California - Environment
The coast of California is lined by long mountain ranges that trend in a generally northwesterly direction. They are collectively called the Coast Ranges. Most are not particularly high--summits are between 1,000 and 1,600 meters. They are heavily folded and faulted as a result of the pressures of plate contact just to the west. The California earthquake fault zones follow the same northwesterly trend as the Coast Ranges.
Earthquakes are common across large sections of the region, especially from the San Francisco Bay area southward to near Bakersfield and from the Los Angeles area southeastward through the Imperial Valley.
To the east of the Coast Ranges for much of their distance lies the Central Valley. This valley is extremely flat, extends 650 kilometers from north to south, and is nearly 150 kilometers wide in places. The Central Valley was originally a massive extension of the Pacific Ocean, open to the sea only at San Francisco Bay. A recipient of the erosion material carried off the western slope of the Sierra Nevada, this former section of the sea has been filled in with sediment. The result is a low-relief landscape rich in potential for large-scale agricultural pursuits.
To the east of the Central Valley, the Sierra Nevada rise gradually and have been heavily eroded. In contrast, the eastern face of the mountains offers a dramatic change in elevation. These are fault-block mountains, large rock masses that rose as a whole unit, and the eastern side was lifted far more sharply than the western face. Because they reach high elevations and contain few passes, the Sierra Nevada have proven a major barrier to movement between middle and northern California and areas to the east.
Two other landscape areas, less identifiably Californian, complete the state's topography. In the north, the mountain-valley-mountain pattern breaks down, and much of the northern tier is generally mountainous. The central plateau portion, directly north of the Central Valley, contains two of the state's major volcanic peaks, Mount Lassen and Mount Shasta. To the southeast of the Central Valley, the Great Basin of the interior is composed of low-lying mountains interspersed with large areas of fairly flat land.
Variations in California's climate and vegetation are nearly as great as the diversity of its topography. Of greatest importance for precipitation is the frequent movement of moisture-laden air southeastward out of the northeast Pacific. The distance southward that storm systems from this air mass can move is influenced substantially by a stable high-pressure center usually found off the west coast of Mexico. Such a stationary center blocks the southward movement of storms emerging from the maritime air mass and forces the moisture-producing system eastward onto shore. This blocking high tends to drift northward during summer and southward during winter.
A definite north-south gradient in average annual precipitation occurs in California as a result of these air masses; the north is much more moist on the average than the south. Furthermore, summers are characteristically drier than winters, especially in the south. In summer, southern California frequently experiences long periods during which there is no rainfall. Consequently, the wooded mountain slopes grow dry as tinder. Forest fires, another of the region's recurring environmental problems, are most frequent in late summer and fall, toward the end of the long dry period. The coast north of San Francisco has temperatures with relatively little seasonal variation, plentiful year-round precipitation, and frequent periods of overcast skies.
The Central Valley is much drier than the coastal margins of the state. Annual precipitation is usually less than half that found at a similar latitude on the western slopes of the Coast Ranges. For example, Mendocino, on the coast north of San Francisco, averages 92 centimeters of precipitation annually, while Yuba City, directly east of Mendocino in the heart of the Sacramento Valley, averages only 52 centimeters. Southward, coastal San Luis Obispo averages 52 centimeters, while inland Bakersfield must make do with only 15 centimeters annually.
Summer temperature differences between coastal and inland points at a similar latitude are equally dramatic. San Luis Obispo's average July temperature is 18C; the average for Bakersfield is nearly 12C higher. Daytime high temperatures in San Francisco in late summer are usually under 27C, while Stockton, 100 kilometers east, is baking in temperatures over 38C. Much of this difference is the result of the moderating effect of the cold ocean current offshore and the usual pattern of afternoon and evening fog in summer along the coast.
To the interior of the Coast Ranges and the Sierra Nevada, in California's southeast, is a broad region of dry steppe or desert environment. During the late summer months, dry winds occasionally press westward to the coast, bringing extremely low humidity and temperatures that may register more than 40C. The southeastern interior area of California receives, on the average, less than 20 centimeters of precipitation annually.
Vegetation patterns closely parallel variations in climate in the state. Nearly all of lowland southern California and the area east of the Sierra Nevada-Cascade ranges is covered with sage, creosote bush, chaparral, and other characteristic desert and semidesert growth. The Central Valley and the valleys of the southern Coast Range are somewhat better watered than areas farther south; they are steppe grasslands. Wrapping around the Central Valley and following the coast from Santa Barbara to Monterey Bay are mixed open forests of live oaks and pines. The coast from Monterey Bay northward is the home of the redwoods, the world's largest trees. At higher elevations in the Coast Ranges and Sierra Nevada are mixed forests of pine and fir, and high in the Sierra Nevada are subalpine hemlock-fir forests, including those of the sequoia.
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