1914} Sumner, et al.: Physical Conditions in San Francisco Bay 25 
figure may seem surprisingly large in view of the slight width of the 
communication between the bay and the ocean. But it is an inevitable 
deduction from the known facts regarding the area of the bay and 
the range of the tide. Moreover, a rough computation of the sectional 
area of the stream passing through the Golden Gate shows us that no 
very swift currents need be assumed in order to account for the dis- 
charge of this vast volume of water. 
Assuming the width of the narrowest part of this strait to be one 
statute mile (1.6 kilometers), and the mean depth along this line to 
be 168 feet (51.24 meters), we have a sectional area of 887,040 square 
feet, or about 20 acres (8.1 hectares). To allow of the passage of 
1,077,600 acre-feet (one and one-third billion cubie meters) of water 
within a period of six hours, we need only assume a mean sectional 
velocity, during this period, of 1.5 nautical miles (2.7 kilometers) 
per hour. Now we are told in the ‘‘Coast Pilot’’ that ‘‘in the Golden 
Gate on the ebb during spring tide a maximum velocity of 6 to 7 
miles per hour has been observed.’’? These last, of course, are maxi- 
mum figures, not average ones, and, furthermore, they are based on 
surface measurements, and do not represent the mean sectional velocity, 
which would be considerably less. 
No measurements of current velocity were made in the Golden Gate 
itself during the present survey. It may be worth while, however, 
to give the results of two series of observations, each extending through- 
out approximately one complete tidal cycle (about twelve hours), which 
were made at no great distance within the Golden Gate, and directly 
in the path of a strong current. At the points chosen, the stream 
was considerably wider than in the Golden Gate, but the depth was 
much less. Hence the current velocity was perhaps not much reduced. 
On the first of these occasions (station 5128, see Table 10 and text 
figs. F and R), the mean current velocity indicated during the flood 
period was 1.66 knots (nautical miles) per hour, the maximum rate 
being 2.63. During the ebb period, the mean velocity obtained was 
1.51 knots, the maximum being 2.23. It will be shown shortly that 
the ebb currents are, on the average, swifter than the flood currents. 
The exceptional condition here found resulted doubtless from the 
relative extent of the rise and fall during this particular period. 
During the first flood phase for the day, according to the ‘‘Tide 
Tables,’’ the water rose 4.1 feet at Fort Point, while it fell only 2.5 
feet during the succeeding ebb. 
occurs at Redwood City 56 minutes later than at San Francisco, and at Mare 
Island 1%4 hours later than at San Francisco. 
