II.

Physical Geography

Illinois ranks 25th in size among the states of the Union, with an area of 149,997 sq km (57,914 sq mi). That includes 1,958 sq km (756 sq mi) of inland water and 4,079 sq km (1,575 sq mi) of Lake Michigan over which the state has jurisdiction. The greatest north-to-south dimension of the state is 610 km (379 mi), and the greatest east-to-west distance is 343 km (213 mi). The mean elevation is about 180 m (600 ft).

A.

Natural Regions

Illinois includes parts of four major natural regions, or physiographic provinces, of the United States: the Central Lowland, the Interior Low Plateaus, the Ozark Plateaus, and the Gulf Coastal Plain. The extensive flatlands of the Central Lowland occupy nearly all of Illinois, whereas the other three regions cover only small areas of the state. The Interior Low Plateaus and the Ozark Plateaus form a strip of hilly land across the southern part of the state. The flat alluvial lands of the Gulf Coastal Plain cover a small section of extreme southern Illinois.

The Central Lowland and the Interior Low Plateaus are subdivisions of a broader region known as the Interior Plains. The Ozark Plateaus form a section of the larger Interior Highlands region, and the Gulf Coastal Plain is part of the Coastal Plain.

The Central Lowland covers all but a small area of Illinois. Most of the Central Lowland is a level or slightly undulating plain, crossed here and there by broad, low ridges. The flatness of the land is a result of glacial action that occurred during the last Ice Age, which ended about 10,000 years ago, when great ice sheets advanced and retreated across the region. After the ice eventually retreated northeastward, the old preglacial landscape was left buried under a thick cover of glacial deposits, called drift, or glacial drift. Much of the drift is made up of clay and boulders, which together are called till, and form moraines. The so-called Till Plains form the typical flat farmlands of Illinois. The fertile Till Plains area of east central Illinois is known as the Grand Prairie. The low ridges found in some areas are terminal moraines that were piled up during the periods of glaciation by stagnant ice sheets.

In some areas, especially in the Grand Prairie and in south central Illinois, the land is so flat that the taller buildings of cities on the plain can be seen for great distances across country. Partly because of the flatness, rainwater does not readily drain away, and before ditches and drains were dug, much of the land was swampy for at least part of the year. There are still large tracts of such wetland along the Wabash, Kaskaskia, and Big Muddy rivers today. Farther west, between the Illinois and Mississippi rivers, lie the Quincy hills, which are broken by deeply cut stream valleys. The extreme northwest is also an area of hills. Together with neighboring sections of Wisconsin and Iowa it forms the Driftless section, or Wisconsin Driftless section. Only the earliest of the ice sheets covered this section, and nearly all of the drift deposited has long since been eroded. Charles Mound, which is 376 m (1,235 ft) above sea level and the highest point in Illinois, lies on a long hilly ridge in the Driftless section. However, this is only 180 to 210 m (600 to 700 ft) above the general level of the Grand Prairie and much of the rest of the state.

The Interior Low Plateaus portion of southern Illinois was not covered by the ice sheets, and its high ridges and bluffs afford magnificent panoramic views across the lowlands. Much of the region is now part of Shawnee National Forest.

The Ozark Plateaus cover a small section of southwestern Illinois. Much of the land is forested and is too rugged and rocky for farming. Limestone, which is soluble in water, underlies much of the region, and small depressions, known as sinkholes, are found where the limestone has been dissolved.

The Gulf Coastal Plain, at the southern tip of Illinois, is an extremely flat area where the land has been built of alluvial deposits from the Ohio and Mississippi rivers. Much of the region is cultivated and is very productive. Because of the fertile soils and the great river there, early settlers in this section of Illinois gave the region the nickname Little Egypt.

B.

Rivers and Lakes

Except for a few short streams flowing into Lake Michigan, nearly all the streams and rivers of Illinois drain westward or southward to the Mississippi River or to the Ohio River, which joins the Mississippi near Cairo in the extreme south. The Mississippi forms the western boundary of the state. Its tributaries in Illinois include the Illinois, Rock, Kaskaskia, and Big Muddy rivers. The Ohio River’s chief tributary in Illinois is the Wabash River. The Embarras and Little Wabash rivers are tributaries of the Wabash.

The largest river entirely within the state is the Illinois River, which is formed by the junction of the Kankakee and Des Plaines rivers. It flows for 680 km (420 mi) across the state before joining the Mississippi River at Grafton. The Illinois has been deepened and straightened and forms part of the Illinois Waterway.

