The Coastal Plain
Four main rivers drain the Coastal Plain: Chattahoochee, Choctahatchee, Conecuh, Alabama, and Tombigbee-Black Warrior (Figure CPR1).
Figure CPR1. Drainage System in the Coastal Plain of Alabama. Dotted Red Line: northern boundary of the Coastal Plain.
With the exception of the Tombigbee, the rivers generally flow south to southwest across the physiographic districts and all cut though the prominent cuestas and hills. In many places the tributaries flow at right angles to the main streams, in the valleys and flatwoods. This pattern can be explained by differential erosive power of the rivers. The higher discharge streams had sufficient erosive power to cut through both resistant and soft materials and formed major southwesterly trending. The smaller tributaries eroded preferentially along the softer materials, thus forming the cuesta-flatwoods landscape.
The largest river is the Alabama, which forms at Wetumka (Elmore County) by the joining of the Coosa and the Tallapoosa. From there it flows southwest, and is joined by the Cahaba near Selma (Dallas County: Figure CPR2) and the Tombigbee near Mount Vernon (Mobile County), where it becomes the Mobile River before empting into Mobile Bay.
Figure CPR2. Junction of the Cahaba River (left foreground) and the Alabama River. Old Cahawaba Dallas County. (Image © Mike Neilson)
The Alabama has a well-defined alluvial plain (a plain made up of material deposited by the river itself) that is up to six miles wide, over which it meanders for all of its course (e.g., sinuosity from Montgomery to Selma is 1.7 and meanders have wavelengths in the vicinity of 10-12 miles south of Selma.. Smaller alluvial plains occur on the Cahaba, Black Warrior, Tombigbee and Chattahoochee (Figure CPR3)
Figure CPR3. Alluvial Plains in the Coastal Plain. Green: alluvial plain. (Cartography Research lab. University of Alabama)
To aid river navigation, three locks occur on the Alabama south of Montgomery. Two, Millers Ferry and Robert F. Henry, are also dams that generate hydroelectricity with 143 MW (megawatts) capacity.
The Alabama--which is the 4th largest river system in the United States based on discharge--brings 5 million tons/year of suspended sediment and 62,500 cfs (cubic feet per second) of water into Mobile Bay. The Mobile-Tensaw delta (Figure CPR4) has formed where the Alabama flows into Mobile Bay.
Figure CPR4. Mobile Bay and the location of the Mobile-Tensaw delta. (from http://www.southalabama.edu/aces/2002Reports/ChenDouglass.pdf)
The delta has a distorted triangular shape, beginning just south of I-65 near Creola (Mobile County) and widening to southwards to the I-10 bridge, where it is about 7 miles wide. The location of the southern terminus of the delta depends on the balance between sediment being transported by the river and deposited in the delta and erosion of that sediment by wave and tidal action and human activities.
The Tombigbee River is now part of the Tennessee-Tombigbee Waterway (Figure CPR5). Approved by Congress in 1946, the aim of the waterway was to shorten the distance (and therefore the cost) of freight transported from the Tennessee River to the Gulf Coast. To that end, the project included a 27 mile 12 foot deep navigation canal to join the headwaters of the Tombigbee to Pickwick Lake on the Tennessee River, 205 miles of canals on the Tombigbee River, 5 dams and 10 locks. Two locks and dams (the Heflin and Bevel) were built in Alabama, the rest in Mississippi. The project cost $2 billion.
Figure CPR5 Locks and dams along the Tennessee-Tombigbee Waterway and the Black Warrior River (http://www.tenntom.org/bigmap.htm)