CHAPTER XXVI. 209 



acres, for which a portion of the supply, estimated at 16 per cent., was 

 taken from the direct flow of the other tributaries of San Diego river, 

 while the reservoir was filling. One-fifth of the water supply of the 

 system is usea for the domestic supply of San Diego, and the remain- 

 der for irrigation. The water is conveyed in a wooden flume, 5 feet 

 wide, 37 miles long, supported on a level bench cut in the hillsides, or 

 on high trestles crossing depressions. It delivers water to the pipe 

 system ten miles east of San Diego, whence it is distributed in iron 

 pipes under pressure throughout the irrigated settlements supplied. 



THE LA MESA DAM. 



At the lower end of the long flumes a reservoir has been constructed 

 on the mesa at an elevation of 433 feet, which serves to impound the 

 surplus flow of the flume, and catch the run-off of a small local shed. 

 The reservoir covers 70 acres, and has a capacity of 1,500 acre-feet. 

 The dam was built by the hydraulic sluicing method, by which the 

 material composing it was loosened, conveyed and deposited by water, 

 which was afterwards caught in the reservoir behind the rising dam. 

 This is one of the most interesting structures of its class in the coun- 

 try, and illustrates a simple and cheap method of building dams of 

 earth and loose rock. It contains 38,000 cubic yards, and was built for 

 $17,000. It serves a useful purpose in economizing the water and in its 

 distribution. 



The cost of the entire irrigation system, including the long flume, 

 was nearly $1,000,000. 



THE OTAY AND MORENO DAMS. 



All of the streams of San Diego county have the characteristics 

 of intermittent flow, which renders them practically valueless except 

 when their floods are controlled and impounded by reservoirs, but they 

 have the compensating advantage of possessing reservoir sites of 

 unusual capacity and excellence. Two of these sites have been utilized 

 by the Southern California Mountain Water Company, by the erection 

 of imposing dams of unusual height on the Otay creek, in Jamal 

 valley, and on Cottonwood creek, at the outlet of Moreno valley, while 

 a third, designated as the Barrett dam, is under construction at the 

 junction of Pine and Cottonwood creeks. 



The lower Otay dam is located some five miles southeast of 

 14 Sweetwater dam, and is an embankment of loose rock, 130 feet high. 



