The Great Valley 135 



the lower valley floor, thus making artesian or flowing wells 

 possible. Porous alluvial deposits, fans, formed by streams 

 that flow from the mountain ranges, absorb the mountain 

 waters and become a source from which artesian flowing wells 

 and pump wells are obtained at lower levels. Large perennial 

 streams flow from the high Sierra Range over the broad slope 

 toward the valley. Shorter streams, more torrential due to the 

 shorter and steeper slopes, enter on the Coast Range side of the 

 valley. These streams from both sides of the valley have built 

 alluvial fans or aprons, delta-like, at their mouths. These fans 

 spread out over great areas on the basin bottom. The fans on 

 the west side of the valley are shorter and more abrupt, due to 

 the torrential character of the streams from the Coast Ranges. 

 On the east side of the basin the fans extend far out upon the 

 valley floor, their sides often coalescing so as to form a continu- 

 ous plain. Streams flow across the deltas or fans built at their 

 own mouths, their waters sometimes soaking into the porous 

 soil and sometimes the fans are channeled by the streams that 

 have built them to a depth as great as 100 feet. Coarser rock 

 debris, gravel and even boulders of considerable size, are thrown 

 down, first on the flanks of the foothills as the streams emerge 

 upon the plain, and the finer sands and silt are carried farther 

 toward the axis of the valley. 



Streams Disappear in Sands of Valley floor 



In the southern part of the Great Valley, south of the big 

 bend of the San Joaquin River, rivers from the Sierra Range 

 spread out upon their own deltas and the waters sink into the 

 porous soils. Kings River, coming from the crest of the Sierra 

 Range, and Los Gatos Creek, coming from the Coast Range on 

 the west, have each built a delta or alluvial fan into the Valley, 

 and these have extended from opposite sides of the Valley till 

 they have met, and so form a ridge of porous water-borne 

 debris entirely across the Valley. Thus a dam has been formed 

 by which the waters from the southern end of the Great Valley 



