CHAPTER III: MISSISSIPPI’S ECOLOGICAL FRAMEWORK-ECOREGIONS OF MISSISSIPPI 72
in the form of salt plugs, domes, and basins. The Mississippi Interior Salt Basin, which extends into this
region, has extensive hydrocarbon reserves that are still largely undeveloped.
Throughout the region, soils are generally acidic with appreciable amounts of clay present. Ultisols,
deeply leached and low in nutrients, are the dominant soil order. Alfisols, less weathered and greater in
fertility, are present in more limited areas, especially associated with loess deposits (a unique type of
windblown silt). Large quantities of loess were probably carried by wind from exposed sediments of the
Mississippi River floodplain and deposited on adjacent uplands during the late Pleistocene and early
Holocene. Loess eventually covered much of the underlying topography under a thick blanket deepest
along the western edge and thinning abruptly eastward. Vertisols (soils with shrink-swell properties due,
in part, to especially high clay content) are uncommon in the southeastern coastal plain but are present in
limited areas of the Black Belt where they were derived from marl and chalk residues.
The UEGCP overlaps several distinctive aquatic ecoregions. The majority of this region has been
considered a priority for freshwater species conservation due to the richness of the fauna present. For
example, rivers in this region provide habitat for over 206 native fish species.
The region also supports relatively large numbers of crayfish and mussel species despite heavily
disturbed conditions in many areas that have likely reduced faunal diversity. The bulk of the regions'
rivers, especially the Mississippi tributaries, have been channelized and/or subjected to headcutting and
heavy sedimentation.
The region includes a diverse assemblage of streams that vary in size, origin, and geology. Particularly
noteworthy rivers of this region include the Hatchie, the longest free flowing tributary in the lower
Mississippi River valley and tributaries of the Pascagoula, America's longest unencumbered river.
The potential natural vegetation of the UEGCP may be characterized as broad bands of different
composition that roughly parallel the coast. From south to north these include southern mixed forests,
oak-hickory-pine forests, and oak-hickory forests, interrupted by occasional southern floodplain forests
and black belt prairies.
Southern mixed forests and oak-hickory-pine forests, the two predominant types in terms of area
occupied, are recognized by the presence of longleaf pine and shortleaf pine. Although longleaf forests
and woodlands were the dominant vegetation type of the southeastern United States coastal plain, they
occur in only limited areas of this region, extending landward into the UEGCP by only about 50 miles.
Northward, longleaf pine is replaced by shortleaf pine.
Bluffs along the eastern edge of the Mississippi River, such as those around Vicksburg, are covered with
up to 200 feet of loess. A number of factors account for the development and maintenance of precipitous
cliffs and ravines where loess is deepest. The vegetation of these loess bluffs is often richer than