The Perdido River, also historically known as Rio Perdido or by its native name of Cassaba, is a 65.4-mile-long (105.3 km) river in the U.S. states of Alabama and Florida; the Perdido, a designated Outstanding Florida Waters river, forms part of the boundary between the two states along nearly its entire length and drains into the Gulf of Mexico. During the early 19th century it played a central role in a series of rotating boundary changes and disputes among France, Spain, Great Britain, and the United States.
It rises in southwestern Alabama in Escambia County approximately 8 miles (13 km) northwest of Atmore. It flows south approximately 5 miles (8 km) to latitude 31°N, south of which it forms the remainder of the Alabama/Florida border. It flows generally east-southeast in a winding course and enters the north end of Perdido Bay on the Gulf of Mexico, approximately 10 miles (16 km) west of Pensacola.
The word "perdido" is Spanish for "lost".

Quelle: Wikipedia