The Chattahoochee River forms the southern half of the Alabama and Georgia border, as well as a portion of the Florida - Georgia border. It is a tributary of the Apalachicola River, a relatively short river formed by the confluence of the Chattahoochee and Flint rivers and emptying from Florida into Apalachicola Bay in the Gulf of Mexico. The Chattahoochee River is about 430 miles long. The Chattahoochee, Flint, and Apalachicola rivers together make up the Apalachicola–Chattahoochee–Flint River Basin. The Chattahoochee makes up the largest part of the ACF's drainage basin.