Estuaries separated from the ocean by barrier islands are numerous on the Atlantic and Gulf of Mexico coastlines in general and on the Florida coastline in particular. Attention is focused on the exchange processes of waters behind barrier islands. Part I deals with the non-dispersive aspects as affected by tides and freshwater discharges. A computational procedure is developed with reference to Lake Worth and Sarasota Bay, Florida. The tide-induced net flow, through one or two inlets which can be viewed as a 'mass-transport' effect, is stressed. This net flow can be significant in water exchange considerations and could be enhanced by the proper design of inlets. Field and/or model corroboration of this conclusion is recommended. Part II of the report considers the renewal of estuary waters resulting from dispersive transport in tide-driven flows. Assuming quasi-steady state conditions, dispersion coefficients are presented, based on field measurements in Lake Worth and Sarasota Bay. (Author)