Because of the complexities of bay and estuarine systems, sophisticated mathematical models are being employed to an increasing degree in water quality management planning. A myriad of inputs are required for these modeling efforts, and one of the most difficult phenomena to define is the exchange of bay and ocean waters which takes place because of tidal action at the mouth of an estuary. Not all of the water which ebbs from a bay is returned to it. Some is transported away by ocean currents and replaced by new ocean water. Simply stated, tidal exchange ratio is that portion of the flooding tide which is ocean water entering the bay for the first time.