The westernmost boundary between Florida and Alabama is formed by the Perdido River along its thalweg (lowest point) up to 31°N.
The northern boundary of Florida follows along 31°N from the Perdido River to the Chattahoochee River, then south to the confluence of the Chattahoochee and the Flint River. It then follows a straight line mostly east to the source of the Saint Mary’s River, along the thalweg to the ocean.