The watershed between rivers that flow into the Mississippi river system and rivers that flow into the Great Lakes is low and in many places is not easily discernible. In what is now the Chicago area, explorers had little difficulty portaging, or carrying, their canoes over the low watershed between the Des Plaines River, which flows into the Illinois, and the Chicago River, which then flowed into Lake Michigan. In 1900 the Chicago River’s flow was reversed to make it carry and dilute the city’s sewage and industrial wastes into the Illinois River, instead of into Lake Michigan, the source of the city’s drinking water.

Except for Lake Michigan, which borders the state on the northeast, there are no large natural lakes in Illinois. Small natural lakes, all in the northern part of the state, include Clear, Crane, Fox, Goose, Grass, Peoria, Pistakee, and Ringwood lakes. The largest artificially created bodies of water within Illinois include Rend Lake, which is held back by a dam on the Big Muddy River, and Carlyle Lake, on the Kaskaskia River near Carlyle.

C.

Climate

The climate of Illinois is characterized by warm to hot summers and cool to cold winters. In winter polar air masses move south or southeast across the state from Canada, bringing cold and crisp weather. In summer warm air masses move up from the Gulf of Mexico, and the weather is often hot and muggy. Lake Michigan tempers the summer heat somewhat for Chicago and other cities along its shores and also delays the date of the first fall frosts nearby.

C.1.

Temperatures

Average July temperatures increase from about 24°C (about 75° F) in northeastern Illinois to more than 26° C (79° F) in the south, which is the hottest part of the state. During July, daytime highs average 29° C (84° F) at Chicago and about 32° C (about 90° F) at East Saint Louis, where a temperature of 47° C (117° F) has been recorded. Summer nights are usually warm throughout the state, ranging from about 19° C (about 66° F) in the north to about 21° C (about 69° F) in the south.

January averages range from less than -4° C (24° F) in the northwest to more than 1° C (34° F) in the south. Chicago has January low temperatures averaging -11° C (13° F) and highs -2° C (29° F). In the north freezing temperatures occur 140 to 145 days a year.

C.2.

Precipitation

Precipitation (rainfall and snowfall) generally increases from north to south. Average precipitation for the state as a whole is about 940 mm (about 37 in) a year. The south is the wettest part of the state, with about 1,220 mm (about 48 in) of precipitation a year in places. The driest sections are in the north, where a few places average about 810 mm (about 32 in). Most precipitation falls in the form of rain, especially thundershowers, in late spring and summer, when it is most needed for crops. Damaging hailstorms sometimes occur in summer, and violent windstorms occasionally sweep across the state during the early spring months. Tornadoes may occur in any time of the year. Snowfall is often heavy in the north but is usually light in the south.

C.3.

Growing Season

The growing season, or period between the last killing frost in the spring and the first killing frost in the fall, increases from less than 155 days in extreme northern Illinois to more than 205 days in the extreme south. Over much of the state the growing season is 190 days. The last killing spring frost occurs in early April in the far south and a month later in the north. The first fall frosts usually occur in early October in the north and in late October in the south.

D.

Soils

Black prairie soils cover the northern sections of the formerly glaciated sections of Illinois. The deep, fertile prairie soils, enriched over thousands of years by humus derived from grasses and other organic matter that once covered them, are today among the most productive soils in the world. However, many miles of underground drains and open ditches must be maintained in the flatter sections in order to drain the land. Farther south, lighter-colored and less fertile soils, called planosols, predominate. They are characterized by a hardpan, or impervious layer, just below the surface, which prevents proper drainage of the cropland. The yield per acre of crops grown on the planosols is generally lower than on the prairie soils. Gray-brown alfisols cover the hilly areas in the western sections of the state. In most cases they are highly acidic and unsuited for crops. The hilly and unglaciated southern tip of Illinois is covered with thin, gray alfisols, which are lacking in the organic matter and mineral elements necessary for producing good crops. Fertile mollisols are found along the valleys of the Ohio, Mississippi, and Illinois rivers. Fine silts, which are extremely productive when properly drained, occur along the margin of Lake Michigan.

E.

Plant Life

Little more than 200 years ago, when nearly all of Illinois was still unsettled, forests covered the southern third of the area and tall grasses and prairie flowers covered most of the northern and central sections. These great stretches of prairie included prairie cordgrass, big bluestem, little bluestem, and switch grass. In many damp areas the grasses grew profusely, sometimes higher than a person on horseback. In early summer wildflowers on the prairie formed a sea of color that stretched away into the distance farther than the eye could see. These great grassland areas were broken only by tongues of woodland along the rivers and scattered upland groves.

