Illinois is in themidwestern United States.Surroundingstates areWisconsin to the north,Iowa andMissouri to the west,Kentucky to the east and south, andIndiana to the east. Illinois also bordersMichigan, but only via a northeastern water boundary inLake Michigan. Nearly the entire western boundary of Illinois is theMississippi River, except for a few areas where the river has changed course. Illinois' southeastern and southern boundary is along theWabash River and theOhio River, whereas its northern boundary and much of its eastern boundary are straight survey (longitudinal and latitudinal) lines. Illinois has a maximum north–south distance of 390 miles (630 km) and 210 miles (340 km) east-west. Its total area is 57,918 square miles (150,010 km²).

• Northwest (Moline) – Rock Island, La Salle, Whiteside, Henry, Lee, Bureau, Jo Daviess, Carroll, Mercer, Putnam
• Northern Stateline (Rockford) – Winnebago, Boone, Ogle, Stephenson
• Northeast (Chicago) – Cook, DuPage, Lake, Will, Kane, McHenry, Kendall, Kankakee, DeKalb, Grundy
• West Central (Quincy) – Adams, Knox, McDonough, Hancock, Warren, Pike, Schuyler, Brown, Henderson
• North Central (Peoria) – Peoria, McLean, Tazewell, Woodford, Livingston, Fulton, De Witt, Mason, Marshall, Stark
• East Central (Champaign) – Champaign, Vermilion, Iroquois, Douglas, Piatt, Ford
• Central (Springfield) – Sangamon, Macon, Macoupin, Christian, Morgan, Montgomery, Logan, Shelby, Cass, Menard, Greene, Scott
• Southwest (Belleville) – Madison, St. Clair, Clinton, Monroe, Randolph, Jersey, Bond, Washington, Calhoun
• Southeast (Charleston) – Coles, Marion, Effingham, Fayette, Crawford, Edgar, Richland, Clark, Lawrence, Moultrie, Clay, Cumberland, Jasper
• Southern (Carbondale) – Williamson, Jackson, Franklin, Jefferson, Saline, Perry, Union, Wayne, Massac, White, Johnson, Wabash, Hamilton, Edwards, Pulaski, Gallatin, Alexander, Pope, Hardin

• Illinois has a maximum north–south distance of 390 miles (630 km) and 210 miles (340 km) east-west.
• Total area is 57,918 square miles (150,010 km²), ranked 25th in size of the 50 states.
• Water area is 2,325 square miles (6,020 km²);Lake Michigan accounts for most of this.
• Charles Mound in the northwestDriftless Area is the highest point in the state at 1,235 feet (376 m) above sea level.
• TheMississippi River atCairo, Illinois is the lowest point, at 279 feet (85 m).
• Average elevation is 600 feet (180 m) above sea level.
• McLean County is the largest county in terms of land area (1,184 sq mi.), whileCook County is the largest county in terms of population (5,275,541 as of 2020), and the largest when land and water area are combined.^[1]

Illinois has three major geographical divisions: Northern, Central, and Southern. Collectively, central and southern Illinois are often referred to within Illinois as "downstate Illinois" but with political developments since World War II "Downstate" now generally refers toall of Illinois outside of the Chicago metro area.

Northern Illinois is dominated by theChicago metropolitan area, including the city ofChicago, its suburbs, and the adjoining exurban area into which the metropolis is expanding. The "collar counties" are the five counties that surround the centrally located Cook County in the Chicago metropolitan area: DuPage County, Kane County, Lake County, McHenry County, and Will County.^[2] The term "Chicagoland" is sometimes used to describe the metro area, including Chicago, its suburbs within Cook County, and the so-called collar counties. The term is said to have been coined byChicago Tribune publisherRobert R. McCormick.^[2] TheChicagoland Chamber of Commerce defines it as all of Cook, and DuPage, Kane, Lake, McHenry and Will counties.^[3] The region is cosmopolitan, densely populated, industrialized, and settled by a wide variety of ethnic groups. As defined by the federal government, the Chicago Metropolitan Area includes two counties in Indiana, two counties in Wisconsin, and the following 11 counties in Illinois:Cook,DeKalb,DuPage,Grundy,Kane,Kankakee,Kendall,Lake,LaSalle,McHenry, andWill counties.

TheRockford Metropolitan Area is north-central nearWisconsin, on theRock River. TheQuad Cities Metropolitan Area is on both sides of theMississippi River separating Illinois fromIowa. The Illinois side includesHenry County,Mercer County, andRock Island County.^[4]

In extreme northwestern Illinois theDriftless Zone, a region of unglaciated and therefore higher and morerugged topography, occupies a small part of the state.Charles Mound, located in this region, is the state's highest elevation abovesea level.

