354 IRRIGATION INVESTIGATIONS IN CALIFORNIA. 



experiences, in order that it may become easier to utilize the public streams and the 

 water going to waste. The obstacles which nature places in the way of executing 

 such works are quite sufficient without having to overcome legal barriers and 

 restrictions of a burdensome nature. These natural obstacles are such as to restrict 

 the number of enterprises which are practicable from an engineering standpoint to a 

 very few, and consist chiefly in a lack of a satisfactorj' combination of conditions 

 essential to success. These requisite conditions ai'e, brieflj' stated: 



First. A dam site, preferably in a narrow gorge, whose foundations are suitable 

 for the erection of any height of dam required, of any type that ma}' be selected. 



Second. A capacious valley above the dam site, with little fall or grade, and 

 affording a reservoir site of capacity commensurate with the size of the stream, with 

 a reasonable height of dam. 



Third. An area of arable lands and a resident population requiring water suffi- 

 ciently extensive to consume all that may be stored in the reservoir, and capable of 

 producing crops of a character and value which will admit of the paj-ment of water 

 rates demanded; these lands being below the elevation of the dam, in its near vicinity, 

 and readil}' commanded by the stoi'ed water through gravity conduits. 



Fourth. A watershed above the reservoir of sufficient area to afford adequate 

 run off from the normal rainfall to fill the reservoir periodicalh' with certainty. 



Fifth. A fairly reliable avprage rainfall. 



It will be seen at a glance, with a moment's reflection, that the conjunction of all 

 these conditions, each of a favorable nature, must necessarilj' be rare. The great 

 majority of the mountain gulches, canyons, and torrents, though abounding in dam 

 sites, have no reservoir sites, except perhaps so near the crest of the mountains as to 

 afford but limited water supply to them. 



For these reasons it becomes all the more essential that there be no friction 

 between such enterprises as are feasible and the laws of the land which control them. 



SWEETWATER RIVEE. 



PHYSICAL CHARACTERISTICS. 



Sweetwater River heads in the Cuj'amaca Mountains northeast of San Diego, at 

 an elevation of about 6,000 feet, and empties into the Bay of San Diego but 7 miles 

 north of the Mexican boundary (PI. XXVIII). Its extreme air-line length is 41 

 miles, and the total area of its watershed above its mouth is 216 square miles. The 

 watershed is a comparatively narrow trough, 2 to 8 miles wide from crest to crest, 

 and over a portion of its length the stream occupies a rockj' gorge that is almost 

 impassable; in fact, there occurs a succession of these gorges from half a mile to 5 

 miles in length all the way down the stream. The lowest of these is 7 miles above the 

 mouth of the stream, and here is located the Sweetwater Dam, a masonry structure 

 which has become quite universally known (PI. XXIX). The reservoir above the 

 dam is 3.5 miles in length, and occupies the whole of a vallej' lying between this lower 

 gorge and the one next above. Both of the lower gorges are cut through the great 

 porphyry dike which traverses the whole of San Diego Countv parallel with the 

 coast, immediately above which and in contact is granite extending back to the crest 

 of the range and beyond. The site of the dam was extremely favorable for the 



