80 University of California Publications in Zoology [ Von. 14 
the proportions above stated, the resulting water would doubtless be 
of a much higher salinity than that actually found. 
In reality, these accessions of fresh and salt water behave quite 
differently from one another. The former gives rise, despite tidal 
fluctuations, to a net southward movement of the waters of the bay, 
from Carquinez Strait to the Golden Gate. There thus tends to be a 
progressive, though intermittent, displacement of the salt water by 
that from the rivers. If the action of the tides should suddenly cease, 
the upper arm, at least, of San Francisco Bay would soon become 
perfectly fresh. 
On the other hand, the water from the sea—itself diluted—enters 
but a comparatively short distance into the bay, after which it passes 
out to the ocean again. Assuming a mean velocity for the flood 
current of 1.2 nautical miles per hour (see p. 26), none of this outside 
water can commonly penetrate more than seven or eight miles beyond 
the Golden Gate.2t This would carry it but a third or a fourth of 
the way to either end of the bay. Within this central area it displaces 
a great volume of the water already present, driving it both to the 
north and the south. The incoming water doubtless mixes, to a consid- 
erable extent, however, with the bay water, along a very irregular 
boundary. But the water which recedes on the ebb tide is largely the 
same as that which enters on the flood, somewhat augmented and 
diluted, to be sure, as a result of the continued influx of fresh water 
at the head of the bay. 
Beyond this central area which may receive water directly from 
the sea outside, the flood stream merely shifts the bay water from 
regions of higher salinity to regions of lower salinity, mixing the two 
together to a greater or less extent. Thus, even in San Pablo Bay, 
which receives the entire discharge of the two great rivers, the mean 
salinity indicates the presence of sixty per cent of sea-water, while 
in Carquinez Strait itself the mean salinity is nearly half that of 
sea-water. 
If we could determine for any considerable period the mean 
salinity of the water entering the bay through the Golden Gate, as 
well as that of the water passing out with the ebb stream, these values, 
together with the mean volume of the water discharged with each 
tide, would give us the rate of inflow of the fresh water during this 
21 In reality such a distance would be reached only in the midchannel and 
near the surface. 