Southward and westward, the second major division isCentral Illinois, an area of mostly flatprairie with the exception of areas along the Illinois and Mississippi rivers that are steeped in hills and bluffs hanging high above the rivers providing commanding views for miles. The western section (west of the Illinois River) was originally part of theMilitary Tract of 1812 and forms the distinctive western bulge of the state. Central Illinois is characterized by small towns and mid-sized cities. Agriculture, particularlycorn andsoybeans, figures prominently. Major cities includePeoria, andSpringfield (the state capital). TheChampaign-Urbana Metropolitan Area is located in the central region of the state and is composed ofChampaign,Ford, andPiatt counties.^[5]

The third division isSouthern Illinois, comprising the area south ofU.S. Route 50, and includingLittle Egypt, near the juncture of theMississippi River andOhio River. This region can be distinguished from the other two by its warmer climate, different mix of crops (includingcotton farming in the past), more rugged unglaciated topography, as well as small-scale oil deposits andcoal mining.

The population in southern Illinois is concentrated in two areas:

• Metro-East is an area east ofSt. Louis and it includesJersey,Madison,St. Clair,Macoupin, and parts ofBond andCalhoun from theSt. Louis Metropolitan Statistical Area, as well asClinton County.^[6]
• Carbondale, Marion, West Frankfort, Benton, Herrin, Murphysboro, Carterville, and the Johnston City area.

TheShawnee Hills arise in southern Illinois.Illinoian Stage, about 132,000 to 300,000 years before present, climbed the northern face of the Shawnee Hills, but did not reach the summit. Most of the Shawnee Hills is therefore unglaciated.

The floodplain on the Mississippi River fromAlton to theKaskaskia River is theAmerican Bottom, and is the site of the ancient city ofCahokia, and was a region of early French settlement, as well as the site of the first state capital, atKaskaskia. The extreme southern tip of Illinois is in theGulf Coastal Plain.

The city of Chicago and some of its older suburbs are heavilyDemocratic. While this tendency has historically been balanced byRepublican voters in the suburbs, Democrats have significantly increased their suburban support in the past decade. Central Illinois's largely rural character helps to sustain a largely Republican voting pattern and widespread antipathy toward Chicago. The combination of poorer soil, coal mining and industrialization has caused the southern region to lean Democratic politically, especially in the urbanized region aroundSaint Louis,Missouri.

Rockford, Illinois is known as a center of liberal"gypsy moth" Republicans.

Illinois is predominantly covered with soil, hundreds of feet thick in many places. Much of this soil is called, appropriately, "Illite", and is the most common type of soil on the planet. The visible geology of the state is mainlyQuaternary, with rocks buried deeply. The wealth of the state mainly arises from this abundance of soil and the favorable agrarian conditions it provides.

Illinois is a majorcoal-producing state. Much of the bedrock surface is ofPennsylvanian age, including "cyclothemes", regular sequences of limestone, shale and coal layers. Newer rocks may have once existed in the State, but these have been removed by repeated epochs of glaciation. Only tiny areas of newer bedrock exist. The chance of finding dinosaur bones in Illinois is almost nil:^{[citation needed]}, the Pennsylvanian rocks were here before the dinosaurs.

Galena, alead ore, has been mined in the northwest corner of the state. The ore was processed aroundPeru, Illinois, where river transport was available to bring the ore to a source of coal. About six tons of coal are needed for each ton of ore.

Fluorite,calcium fluoride, is the state mineral. It is mined in the southeastern portion of the state, aroundMetropolis. Fluorine gas is produced from fluorite. This is used in the production ofuranium hexafluoride in Metropolis, Illinois for use at thegaseous diffusion plant inPaducah, Kentucky.

Geodes can be found in the olderMississippian outcroppings along theMississippi River.

Igneous intrusions are found in the far southern areas of the State. Ages ago North America began to split roughly along the lines of the Ohio and Mississippi Rivers. Igneous rock flowed up into fissures, as a new ocean began to form. Had this continued, Illinois might have resembled theArabian Peninsula, a broken terrain between two continents. This split, however, failed: the continent reclosed, leaving only a few igneous intrusions, and some oil fields, as evidence that it happened.

North of theIllinois River the bedrock surface is much olderOrdovician sandstone. This sandstone yields ancient sand that is very useful in the production of glass, and in making molds for metal castings.

