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CHAPTER III: MISSISSIPPI’S ECOLOGICAL FRAMEWORK-ECOREGIONS OF MISSISSIPPI 66


Part 2: Mississippi River Alluvial Plain Ecoregion


The Mississippi River Alluvial Plain (MSRAP) is a 23,968,700 acre ecoregion that includes several
uplands and most of the Atchafalaya Basin, but excludes the Red and Ouachita River Alluvial Plains and
coastal areas south of the forested portions of the Atchafalaya Basin. Its most defining feature is the
Mississippi River which flows south over the Mississippi Embayment, a structural trough in the earth's
crust that, over the past 100 to 200 million years, has thrust alternately upward and downward relative to
the sea. MSRAP is a geologically complex area, with Coastal Plain sediments having been deposited by
a retreating Gulf of Mexico during the Tertiary Period of the Cenozoic Era. The melting of the glaciers
during the Pleistocene forced the upper Midwest and the current Ohio River Basin to drain southward
and, over time, form the modern-day Mississippi River. Retreating glaciers left behind glacial outwash
that, through time, was reworked by the energy of the river and overlaid by deep alluvium deposited
through annual overbank flooding. Several distinct landforms in MSRAP represent an accumulation of
coarse, glacial sediments that have not been fully subjected to the erosional forces of big river systems,
and thus remain tens of feet above floodplain elevations. Crowley's Ridge in Arkansas is hundreds of
feet above the floodplain and is comprised of Tertiary deposits. Well-drained, highly-erodable, wind-
blown deposits (loess) originating from glacial outwash are characteristic of these landforms. Upland
pine hardwood plant communities and, in areas of clay-pan formation, prairie communities, characterize
these upland areas.


The bottomland hardwood forest is by far the dominant natural plant component of MSRAP. It is
maintained by regular back- and headwater flood events and localized ponding on poorly drained soils.
Headwater or mainstem flooding results from rainstorms over the watersheds of the Mississippi's
tributaries, and produces the great spring floods characteristic of MSRAP. Backwater flooding is a


Mississippi River Alluvial Plain
Counties
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