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Show Chapter 2 Streams and Ground Water The Central Valley is divided into three drainage basins. The two principal streams, the Sacra- mento and San Joaquin, drain the northern and southern sections of the valley respectively. The extreme southern end of the San Joaquin Valley, however, is a closed basin, separated from the San Joaquin system by a low ridge of erosional debris spread out on the valley floor by the Kings River. The Sacramento River rises in the vicinity of Mount Shasta and flows south and southwest to the delta. In its upper course it is a swift-flowing moun- tain stream enclosed by steep walls. North of Red Bluff it breaks out onto the valley floor and proceeds southward in a winding, sluggish course. Numer- ous small tributaries, mostly from the Sierras, enter it. The Pit and McCloud Rivers, which drain the extreme northeast section of the valley, join the main stem at Shasta Reservoir. The Feather, Yuba, Bear, and American Rivers join the Sacra- mento on the valley floor. Three smaller tribu- taries drain the east flank of the Coast Range. In addition, numerous small tributaries contribute a large flow to the Sacramento. The San Joaquin River rises in the Sierras north- east of Fresno, flows westward to the center of the valley floor and there turns northwest to the delta. The Fresno, Chowchilla, Merced, Tuolumne, and Stanislaus Rivers are the principal tributaries drain- ing the Sierra Nevada. Three other streams from the Sierras, the Galaveras, Mokelumne, and Cosumnes Rivers, enter the delta directly. At the southern end of the San Joaquin Valley is a closed basin. Tulare Lake lies in the bottom of the depression. In wet years, prior to extensive irrigation, this lake discharged into the San Joaquin. A large part of this lake bed is now leveed to form a reservoir. During flood years water flows into the lake, then is pumped out during the following irrigation season or seasons. In most years, Tulare Lake is dry and the lake bed fanned. The lake is fed by floods from the Kings, Kaweah, Tule, and Kern Rivers. The annual regimen of the streams is highly ir- regular and is related to precipitation. During winter, if prolonged rains occur over the basin, runoff can be heavy and rapid. Under such cir- cumstances flooding of the lowlands is apt to be widespread. Also, snow accumulates to great depths in the high Sierra Nevada. In the spring the probability of flood-producing rains decreases before the high snow fields begin to melt. This is of major importance. It permits storage reservoirs which are used for flood control during the winter rainy season to be used also for storage of water released by snow melt during the summer. As stream flow from the mountain snow pack can be forecast with considerable accuracy, reservoir stor- age permits a maximum of water control and con- servation. The great flows from the high snowfields are confined largely to the months of May, June, and July. By the end of July many mountain streams have low discharge rates, which continue until the beginning of the winter rains. Consequently, stor- age either in reservoirs or underground is absolutely essential if large-scale irrigation is to be practiced in the late summer and early fall. The streams of the low-lying delta lands are subject to tidal action 40 miles upstream from the mouth. Under natural conditions the flow of the rivers was sufficient to keep salt concentrations at the delta margins to a minimum. As larger and larger quantities of water have been diverted from the natural flow for other purposes, the salinity concentrations have increased in the delta area. 87 |