Today most of the original vegetation of Illinois has been cleared for farming or otherwise modified by human activity. However, the term prairie, originally a French word meaning “meadow,” has been retained to describe the former grassland regions of Illinois and other Midwestern states. Forests and woodlands now cover only 12 percent of Illinois. The chief deciduous trees found in the southern part of the state include white oak, which is the state tree, shingle oak, post oak, sweet gum, river birch, and maple. Other trees common to Illinois include black walnut, honey locust, black cherry, basswood, cottonwood, Kentucky coffee, hackberry, hickory, ash, and sycamore. Illinois has few conifers, but there is a notable stand of white pine in White Pines Forest State Park, near Rockford. Other coniferous species, found mostly in northern Illinois, include red cedar and juniper.

The violet, which is the state flower of Illinois, grows wild throughout the state. Other wildflowers that still grow in profusion in undisturbed woodlands and roadside areas include Dutchman’s-breeches, blue phlox, black-eyed Susans, goldenrods, dogtooth violets, bluebells, trilliums, buttercups, bloodroots, prairie docks, toothworts, blazing stars, and asters.

F.

Animal Life

Bison, elk, black bear, and gray wolves once inhabited Illinois but had largely disappeared by the beginning of the 19th century. Furbearing animals such as muskrat, mink, and weasel have declined in number but are still common. A variety of small animals, including squirrel, woodchuck (groundhog), raccoon, opossum, and skunk, are still thriving. White-tailed deer, once extinct in Illinois, have through human intervention and migration from adjacent states returned in considerable number. Although never extinct, the beaver and coyote also declined in number but have since rebounded to become fairly common.

Illinois lies in the Mississippi Flyway, a route followed by millions of birds during spring and fall migrations. Species of waterfowl commonly seen in the state during the migrations include the Canada goose, common merganser, pintail, lesser scaup, shoveler, blue-wing teal, green-wing teal, ruddy duck, and mallard. Some migrant waterfowl, such as the mallard, breed and nest in Illinois if conditions permit. Upland game birds include the ring-necked pheasant, woodcock, northern bobwhite, and wild turkey. Among other birds found in the state are the meadowlark, robin, flicker, herring gull, American crow, blue jay, white-breasted nuthatch, starling, ruby-throated hummingbird, and cardinal (the state bird). There are also several species of warblers, sparrows, hawks, owls, thrushes, woodpeckers, wrens, flycatchers, and swallows. American bald eagles winter on the bluffs along the Mississippi River near Nauvoo.

Among the most abundant fish in Illinois waters are carp and catfish. Other fish include largemouth and smallmouth (black) bass, freshwater drum, bowfin, gizzard shad, suckers, gars, and sunfish. Pollution and siltation of the state’s streams and lakes in the 20th century have negatively affected the fish.

G.

Conservation

The principal agencies responsible for protecting Illinois’ forests, soils, water, and fisheries are the Natural Resources Conservation Service, the United States Forest Service, and the Illinois Department of Natural Resources. Soil erosion, poor drainage, and floods are major, and sometimes inter-related, problems in southern Illinois. Reforestation and other conservation techniques are practiced in both western and southern Illinois, where land too steep for farming was cleared in early times and is now seriously eroded. Much of the topsoil there has been lost through erosion, and only the underlying and impervious clay pan remains. In wet weather, rain cannot sink into the ground because of the clay pan and it runs off the surface in sheets, thus contributing to further erosion and to floods along the state’s major rivers. Many dams and flood-control projects have been built in the state. Fish and game resources, greatly depleted because of water pollution and destruction of the natural habitat, are being increased by the Illinois Department of Natural Resources.

Two state agencies have responsibilities for air, land, and water quality. The Pollution Control Board institutes policies and regulations, hears pollution cases, and sets penalties. The Illinois Environmental Protection Agency administers the emissions permit system, prosecutes pollution cases before the Pollution Control Board, and investigates environmental problems such as unlawful dumping.

In 2006 the state had 41 hazardous waste sites on a national priority list for cleanup due to their severity or proximity to people. Progress was being made in efforts to reduce pollution; in the period 1995–2000 the amount of toxic chemicals discharged into the environment was reduced by 14 percent.