1914] Sumner, et al.: Physical Conditions in San Francisco Bay 21 
According to other estimates, the total area of San Francisco Bay 
is somewhat greater than is here stated. Thus Professor George Dav- 
idson, according to some manuscript notes, placed at our disposal by 
Professor Holway, calculated this as 435 square miles, while McAdie 
(1913) gives 420 square miles. These differences are probably due 
to the inclusion in the other estimates of salt marshes or estuaries 
which have been excluded in our computations. 
Tt is evident that, throughout the greater part of its extent, San 
Francisco Bay is a very shallow body of water. Seventy per cent 
of its area is less than three fathoms (5.5 meters) deep, while more 
than 80 per cent is less than five fathoms (9 meters) deep. In ex- 
tensive regions within the upper and lower sections of the bay navi- 
gable water is confined to a comparatively narrow central channel, 
Considerable depths occur, however, in the middle section of the 
bay, nearest to the Golden Gate. Thus depths of from 25 to 30 fath- 
oms are reached between Angel and Aleatraz islands, while 36 fathoms 
(66 meters) are recorded at either end of Raccoon Strait, and 63 
fathoms (115 meters) in the narrower part of the Golden Gate. The 
great depth of these straits is plainly due to the scouring action of 
the tidal currents. Outside the Golden Gate the water rapidly be- 
comes shallow again, soundings of only five fathoms being encoun- 
tered at a distance of about five miles off shore. Here we find an 
elongated curved shoal, known as ‘‘The Bar,’’ surrounding the outer 
entrance to the Golden Gate, throughout an are of about 270°. 
The mean depth of San Francisco Bay at mean low tide has been 
computed according to the following method: It has been seen above 
that the area comprised between the three-fathom line and the zone 
exposed at low tide is 226.0 square miles. Since the depth throughout 
this area may be supposed to be graduated fairly uniformly from 0 
to 3 fathoms, a mean depth of 114 fathoms (= 9 feet) may be fairly 
assumed. 
There are, as has been shown, 119.4 square miles of water having 
a depth greater than three fathoms. In order to determine the mean 
depth of these portions of the bay, they were divided on the chart 
(C. and G. S. chart No. 5530) into small rectangles of an arbitrary 
size.” The mean of the published soundings within each rectangle 
was obtained, and then the average of these various averages was 
7 These rectangles, 204 in number, covered 1 minute of latitude, and were 
1 em. in width. Partial (marginal) rectangles were included in the compu- 
tations, provided that over half of their area was occupied by water more than 
three fathoms deep. 
