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CHAPTER XIV. 



SOURCES OF WATER SUPPLY. 



Our water supply is of two distinct types; 



First: That derived directly from springs or streams in the 

 mountains. It is plain that this type of supply is due to a rainfall In 

 the mountains. Where the water-sheds are forested, other things be- 

 ing equal, the supply is largest and most regular. 



The rainfall by the aid of trees, brush and humus gets into the 

 mountains. These mountains are porous and have a great absorbative 

 power for water. The water percolates slowly through these rocks and 

 broken quartz veins or fissures, until it reaches an open vein. Through 

 this, it flows to a low point and appears as a spring. The mountains 

 are a reservoir and require the forest to detain the rainfall long enough 

 to allow it to get into the rocks and soil. Our canyons carry summer 

 water only because they cut at the lowest places the water carrying 

 strata. It is the water-shed of the strata and its size and condition 

 that determines the summer flow of every spring and stream in the 

 Southern Sierra. 



The water-shed of the canyon carrying the stream is not the govern- 

 ing factor. In a climate like ours, with usually six to eight months 

 without rain, and sometimes without rain of importance for a year or 

 more, there can be no reliable surface flow to supply springs and 

 streams. 



A careful examination of our mountain springs and streams dem- 

 onstrates the fact that all of them have their summer flow from cut 

 strata. These sources are sometimes masked in cienegas or deposits 

 of detritus and silt. I have demonstrated by experimental tunnels in 

 three of my own canyons that these mountain cienegas in canyons are 

 supplied by quartz ledges, cut by the canyon and not by a bed-rock 

 flow in the canyon. There are canyons, where there is a bed-rock flow, 

 but such flow also originates In an open vein above from the rock. In 

 the high Sierras, there are places like Bear Valley, where from the ex- 

 tensive and comparatively level district, a good year would doubtless 

 rurnish a prolonged summer supply of not deep seepage water. This 

 is never the case in our canyons. 



