Max flood and max ebb current velocities are not be equal, i.e. the current averaged over a complete tidal cycle is not zero.

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.