The inequality, at least in the central part of the bay, is not due to the water flowing down the Sacramento River. Even in the spring, when the rain runoff is maximum, the river flow contributes little to the flow through the Golden Gate. If water is not created or lost, one might think the average flow past every location must be zero: the water that flows past the location during ebb must be the same as that flowing back during flood.
However, the spatial pattern of currents is typically differnt during the flood and ebb. As an example, while the current in the middle of the Golden Gate tends to be larger than currents at the sides, currents on the North side are different than those on the South, i.e. the current profile is quite assymetric. What is important for the present issue, is that this current profile changes from the flood to the ebb current cycle. In addition, the time profile of current velocity is different from flood to ebb.
If the amount of water that flows in the Golden Gate at flood is equal to the amount that flows out at ebb, it is only required that the total flow be the same in each direction. The total flow is calculated by adding up the flow at each location accross the Golden Gate, and this process must be repeated at short time intervals, e.g. each minute, since the flow is changing with time. After this laborous calculation (integration over space and time) we will see that the total flow in each direction is the same.
|If the max flood and max ebb currents are very different, the times of slack currents are not evenly spaced between times of max flood and max ebb.|
|The solid red line represents a current with a mean value of zero (horizontal solid black line). The dashed red line represents a current with a large negative mean value (horizontal dashed black line).
The times of maximum flood and ebb are not different for the two currents. However, for the current with a negative mean, the time of slack after the flood is shifted to an earlier time, and the time of slack after the ebb is shifted to a latter time. Each of these shifts is indicated by the pair of grey vertical lines.
In the table of currents, the biggest offset was for the current South of Angel Island: 0.7 hours or 42 minutes.