76 University of California Publications in Zoology [ Vou. 14 
found in the second period, when the largest amount of fresh water 
was being discharged into the northern end of the bay. On the other 
hand, the range fell to nearly its lowest point in Oetober, when the 
salinity throughout the bay as a whole was greatest. Why the actual 
minimum oceurred in January, rather than in October, is not so clear. 
We regret our inability to compare our annual salinity curve with 
one representing the discharge of the rivers for the entire period during 
which our water samples were collected. Unfortunately, at the date 
of writing this report, the figures for the river flow have been com- 
puted only to September, 1912. The estimated run-off of the Saecra- 
mento and San Joaquin rivers is, however, given herewith for the first 
nine months of that year. We are indebted to the United States Geo- 
logical Survey for the use of the unpublished records for the last three 
months. 
Acre-feet 
January 1,266,000 June 2,498,000 
February 972,000 July 772,000 
March 1,714,000 August 471,000 
April 1,616,000 September 498,000 
May 3,446,000 
It will be seen that in this particular year, which was, in that 
respect, typical, the month of maximum river flow was May. It was 
during the second period of our hydrographic observations. which 
fell in late April and early May, that the lowest mean salinity for 
the year was found. In another year this minimum might have been 
found during the February period of observations. An examination of 
the various years’ records for the run-off of the Sacramento and San 
Joaquin systems discloses not only very great differences in the total 
discharge from year to year, but equally great differences in the 
distribution of their discharge according to seasons. In some years 
the maximum river flow falls in January, in others the maximum is 
not reached until May. It is obvious, therefore, that no typical 
salinity curve for the bay could be based upon a single year’s 
observations. 
Since one of the most important of the physical factors affecting 
life in this bay, its salinity, varies inversely with the amount of water 
discharged by the two great river systems emptying into it, a some- 
what detailed consideration of the flow of these rivers seems desirable. 
Our estimates of river flow have been computed from Water Supply 
Papers 298 and 299 of the United States Geological Survey (Me- 
