1914] Sumner, et al.: Physical Conditions in San Francisco Bay 27 
As regards seasonal variations, the following figures are offered : 
2nd period 4th period 5th period 
(April—May ) (October) (Nov.—Dec.) 
10} 0) Npeeceerr ee sete 1.72 1.62 1.61 
I foyey ll eee 1.27 1.33 0.93 
The differences here shown may have been due either to differences 
in the range of the tide during the period of observation, or to dif- 
ferences in the amount of fresh water coming from the rivers. The 
distinctly higher averages for the ebb current are probably due to 
the latter influence. In the ‘‘Coast Pilot,’ p. 74, we read: ‘‘At 
periods of great freshets in the Sacramento and San Joaquin rivers 
there have been instances of very slight surface current, or none at 
all, on the small flood in the Golden Gate.’’ 
The rate of the tidal current in a given locality may have an 
important effect upon the occurrence of various sedentary organ- 
isms. The powerful streams passing through Raccoon Strait and the 
Golden Gate prevent the accumulation of mud or fine sand, and thus 
are responsible for the stony or gravelly bottoms which prevail there. 
On the other hand, fixed or slow-moving organisms are largely de- 
pendent for their food upon particles which are passively brought to 
them in the tidal currents. It is perhaps worth while to devote a few 
moments to computing the mean rate of tidal flow over the bottom of 
San Franciseo Bay as a whole. 
The mean current velocity, as derived from the above discussed 
observations at the regular stations, was about 1.4 knot per hour. It 
must be recalled, however, that the meter was placed but a few feet 
below the surface. This is about the level at which the maximum rate 
of flow is found. As the bottom is approached the effect of friction 
increases, until just above the bottom the rate must be very materially 
reduced. Now it is evidently this reduced rate of flow at the bottom 
which conditions the supply of food and oxygen for the various organ- 
isms of the benthos. 
It must also be borne in mind that our current readings in the 
northern and southern sections of the bay were all made in the navi- 
gable waters of the central channels. where the currents were naturally 
swifter than in the marginal shoal regions. Another circumstance 
which has doubtless contributed toward giving us too high a mean 
figure for the current velocity throughout the bay is the fact that no 
readings were taken during the period of slack water. The meter was 
used only during times of visible flow. 
