WATEB STORAGE ON SWEETWATER AND SAN JACINTO RIVERS. 



355 



construction of a safe masoniy wall, and its onl3' drawback is its low elevation, the 

 crest of the dam being but 215 feet above sea level, and its lowest outlet 140 feet. 

 The mesa lands along the coast, which are the most desirable for residence and 

 cultivation, on account of their freedom from frost, reach above these elevations, 

 and there has been difficult}- in supplying man}- of those needing water. The water- 

 shed area above the dam is 186 square miles. This area has been classified by Mr. 

 J. B. Lippincott, in Bulletin No. liO of the United States Geological Survej- for 

 1895, as follows: 



Square miles. 



Steep and rocky mountains, favorable to large run off 26 



Lower rolling mountains, usually covered with brush 99 



Rolling hills, covered with soil and disintegrated granite 30 



Agricultural lands and river bottom (17 per cent) 31 



Total 186 



The same authorit}' estimates the mean elevation of the watershed at 2,200 feet, 

 and says, regarding its topograph}-: 



The most noticeable feature of this ba.«in influencing the run off is the abrupt flattening of the 

 slopes at the base of the mountains into agricultural fields. The drainage lines, deeply cut into the 

 steep hillsides, are quickly lost or are poorly defined in crossing the flat alluvial cones or partly filled 

 valleys on the low grounds. In turn, the nearly level fields or parks drain into deep canyons. The 

 occasional flood, caused by a heavy rain rushing down the mountain side, spreads out over the flat 

 lands, much of it disappearing before it can reach the lower canyon. The steady percolation which 

 might be expected at points- telow is, to a large extent, cut off by the high rate of evaporation, and 

 thus the percentage of run off, taking the basin as a whole, is small. 



This watershed area in a region less arid would be considered a sufficient one to 

 afford a reliable supply for a I'eservoir of even greater capacity than the one now 

 back of the dam. The extreme irregularity of the run off is shown by the following 

 table, made up from measurements taken during the entire period since the completion 



of the dam: 



Bun off of the Sweetwater uxUershed. 



