36 



PRECIPITATION, FLOOD HABITS AND WATER DELIVERY, 

 SACRAMENTO AND SAN JOAQUIN RIVERS 



Some idea of the relative amount of fresh water reaching the bay region can be 

 obtained from the rainfall records, if the fact is borne in mind that run-off is not 

 proportional to rain, but that in wet years, that is in years of much rainfall, the portion 

 thereof which reaches the streams is larger than in dry years or years of light rainfall. 

 The climatic years and not the calendar years are here referred to. A diagram is 

 presented (fig. 17) showing the relative annual precipitation in central California 

 since 1849, in per cent of the normal annual rainfall. The beginning of the climatic 

 or seasonal year was taken at Juls' 1st. 



The normal annual delivery of fresh water into San Francisco Bay by the two 

 principal rivers which discharge into the bay is approximately 35,000,000 acre feet. 

 About 85 per cent of this water reaches the bay in the first six months of the year, 

 and rather less than 15 per cent thereof in the last six months. Fig. 17 also shows the 

 seasonal water output of Sacramento River, based on a summation of the discharges 

 of the Sacramento River at Red Bluff and its principal tributaries at foothill points, 

 not including Stony, Cache and Putah creeks, or other lesser tributaries. For the 

 years preceding the period covered by records of the U. S. Geological Survey, the 

 discharge for the climatic year has been approximated from rainfall and it has thus 

 been possible to give the eye a picture of the relative amount of water annually dis- 

 charged into Sacramento Valley by these streams for the full period of 70 years 

 covered by rainfall records. 



What is presented in the diagram, although applying to only a part of the area 

 tributary' to the upper end of the bay, being expressed in per cent of normal flow, 

 may, nevertheless, be accepted for the present as fairly well representing the relative 

 annual fresh water accession of the bay from both the Sacramento and the San Joa- 

 cjuin Rivers, subject, however, to some correction for greater diversion for irrigation 

 in recent years than formerly. 



The normal low water output of the Sacramento and San Joaquin Rivers at the 

 point where they discharge into Suisun Bay, as it was before disturbance by human 

 activities, is not ascertainable with any great degree of precision. In the early history 

 of the State, measurements of the stream flow were not made, and now the effect of 

 human activities upon this flow is material. 



To give some idea of the amount and range of the annual water delivery into 

 Suisun Bay by the Sacramento River, which delivers about four times as much water 

 as the San Joaquin, table 1 has been prepared showing the flow of the river, by months, 

 in selected recent years. 



The figures presented in the table, being a summation of the discharges of 

 Sacramento River at Red Bluff, of Feather River at Oroville, of Yuba River at Smarts- 

 ville, of Bear River at Van Trent, and of American River at Fair Oaks, will give 

 some idea of the normal discharge of Sacramento River and the range of its mean 

 monthly discharge, all amounts noted being based on the estimates of the U. S. 

 Geological .Survey. 



Not only should the facts relating to the variation in the seasonal delivery of 

 water by the rivers into the bay be given consideration when the degree of salinity 

 in different years is compared, but it is likewise important to bear in mind that the 

 mean monthly discharge of the rivers from the mountains into the \alley varies 

 within wide limits. The natural normal combined flow of the two rivers for the several 

 months shows a range from a minimum monthly mean of about 7000 second-feet to 

 a maximum of about 70,000 second-feet, while in certain individual years the range 



