Apalachicola Bay is an estuary and lagoon located on the northwest coast of the U.S. state of Florida. The Apalachicola Bay system also includes St. Georges Sound, St. Vincent Sound and East Bay, covering an area of about 208 square miles (540 km²). Four islands St. Vincent Island to the west, Cape St. George Island and St. George Island to the south, and Dog Island to the east separate the system from the Gulf of Mexico. Water exchange occurs through Indian Pass, West Pass, East Pass and the Duer Channel.