The San Joaquin Valley includes roughly the southern two-thirds of the Great Central Valley of California. It is a broad structural trough surrounded by mountains. The northern part of the valley drains through the San Joaquin River northward to San Francisco Bay ; the southern part of the valley normally is a basin of interior drainage tributary to evaporation sumps in the trough of the valley, chiefly Tulare and Buena Vista Lake beds. In years of normal discharge most of the streamflow in the southern part of the valley not diverted for irrigation finds its way to Tulare and Buena Vista Lake beds. In the historic past, however, during years of heavy floods the low divide between Buena Vista and Tulare Lakes and the low divide between Tulare Lake and the San Joaquin River were overtopped and through-flowing drainage occurred over the full length of the valley. Because the Tulare Lake bed is the lowest point and also the largest sump, this whole basin of interior drainage is commonly referred to as the Tulare Lake drainage basin. Average annual precipitation ranges from more than 15 inches in the north- eastern part of the valley to less than 4 inches in the southwestern part. The precipitation decreases from north to south and from east to west across the valley. Streamflow, the critical quantity in the water supply, depends almost wholly on the amount and distribution of precipitation in the Sierra Nevada to the east. Much of this precipitation falls as snow, and the snowpack acts as a natural reservoir retaining much of the annual runoff until late spring and early summer. The mean seasonal runoff to the San Joaquin Valley is nearly 10 million acre- feet, of which about two-thirds is tributary to the San Joaquin River; the remaining third is tributary to Tulare Lake drainage basin. In 1952 about 8.5 million acre-feet of surface water was diverted for irrigation. Withdrawals of ground water for irrigation in 1952 approximated 7.5 million acre-feet. The surface of the San Joaquin Valley is not a featureless plain but is characterized by various types of physiography such as dissected uplands, low alluvial plains and fans, river flood plains and channels, and overflow lands and lake bottoms. The dissected uplands fringe the valley along its mountain borders. They are underlain by unconsolidated to semiconsolidated continental deposits of late Tertiary and early Quaternary age which have been moderately tilted and folded. The topography of these uplands ranges from deeply dissected hill land having a relief of several hundred feet to gently rolling land whose relief Is only a few feet. The low plains and fans border the dissected uplands along their valley- ward margins. They are generally fiat to gently undulating and featureless and are underlain by undeformed to slightly deformed alluvial deposits of Quaternary age. The river flood plains and channels lie along the San Joaquia and Kings Rivers in the axial part of the valley and along the major east-side streams. Where the rivers are incised below the general land surface, the flood plains are well defined; but in the axial trough of the valley, where the rivers are flanked by low-lying overflow lands, the flood-plain and channel deposits are confined to the stream channel and to the natural levees that slope away from the river. Overflow lands and lake bottoms include the historic beds of Tulare, Buena Vista, and Kern Lakes in the southern part of the valley, and the low-lying lands in the axial trough between the low alluvial plains and fans and the natural levees of the San Joaquin River and its major tributaries. They are level and featureless and are underlain by lake and swamp deposits of Recent age. The San Joaquin Valley is a great structural downwarp between the tilted block of the Sierra Nevada on the east and the complexly folded and faulted Coast Ranges on the we