The majority of Illinois surfacebedrock belongs to thePennsylvanian period, particularly the downstate region. About 68% of the state hascoal-bearing strata of this geologic period. According to theIllinois State Geological Survey, 211 billion tons ofbituminous coal are estimated to lie under the surface, having a total heating value greater than the estimated oil deposits in theArabian Peninsula. However, this coal has a highsulfur content and as such requires special equipment to reduce air pollution.

Illinois is mainly bounded by rivers, and theIllinois River bisects the state. A few rivers, including theFox andRock Rivers enter the State from Wisconsin. TheIroquois andKankakee arise inIndiana, but lose their identity before reaching theMississippi. The North Fork of the Vermilion River arises in Indiana, but joins theVermilion, which exits the state before joining theWabash River in Indiana. Other rivers include theOhio River, theKaskaskia River, theSangamon River and theChicago River. TheCashe River area in southern Illinois forms the basis for aRamsar ConventionWetland of International Importance.

Subsurface water is generally plentiful in Illinois. However, irregular gravel deposits from glaciation can lead to difficulties predicting depth of the water table.

TheMahomet Aquifer is a large aquifer used as a source for public water supply by many communities in Central Illinois.

Largemammal species currently found in Illinois includewhitetail deer andcoyote, with the latter becoming urbanized in theChicago area, as well as common in the rural areas. Furbearers commonly found includeopossum,raccoon,mink,red andgray foxes, andmuskrat. Commonly seen lagomorphs include thecottontail rabbit and commonly seen rodents includesquirrels. Game birds found includeCanada goose,mallard duck,ruffed grouse,wild turkey, andbobwhite quail. Thenorthern cardinal is the official state bird. The state has approximately 300 species of birds, many of which migrate on theMississippi Flyway. Extensive wetland restoration along theMississippi River andIllinois River are attracting many species.

About two thirds of the state was originally prairie, with much of the area converted to rowcrop farming, while the southern third of the state remains heavily forested withoak,hickory, andmaple. Also, several prairie plants.

Especially in the northern third of the state,intensive agriculture has made some species rare, while population numbers are increasing in the less-developed south. Large mammals generallyno longer found in the wild in Illinois includeAmerican bison,gray wolf,American black bear, andelk.

Illinois has acontinental climate, with large temperature extremes not moderated by either mountains or oceans. Weather is influenced primarily by cold Canadian Arctic air in the winter, and warm, humid air from theGulf of Mexico in the summer. High temperatures in the south of the state are about 10 to 12 °F. warmer than the north. Average annual temperature is 47 °F. in the north and 58 °F. in the south. Temperatures greater than 90 °F. occur about 45 days per year in the south, and 12 days per year in the north. Temperatures below zero °F. occur one day or less per year in the south, and 12 days per year in the north. Annual rainfall is 34 inches (860 mm) in the north and 46 inches (1,200 mm) in the south.

Prevailingwinds are from the west. Average wind speeds "blowing off theprairie" are greater in theChicago area. Statewide, the wind speeds average 8 to 12 miles per hour (13 to 19 km/h), and is greater in the winter than in summer.

Snowfall averages 30 inches (760 mm) in the north and 10 inches (250 mm) in the south, although the actual snowfalls vary greatly by year. In some years, southern Illinois receives very little snowfall, and may never get typical winter weather.

Most of the state has four distinct seasons. The southernmost part of the state is heavily forested and the trees are noted for having beautifulfall colors.

Tornadoes are a common hazard, primarily in the central part of the state, with peak months in June and April. Freezing rain is most common in central Illinois.

TheMississippi River is the nominal western boundary of the state, except for locations where the river has changed course.Kaskaskia, Illinois, the first state capital, is now west of the river. The Mississippi, due to a flood, destroyed the old town of Kaskaskia, and occupies what was the lower channel of theKaskaskia River. A portion ofMissouri is east of the river, nearGrand Tower, Illinois. Similar "avulsions" are found along theOhio River border withKentucky and theWabash River border withIndiana.

• List of ecoregions in Illinois

• Harris, Stanley E.; C. W. Horrell; Daniel Irwin (1977).Exploring the Land and Rocks of Southern Illinois: A Geological Guide. Carbondale: Southern Illinois University Press.ISBN 0809307405.
• Wiggers, Ray (1997).Geology Underfoot in Illinois. Missoula, Mont: Mountain Press Pub. Co.ISBN 087842346X.

• About Illinois Data and Statistics^{[permanent dead link]}

• Geography of Illinois

• Webarchive template wayback links
• Articles with short description
• Short description is different from Wikidata
• All articles with unsourced statements
• Articles with unsourced statements from October 2012
• All articles with dead external links
• Articles with dead external links from August 2025
• Articles with permanently dead external links