Wetland and Habitat
The Mobile-Tensaw River Delta is one of the largest intact wetland ecosystems in the United States. Overall, it comprises approximately 260,000 acres of wetland habitats, ranging from submersed grass beds to cypress-gum swamps and seasonally flooded bottomland hardwoods, and is greatly influenced by numerous local conditions which together comprise the physical environment of the wetlands. Residents of south Alabama can attest to the effect a very mild and almost subtropical climate, coupled with 50-60 inches of annual rainfall, has on plant growth.
The wetland soils are also rich in phosphorous, nitrogen, as well as high levels of organic matter deposited during floods. However, the two most important environmental factors that affect the wetlands, and ultimately the whole system, are: 1) the hydrologic regime, or manner in which the rivers flood and move water across the landscape, and 2) the geomorphic features, or the physical characteristics which result from movement of the water across the Delta.
While the immediate area surrounding the Delta receives annual rainfall in excess of 50 inches, the more important rainfall events are those that take place within the Delta’s entire watershed. A 1982 Alabama Geological Survey report described the flows of water from the Mobile Bay Drainage and stated “the Mobile River below the confluence of the Tombigbee and Alabama Rivers has an average flow of 39,300 million gallons of water per day.” The timing, intensity, and duration of the river’s flooding of the Delta, collectively known as the hydroperiod, ultimately have the foremost impact on the species composition, structure and function of the wetland’s plants. Floods in the spring and summer months have a greater impact on tree survival and plant growth than flooding in the fall and winter.
The movement of water traveling along rivers through the floodplain causes a constant process of natural sediment erosion and deposition, producing numerous variations in wetland habitats. The meandering river channel produces oxbow lakes that develop into deepwater alluvial swamps. Sloughs form in meander scrolls. Sand bars form on the inside of river curves. Natural levees form where coarse sedimentary material settles out along the river channels as floodwaters recede, while backwater areas, or backswamps, often form in depressions located between the natural levees and valley walls. Each of these variations of wetland habitat has specific soils and elevations, and thus distinct periods of inundation by water. The combination results in specific plant species, and hence discrete plant and animal communities.
Within the Mobile-Tensaw Delta, several terrestrial habitats are readily discernable to casual human observation as you move in a general north/south direction along the rivers. Many of these same habitats, and others, are less discernable to casual observation as you move laterally or in a general east/west direction along the rivers.
Starting at the northern end of the Delta, the Alabama and Tombigbee Rivers form a long peninsula of over 36,000 acres of the most extensive area of seasonally flooded natural levee bottomland hardwoods in Alabama. This area, called Fork of Rivers, is perhaps one of the most extensive intact examples of this wetland community type left in the entire southeastern United States. The seasonally flooded natural levee forests grades into deep alluvial swamp forests of bald cypress and tupelo gum as you move south. These deepwater swamps have surface water throughout all or most of the growing season but water levels vary seasonally and annually.
Moving laterally within this area, along the banks of the present day channels of the Middle and Tensaw Rivers, bald cypress and tupelo dominate the species which are common along the open water river corridors. They transition into the river swamp forest, or deepwater alluvial swamp, which naturally supports bald cypress and water tupelo in equal numbers. Common shrubs include common alder, buttonbush, swamp privet, fetterbush, and black willow. Typical herbs include golden club, swamp lily, and fragrant water lilies. Spanish moss is a common epiphyte growing among the crowns of trees. These habitats can easily be seen from the I-65 bridge that dissects the Delta north of Mobile.
Moving further laterally from the river channels, oak dominated bottomland hardwood forests occur in areas where temporary flooding is common. The lower hardwood swamp forest, sometimes referred to as the first bottom, is characterized by a greater diversity of woody species. These include overcup and laurel oak, water hickory, river birch, red maple, and green ash, as well as cottonwood in disturbed areas. Non-woody plants include lizard’s tail, jewelweed, and several species of ferns. Lower hardwood swamp forests do not stay wet during the entire growing season, nor do they flood every year.
