Mississippi River Alluvial Plain Ecoregion
Description & Physiography
Sprawling across parts of seven states, from southern Louisiana to southern Illinois, the chalk-outline of the Mississippi River Alluvial Plain (MSRAP) delineates multiple storylines: a long and compelling geological record, an ever-present cycle of floodwater and sedimentation, and a habitat formed in equal measure by these natural forces and those of humankind. If that sounds like exaggeration, it is helpful to remember that the entire watershed drains all or parts of thirty-one states, two Canadian provinces, and over 1,000,000 square miles before it finally reaches the Gulf.
A geologically complex area, most defining feature of the MSRAP is the Mississippi River itself, which flows south over the Mississippi Embayment, a structural trough in the earth’s crust that, over the past one-to-two hundred million years, has thrust alternately upward and downward relative to the sea. Headwater or mainstem flooding results from rainstorms over the watersheds of the Mississippi's tributaries, and produces great spring floods that have been a boon to agriculture and the bane of riverfront communities.
Winters are mild and summers are hot, with temperatures and annual average precipitation increasing from the north to south. (http://www.eoearth.org/article/Ecoregions_of_the_Mississippi_Alluvial_Plain_%28EPA%29) Precipitation averages 45 to 65 in (1,150 to 1,650 mm) annually. Temperature averages 56 to 70F. (14 to 21C). The growing season lasts 200 to 340 days.
Plants & Animals
On the Mississippi River Alluvial Plain, changes in elevation of only a few inches can result in great differences in soil saturation characteristics and thus the species of plants that thrive. As a result, there is much variability within a bottomland hardwood ecosystem, ranging from the bald cypress/tupelo swamp community that develops on frequently inundated sites, to the cherrybark oak/pecan community found on sites subjected to temporary flooding. Between these rather distinct community types are the more transitional, less distinguishable overcup oak/water hickory, elm/ash/hackberry, and sweetgum/red oak communities.
As for animals, river floodplain systems are highly productive and provide exceptional habitat for a variety of vertebrates including foraging and spawning fish, amphibians, and reptiles. Over 240 fish species, 45 species of reptiles and amphibians, and 37 species of mussels depend on the river and floodplain system of MSRAP. In addition, 50 species of mammals and approximately 60 percent of all bird species in the contiguous United States currently utilize the Mississippi River and its tributaries and/or their associated floodplains. Although elk, mountain lion, wolf, Carolina parakeet, and ivory-billed woodpecker once inhabited this area, current resident animals include white-tailed deer, black bear, bobcat, gray fox, raccoon, cottontail rabbit, gray squirrel, fox squirrel, striped skunk, swamp rabbit, and many small rodents and shrews. (http://www.fs.fed.us/land/pubs/ecoregions/ch22.html) A number of species inhabiting MSRAP are threatened or endangered including the interior least tern, the fat pocketbook pearly mussel, the pallid sturgeon, the ring pink mussel, the orangefoot pimpleback mussel, the pink mucket, pondberry, and the Louisiana black bear.
Humans & History
At one time an impenetrable blanket of forest cover, occasionally interrupted by dense thickets of cane or prairie terrace, the MSRAP stretched across 9.7 million hectares of rich floodplain. The diverse plant species and complex forest structure supported wildlife so exotic in form and habit that many settlers likened this New World environment – the largest forested wetland in North America – to the floodplain forests of the Amazon.
In the 20th century, a series of socio-political events, technological advances, and environmental disasters made possible widespread drainage and clearing of the area. Some 4,300 miles of levee were erected along the river and its tributaries. Hundreds of thousands – perhaps millions – of miles of ditches were dug. The levees now separate much of the river and its immediate habitat from the rest of the hydrologic system.
During this same time, nearly eight million acres of forests, roughly 80%, were cleared for agricultural production. Since 2000, conservation organizations and agencies from throughout MSRAP have focused tremendous attention and allocated substantial resources to address the ecological consequences of widespread clearing and hydrologic alteration.
Presently, most of the northern and central sections of the region are in cropland and receive heavy treatments of insecticides and herbicides. Soybeans, cotton, and rice are the major crops; commercial catfish farms have been a boon to local economies. In addition, large river channel dredging projects remove silt and sediment accumulations from the river channel to facilitate navigation along the Mississippi River. These factors, and the large concrete river revetments and channelization policies, have all contributed to the decrease of sediment mobilization within the system, thus altering the delta formation at the mouth of the river and contributing to the loss of habitat for many coastal and estuarine species. This region is also a major bird migration corridor used in fall and spring migrations. Degradation and destruction of forest and wetland habitats and the construction of navigation and flood control systems have had detrimental effects on many of these bird populations.