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. George 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. The lagoon has been designated as a National Estuarine Research Reserve and the Apalachicola River is the largest source of freshwater to the estuary. Combined with the Chattahoochee River, Flint River, and Ochlockonee River they drain a watershed of over 20,000 square miles (50,000 km²) at a rate of 19,599 cubic feet (550 m³) per second according to the United States Geological Survey in 2002.