22 University of California Publications in Zoology [ Vou. 14 
found. As thus computed, the mean depth of the bay, beyond the 
three-fathom line, was 8.10 fathoms, or 48.6 feet at low tide. From 
this average and that for the shoal water zone, the mean for the entire 
bay may readily be computed. This is found to be 22.7 feet (6.9 
meters) .§ 
It is of interest to determine, if only approximately, the volume 
of water in San Francisco Bay. As just shown, 226 square miles 
(= 144,640 acres) have a mean depth of 9 feet at low tide. This 
gives us 1,301,760 acre-feet as the volume of water in this shallower 
zone. The deeper portions of the bay comprise 119.4 square miles 
(= 76,416 acres), having a mean depth of 48.6 feet. These deeper 
portions thus contain 3,713,818 acre-feet. The total volume of the 
bay at mean low tide is, according to this computation, 5,015,578 aecre- 
feet (6,187,000,000 cubic meters). The amount of water present at 
high tide is greater than this by 1,077,568,° i.e., there are 6,093,146 
acre-feet (7,516,000,000 cubic meters) of water present at mean high 
tide. 
For certain purposes which will be discussed later, the bay as a 
whole has been divided into four quite unequal segments. The first 
of these includes San Pablo Bay, which is bounded below by a line 
passing from Point San Pablo to Point San Pedro. The second 
division extends from the latter line to one drawn from Bluff Point 
(Raecoon Strait) to Point Richmond. The third division extends 
from the preceding to a line through the Ferry Building and Goat 
Island Light. Finally, the fourth division extends from the latter 
line to the lower end of the bay. 
The mean depths of these divisions are as follows: 
1. 7.4 fathoms 
2. 8.7 fathoms 
3. 14.8 fathoms 
4. 7.8 fathoms 
The percentages of water by volume, at low tide, are as follows: 
. 17 per cent 
. 10 per cent 
. 36 per cent 
. 37 per cent 
HR oo DO 
8 This system is obviously a more exact one than that of taking a simple 
average of all recorded soundings for the bay. Since these soundings are much 
more closely grouped in the deeper parts of the bay than in the shoaler ones, 
the resulting figure would undoubtedly be too high. 
2 Derived as follows: There are 221,056 acres of water in the bay below the 
low-tide line. The mean tidal range being 4.52 feet, we have, for this area, a 
mean addition of 999,173 acre feet. There are, between high and low water 
mark, 34,688 acres. Here we may assume a mean depth, at high tide, of 2.26 
feet, and thus a volume of 78,395 acre feet. The sum is 1,077,568 acre feet. 
