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Natural Division Overview
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.
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 River and 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 Illinoisan 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 extension 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 tree frog 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.
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.