The Cahaba River is Alabama’s longest free flowing river, meaning there are no dams on the river for hydroelectric power. However, there are water control structures than can limit the migration of some fish and mussel species. The entire river is over 190 miles long, traveling through 5 counties and 3 of Alabama’s physiographic regions. The Cahaba River begins in the Valley and Ridge region bounded by the Piedmont to the southeast and the Cumberland Plateau to the northwest. The biological characteristics of the river are divided into two parts at the fall line; the upper and lower Cahaba, each part of the river about 100 miles long. The fall line near Centerville, Alabama separates hard Piedmont rocks of the Appalachian Highlands from the softer Atlantic Plain rocks. The 100-200 foot elevation change at the fall line also divided aquatic animal populations.
Two rivers make the main channel of the Cahaba River in Birmingham where the Little Cahaba River – generating from the Birmingham Water Works Board secondary drinking water source at Lake Purdy– meets the main Cahaba River flowing from St Clair County. Near Centerville, Alabama the water enters the Gulf Coastal Plain from the Appalachian Ridge and Valley region. The Cahaba joins the Alabama River southeast of Selma.