GROUND WATER SUPPLY AND NATURAL LOSSES IN THE 



VALLEY OF SANTA ANA RIVER BETWEEN RIVERSIDE 



NARROWS AND THE ORANGE COUNTY LINE 



By Harold C. Troxell * 

 Description of area. 



One of the most useful streams in southern California is the Santa 

 Ana River. It rises in the heart of the San Bernardino Mountains 

 above San Bernardino and flows westward across the San Bernardino 

 Valley, southwestward through the Jurupa Basin and along the south 

 edge of the Chino Basin, through the lower canyon in the Santa Ana 

 Mountains, and across the coastal plain to the Pacific Ocean near New- 

 port Beach. Flow through this channel from the mountains to the 

 ocean is continuous only during the winter flood periods. The section 

 of the river channel involved in this report extends from Riverside 

 Narrows to the Orange-Riverside County line. 



South of the Jurupa Mountains and northwest of the city of 

 Riverside the Santa Ana River passes through a granite canyon known 

 as Riverside Narrows. This bedrock obstruction forces most of the 

 water to the surface, forming a stream that flows continuously as far 

 as the intake of the canal companies in Orange County. Most of this 

 water at Riverside Narrows is return water from the irrigated areas 

 around Riverside. According to old settlers the channel was drj^ dur- 

 ing summer periods prior to the irrigation developments. The earlier 

 measurements of discharge at this point showing the increase in dis- 

 charge are given by W. C. Mendenhall in "Hydrology of San Ber- 

 nardino Valley," (U. S. Geol. Survey Water-Supply Paper 142). 



About 16 miles down stream from Riverside Narrows the Santa 

 Ana River passes through a secondary coast range, the Santa Ana 

 Mountains. These mountains are made up of shale and sandstone and 

 form a barrier that concentrates and forces to the surface most of the 

 underflow. In this 16-mile stretch the Chino Basin drains into the 

 river along the north bank. The mountain streams of this area are 

 exceedingly steep and flow over bare rock. Many of the storms that 

 occur in southern California are violent and, falling on these mountain 

 drainage basins, produce floods that rush across the plains, carrv^ing 

 large quantities of granitic detritus. In this way the Chino Basin 

 has been built up. These great beds of gravel and boulders have a 

 high percentage of voids, and the flood waters passing over them are 

 greatl}^ reduced, if not entireh- absorbed, adding to the supply in the 

 imderground reservoir. The outlet of this underground reservoir is 

 by seepage into the Santa Ana River between Riverside Narrows and 

 Prado and by evaporation and transpiration in the bottom land of this 

 stretch of the river. The velocity with which the water passes through 

 the gravel is very slow, and the water is doubtless delivered to the river 

 valley at a nearly uniform rate. 



The Santa Ana Mountains and Temescal Basin, which drain into 

 the river along the south bank, undoubtedly make a much smaller 

 contribution, except possibly during storm periods. 



* Associate Engineer, Water Resources Branch, U. S. Geological Survey. 



(147) 



