74 University of California Publications in Zoology [ Von. 14 
observers at three points: (1) at Red Bluff, representing the Saecra- 
mento Valley; (2) at Fresno, representing the San Joaquin Valley, 
and (3) at San Francisco, representing the bay region."® 
Nn 
o = 
JI 
30 
29 
od 
ey 
a ee a 
Jan. Feb. Mar Apr. May Jun. Jul Aug. Sept. Oct. Nov. Dec. et 
isle 
Fig. 8S. Relation between the salinity of San Francisco Bay and the rain- 
fall of the regions tributary to it. The curve for rainfall (upper) is based upon 
the mean of the monthly precipitation (in inches) at Red Bluff, Fresno and San 
Francisco, during the year covered by the hydrographic observations. 
The relations which are discoverable between these curves are ones 
which might have been anticipated. There is plainly a general cor- 
respondence between the period of greatest rainfall and that of lowest 
salinity, and between the period of least rainfall and that of the 
highest salinity. That the period of greatest salinity should lag 
somewhat behind the period of least rainfall is likewise a condition 
which we should have expected to find, since the effect of the early 
fall rains would not be evident immediately in the discharge of the 
rivers. In the diagram it is plain that the salinity of the bay con- 
tinued to increase between the July period of observations and the 
October period, although there was a mean rainfall at the three points 
named of nearly two inches in September (chiefly during the first 
week of that month). 
18 While the three points chosen are doubtless not situated in the partieular 
localities which supply the greatest amounts of water to San Francisco Bay, 
their fluctuations of rainfall are probably proportionate to those at the chief 
sources of this water. ; 
