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Natural
Divisions of Illinois.
A common classification scheme used in Illinois, developed by John Schwegman
and coworkers, recognizes fourteen Natural Divisions, plus Lake Michigan--
geographic regions having similar topography, soils, bedrock, plants,
and animals. Natural Divisions are an important tool for recognizing biological
variation across Illinois, and organizing regional needs, objectives and
strategies of the Illinois Wildlife Action
Plan.
The Wisconsin Driftless Natural Division is part of an
area extending from the northwestern corner of Illinois into Iowa, Wisconsin
and Minnesota that apparently escaped Pleistocene glaciation. Bordered
by the Mississippi River Bottomlands on the west and characterized by
rugged terrain that was originally mostly forested, the division contains
northern and pre-Ice Age relict species (e.g., Iowa Pleistocene snail),
dolomite outcrops and caves.
The Rock River Hill Country Natural Division of north-central
and northwestern Illinois is a region of rolling topography drained by
the Rock River. Prairie formerly occupied the larger expanses of level
uplands, with forest equally abundant along water courses and in the dissected
uplands.
The Northeastern Morainal Natural Division is the most
recently glaciated in Illinois. Drainage is poorly developed, thus abundant
marshes, natural lakes, and bogs are distinctive features. With diverse
wetland, prairie, forest, savanna, and lake communities, this northeastern
section of Illinois hosts the greatest biodiversity in Illinois, and the
largest human population. As is true statewide, natural land cover has
been extensively altered, though urbanization is considerably more extensive
than elsewhere.
Lake
Michigan, about 6% or 1 million acres of which occurs in Illinois,
is one of the Great Lakes and part of the largest freshwater ecosystem
in the world. While water quality in Lake Michigan has improved in recent
decades, declining water levels and invasive animals now pose the greater
threats to the ecosystem. Characteristic fishes of the Lake Michigan Natural
Division include yellow perch and lake trout.
The Grand Prairie Natural Division of central and east-central
Illinois is a vast plain formerly occupied primarily by tallgrass prairie,
now converted extensively to agriculture. Natural drainage of the fertile
soils was poor, resulting in many marshes and potholes. Bison, Blanding’s
turtles, and Franklin’s ground squirrels are distinctive animals
of the Grand Prairie, but are now extirpated or imperiled–as is
the native prairie.
The Upper Mississippi River and Illinois River Bottomlands Natural
Division of western and west-central Illinois encompasses the
river and floodplains of the Mississippi River above the confluence with
the Missouri River, and of the bottomlands and backwater lakes of the
Illinois Riverand its major tributaries south of LaSalle. Much of the
division was originally forested but prairie and marsh occurred. Agriculture
is the primary land use in the floodplains today. The big rivers, their
fish and mussel communities, and the backwater lakes of the Illinois River
are distinctive.
The Illinois River and Mississippi River Sand Areas Natural Division
are several discrete patches of sand areas and dunes in the bottomlands
of the Illinois and Mississippi rivers, and ‘perched dunes’
atop bluffs near Hanover in JoDaviess County. Several relict western amphibians
and reptiles are known only from these sand areas, including the plains
hognose snake, Illinois mud turtle, and Illinois chorus frog.
The Western Forest-Prairie Natural Division of west-central Illinois
is a strongly dissected glacial till plain of Illinoian and Kansan age.
Forest was the predominant vegetation, with considerable prairie on the
level uplands. This character is retained with forests in riparian zones
and on steep hillsides, and agriculture and rural grasslands in upland
areas. Land use patterns of this division and the Southern Till Plain
are similar, and five-lined skink, ground skink and ornate box turtle
are animals characteristic of these two divisions.
The Middle Mississippi Border Natural Division of west-central
Illinois consists of a relatively narrow band of river bluffs and rugged
terrain bordering the Mississippi River floodplain from Rock Island County
to St. Clair County and the lower Illinois floodplain. Forest is the predominant
vegetation with interspersed hill prairies common on west-facing bluffs.
Limestone cliffs are common features, and the dark-sided salamander and
western worm snake are restricted to this division. Forests of this division,
close to river foraging areas, are important winter roosting sites for
significant concentrations of bald eagles.
The Southern Till Plain Natural Division of south-central
Illinois is a dissected Illinoian till plain south of the terminal Wisconsinan
moraine. Forest was found along streams and prairie occupied the level
uplands. Soils are poor because of high clay content and frequent “claypan”
subsoil. Post oak flatwood is a characteristic community. Northern crayfish
frog, ornate box turtle and remnant populations of greater prairie-chickens
are characteristic of the Southern Till Plain Natural Division.
The Wabash Border Natural Division includes the bottomlands
and the loess-covered uplands bordering the Wabash River and its major
tributaries in southeastern Illinois. Lowland oak forests with beech,
tuliptree and other eastern species are characteristic, and the Wabash
River drainage contains several distinctive fishes, including river chub,
greenside darter,
bluebreast darter and harlequin darter.
The Ozark Natural Division consists of the part of the
Ozark uplift that extends into southwestern Illinois. Topography is of
a maturely dissected plateau with bluffs along the Mississippi River,
and a sinkhole plain in the northern section. Natural vegetation of the
area is mostly forested with many hill prairies. Several Ozark, southern
and southwestern animals are
present only within this division in Illinois, such as plains scorpion,
spring cavefish, eastern narrow-mouthed toad, coachwhip, and northern
flat-headed snake.
The Lower Mississippi River Bottomlands Natural Division
includes the Mississippi River and its floodplain from Alton to the Thebes
Gorge. The Mississippi River, silt-laden below the confluence with the
Missouri River, contains a distinctive fish assemblage of silt-tolerant
plains species (plains minnow, sturgeon chub, flathead chub, sicklefin
chub). Natural vegetation
included prairies, marshes and rich forests with several southern lowland
tree species.
The Shawnee Hills Natural Division extends across the
southern tip of Illinois. The unglaciated hill country is characterized
by an east-west escarpment of sandstone cliffs and a series of lower hills.
Originally, the division was mostly forested, and is presently the most
heavily forested of Illinois’ natural divisions. Like the Northeastern
Morainal Natural Division, the Shawnee Hills hosts outstanding biodiversity.
The Coastal Plain Natural Division of extreme southern
Illinois is a region of swampy forested bottomlands and low clay and gravel
hills that is the northernmost extention of the Gulf of Mexico Plain Province
of North America. Bald cypress-tupelo swamps are a unique feature of the
natural division, as are many southern animals such as bird-voiced treefrog
and cottonmouth. The floodplain at the confluence of the Mississippi and
Ohio rivers and Cache and Ohio rivers host rich bottomland forests, while
the “Cretaceous Hills” section is a steep to rolling area
of unconsolidated sand, gravel and clay hosting Cretaceous period fossil
beds.
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