Between the first bottom and the uplands reside two additional habitat types: backwaters and flats, sometimes referred to as the second bottom, and the upper hardwood swamp – a transitional area to true uplands. Backwaters and flats are intermediate in the elevation profile of the system. Trees include those which are common in first bottoms as well as American elm, water oak, willow oak, sweetgum, and sycamore. Vines such as greenbrier, poison ivy, trumpet creeper, and grape species become more common. Grasses and sedges also become more prevalent. This habitat, which is most prevalent in the north and central portion of the Delta and fades as you move south towards Mobile, has standing water during less than 25% of the growing season.
The upper hardwood swamps lie next to the true uplands that indicate an exit from the delta. It includes the highest elevations of the floodplain, and encompasses the natural levees and terraces as well as very old ridges and dunes. Tree species composition includes more upland species than other Delta habitats, due to their tolerance for periodically saturated soils. Species include white oak, swamp chestnut oak, cherrybark oak, bitternut hickory and spruce pine. Holly, pawpaw, spicebush, wax myrtle and saw palmetto are common beneath the trees. Upper hardwood swamps are only covered with water for very brief periods during the growing season, flood no more than 50% of the years within a 100-year cycle, and have a water table which is below the soil’s surface.
Continuing further downstream, a maze of bayous, creeks, distributary channels and other watercourses bisect the Delta between the Tensaw River and Big Lizard Creek adding to the hydrologic and ecologic diversity of the system. Along the lower stretches of the blackwater streams which flow into the lower portion of the Delta, bay forests occur on sandy, acidic soils bordering the streambanks. High water tables along with low relief provide conditions for the development of organic peat layers which are very acid and saturated almost year-round.
Tidal freshwater marshes occur in the extreme lower portion of the Delta near the mouth of Mobile Bay. They occupy large expanses of the low flats at the mouth of the Delta and in recently emergent bottoms along the margins of creeks and rivers upstream five to ten miles. Marshes represent a pioneer habitat type, being the first established emergent plant community in the slow velocity, shallow depositional setting of the Delta mouth. Many marshes have been shown to be nutrient traps that purify water flowing through them. The marshes are subject to a daily tidal fluctuation of approximately 1.5 feet, and during low river flows in late summer they are inundated by increasingly brackish waters which move up Mobile Bay from the Gulf of Mexico. Submersed aquatic vegetation also occurs in the large shallow bays in the southern portion of the Delta and in small areas located along the smaller tributaries and in pockets along the margins of the large rivers.
The underlying theme to this wetland landscape is that water and soil elevations govern the dynamics of the plants that grow there. The Mobile-Tensaw River Delta is an outstanding wetland resource representative of the forested expanses of the past. The Delta and the Mobile Bay are clearly linked together hydrologically and ecologically via the flow-through system of water and nutrients.
The importance of the Delta wetlands can also be defined in economic terms through its contribution to the health of the downstream estuary. The commercial and sport fisheries in Mobile Bay and the adjacent Gulf of Mexico contribute significantly to the Alabama coastal economy. The crab, oyster, shrimp and finfish populations of Mobile Bay are dependent on a healthy Delta ecosystem, including the bottomland hardwoods. Without the bottomlands to retain floodwaters and receive their sediments, the soil and large pulses of freshwater could have disastrous effects on the downstream bay. The bay is adapted to periodic and gradual increases in freshwater and silt, but too much at once could kill many of the plants and animals vital to the estuary’s functioning. The bay ecology is also dependent on the abundance of rich organic matter that provides the energy and nutrients for the aquatic food chain. Much of this organic matter is produced in the forested wetlands of the upper Delta and brought downstream to Mobile Bay.
One should also consider that the habitats described above are complex conglomerations of neighboring habitats, interwoven with numerous drains and small streams. It is critical that the forested wetlands of the Delta, especially the seasonally flooded communities, be protected and managed properly for the long-term maintenance and future preservation of this ecosystem and our economy.