific storms to move inland. These storms usually pass to the north of the Central Valley Basin so that the majority of the moisture falls in the northern part of the basin. The result is that as one moves towards the southern basin, the amount of precipitation decreases. The annual average rainfall in the valley decreases from about 23 inches in the north, to around 6 inches in the south. In the Sierra Nevada, the average annual precipitation in the north is about 80 inches, and falls to about 35 inches in the southern mountains.

The moderating influence of the Pacific Ocean to the west, and the Sierra Nevada that prevents cold air from entering the valley from the east, gives the valley its mild winter climate. As a result, there is no place in the valley that averages more than fifteen days per year where the temperature falls to thirty-two degrees or less. The average period between the last frost of Spring and the first frost of Fall is more than 7 1/2 months.

Sources

Johnson, Stephen, Robert Dawson, and Gerald Haslam. The Great Central Valley, California's Heartland. Berkeley: University of California Press, 1992.

United States Department of Interior, Bureau of Reclamation. Central Valley Basin, A Comprehensive Report on the Development of the Water and Related Resources of the Central Valley Basin for Irrigation, Power Production, and Other Beneficial Uses in California, and Comments by the State of California and Federal Agencies. Senate Document 113, 81st Congress, 1st Session. August 1949.