Illinois Geography, Climate & Nature

Illinois covers 57,918 square miles, the 25th-largest state, and runs 390 miles from the Wisconsin line to the confluence of the Ohio River and the Mississippi River at its southern tip and 210 miles east to west. Its borders are mostly water: Lake Michigan and nearly 50 miles of shoreline to the north-east, the Mississippi River to the west against Iowa and Missouri, the Ohio River to the south against Kentucky and the Wabash River to the south-east against Indiana. The Mississippi River has moved: Kaskaskia, the first state capital, now lies west of it and is reachable only from Missouri. The Illinois River, 273 miles long, cuts the state diagonally from the Chicago suburbs to the Mississippi River above St. Louis, past Starved Rock, Peoria and the Emiquon wetlands, and once yielded more freshwater fish than any American river but the Columbia River.

Glaciers made the rest. The Laurentide ice sheet covered up to 85 per cent of the state and left it flat, fertile and drained by slow rivers; the highest natural point, Charles Mound in the unglaciated Driftless corner of the north-west, is 1,235 feet, and the lowest is the levee-ringed river town at the southern confluence. Only the far south, the Shawnee Hills, escaped the last glaciation and is rugged, forested and warmer, with sandstone bluffs like the Garden of the Gods, coal and a little oil. In between lies the prairie that John Deere's steel plough of 1833 turned into some of the most valuable farmland on earth, now corn and soybeans from horizon to horizon.

The people are in the north-east: the Chicago metropolitan area, 9 per cent of the land and 65 per cent of the 12.8 million residents, with 9.8 million people across the state line into Indiana and Wisconsin; Chicago itself holds 2.74 million. Beyond it the cities are small -- Rockford, Joliet and Naperville around 150,000, Springfield the capital at 114,000, Peoria on the river, Champaign–Urbana with the university, the Quad Cities on the Mississippi River, the Metro East suburbs of St. Louis -- and everything outside Chicagoland is called downstate.

Illinois has a continental climate with no mountains or oceans to moderate it, and 390 miles of latitude to vary it. Most of the state is humid continental: hot, humid summers and cold winters, with Springfield averaging 87 °F by day in July and 20 at night in January, 38 inches of rain and 22 of snow. From about Carbondale south the climate is humid subtropical, Carbondale averaging 88 °F in July and 23 on January nights, with 49 inches of rain and only 11 of snow; precipitation runs from 48 inches at the southern tip to 35 in the north, and snowfall from over 38 inches around Chicago to under 14 in the south. The extremes are Midwestern: 117 °F at East St. Louis on 14 July 1954 and −38 °F near Mount Carroll on 31 January 2019, the winter of 1830–31 remembered as the Winter of the Deep Snow, Chicago's snowiest winter in 1978–79, and heat waves that killed 2,656 Illinoisans in July 1936 and 739 Chicagoans in five days of July 1995, most of them elderly and poor without air conditioning.

Spring is tornado season: the state averages 54 a year, about 50 days of thunderstorms, and in 2023 produced 136 tornadoes, more than any other state; the deadliest tornado in American history, the Tri-State of 18 March 1925, killed 613 people in Illinois alone, 234 of them in Murphysboro. Both advisories add flash floods, and snowstorms that shut O'Hare and the interstates. The Great Flood of 1993 on the Mississippi River and the 2011 evacuation of the town at the southern confluence are the river's reminders. Lake Michigan cools the shore in summer and warms it a little in winter, and it makes the snow.

Best Destinations by Month has the practical reading.

Illinois was tallgrass prairie, oak savanna and river-bottom forest, and it is now mostly farmland: very little of the prairie survives, which makes what does -- the 19,000 acres of Midewin near Joliet, the first national tallgrass prairie, restored since 1996 and grazed by a conservation herd of bison since 2015 -- the state's most important nature. The Shawnee National Forest in the unglaciated south, 498,615 acres and the largest public land in the state since 1939, holds seven wilderness areas including the Garden of the Gods, whose sandstone was laid down 300 million years ago; the Cache River at the southern tip is a Wetland of International Importance with cypress swamps; Emiquon on the Illinois River, another Ramsar site, is the second-largest wetland restoration in the country after the Everglades; Volo Bog is the state's only quaking bog; and Starved Rock's 2,630 acres hold canyons cut by a glacial flood 14,000 to 19,000 years ago. Illinois Beach is the only beach-ridge shoreline left on the state's Lake Michigan coast.

About 300 bird species pass through or breed, most on the Mississippi Flyway, with at least 150 recorded along the Wabash; bald eagles winter on the big rivers and now nest in every state; the Illinois River was once the richest freshwater fishery in the country after the Columbia River. The state's coal -- 211 billion tons of it, high in sulphur -- and its six nuclear plants, which made it first in the nation for nuclear generation, sit under and beside the farms, and its corn makes 40 per cent of the country's ethanol.

The threats are the plough, the levee and the drainage tile that took the prairie and the wetlands, the carp and the mussels of the canal that joins the Great Lakes to the Mississippi River basin, and a climate of hotter summers and heavier rain. No fuller species inventory was consulted beyond the sources named in the data file.

The animals a visitor can plan on are the white-tailed deer, which number around 30 million nationally and are the state's most common large mammal and its most common road hazard; the bison of Midewin, a small conservation herd seen from the trails; the bald eagles that gather on the open water of the Mississippi River and the Illinois River in winter, below the locks and dams; the migrating waterfowl, cranes and warblers of the Flyway at Emiquon, the Cache River and the lakefront in spring and autumn; and the coyotes, raccoons and geese that have made themselves at home in Chicago's parks. The Shedd Aquarium's 32,000 animals and the Field Museum's T. rex are the indoor wildlife of a city built on a lake.

The practical zoology is modest: rabies may be present in wildlife including bats, the advisories note; deer on the roads at dusk are the real danger; ticks in the southern forests in summer; and the Lake Michigan rip currents that take swimmers off the Chicago beaches every year.