Hydrologic and beach dynamics studies were conducted in and near Rookery Bay Sanctuary south of Naples on the southwest coast of Florida. These studies determined water exchange rates, tidal range variations, the characteristics of the tidal prism, bottom contours, the basin capacities and changes occurring in the geometry of the channels connecting the Sanctuary system with the Gulf of Mexico. Rookery Bay has an average depth of 3.0 feet, a mean volume of 135 million cubic feet and an effective mean tidal prism of 154 million cubic feet. Data on mean high tide levels at four stations for 23 months were presented. The changing configurations of Keewaydin and Little Marco Islands are illustrated and the implication for the Sanctuary discussed.