Physiography


The Mobile-Tensaw River Delta is the second largest river delta in the U.S., second only to the Mississippi River Delta. It is characterized by many distributary rivers, streams, bayous and creeks which form a maze of waterways. The Delta is formed by the confluence of the Alabama and Tombigbee Rivers which combine to form the Mobile River, which in turn divides into several major distributaries: the Tensaw, Appalachee and Blakeley Rivers. The Delta itself covers over 400 square miles or 300,000 acres of swamps, marshes, and river bottomlands that are among the most impressive in the world, so impressive that Congress recently named the Delta a National Natural Landmark.
 
The Mobile-Tensaw River Delta receives a lot of water, from roughly 100,000 miles of rivers and streams in the Delta’s drainage area, the Mobile River Drainage Basin. The Mobile River Drainage Basin is one of the largest in the world and has the greatest yield of water per square mile than any river basin in the country. It is nearly 44,000 square miles and includes portions of Mississippi and Georgia as well as sixty-seven percent of Alabama. It is the funnel to the Gulf for seven major river systems (the Tombigbee, Black Warrior, Coosa, Tallapoosa, Cahaba, Alabama, and Mobile-Tensaw) draining portions of four states (Mississippi, Tennessee, Georgia, and Alabama) through ten different physiographic regions (including Alabama’s Cumberland Plateau, Valley and Ridge formation, Piedmont Uplands, and Coastal Plain.)
 
Such a bountiful realm of water is further enhanced by the Delta’s unique geology. Most river deltas are broad areas of alluvial deposit where the mouth of a river fans out meeting a lake or ocean. But the Mobile-Tensaw River Delta is quite different. It is believed to be a rare geological phenomenon. Rather than fanning out broadly to the ocean, the Mobile-Tensaw is instead an elongated delta formed, some experts say, as a result of a depression or sinking of the earth between two geological faults. Thus, the Mobile-Tensaw Delta is bounded on both sides by relatively high ground.
 
The Delta’s dimensions are about 10 miles wide and nearly 50 miles long and elevations range from six to zero feet above sea level, which has a very dramatic influence on soil moisture, and thus the plants that grow there. Over many thousands of years, alluvial sediments have been deposited throughout the Delta and expansion continues today as sediments gradually fill in Mobile Bay. The Delta lies well inland, more than 30 miles north of the Gulf; therefore, it is largely a freshwater system and one that has long been attractive for many forms of life.

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