Tidal Exchange at Golden Gate

Tidal exchange can be a powerful mechanism for flushing pollutants from bays and estuaries. Tidal exchange is defined by a ratio describing the portion of the flooding tide which is ocean water entering the bay for the first time. An interagency program, headed by Brown and Caldwell and the State of California Department of Water Resources, was organized to undertake the measurement of tidal exchange ratio at the mouth of the Golden Gate. A theory was developed which allowed tidal exchange to be derived from measurements of a naturally occurring tracer, chloride. Field measurements of the tracer were made on two occasions for an ebb tide and a following flood tide, and for background ocean conditions. An exact description of flow through the ocean-bay interface is not required. Tidal exchange during the dry weather period of low fresh water inflow to the estuary was found to correlate with flood tide range and to average 24%.