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.
