ILIllinois Geography

In 1818 the Northwest Ordinance would have drawn the top of Illinois at the latitude of the southern tip of Lake Michigan, roughly 41 degrees 37 minutes north. Nathaniel Pope, the territory's delegate in Congress, argued for moving it to 42 degrees 30 minutes so the new state would touch the lake and be bound commercially to the north rather than to the slaveholding south. Congress agreed. The amendment added about 8,500 square miles, fourteen future counties, and the ground on which Chicago now stands.

Everything distinctive about Illinois geography sits somewhere between that engineered northern line and the swampy southern tip at Cairo, 385 miles away.

A state fenced almost entirely by water

Only the Wisconsin border is a pure survey line. Everywhere else water does the work. The Mississippi runs the entire western edge against Iowa and Missouri. The Ohio wraps the south against Kentucky. The Wabash forms most of the southeastern boundary with Indiana. Lake Michigan supplies about 63 miles of shoreline in the northeast, plus a water boundary shared with Michigan itself.

Rivers move, which creates problems that surveyors never anticipated. Kaskaskia was the first state capital, seated on the east bank of the Mississippi. In 1881 the river broke through and carved a new channel to the east of the town. Kaskaskia is still legally part of Illinois but sits on the Missouri side of the water, reachable by road only through Missouri, with a population now in the low double digits.

Charles Mound to Cairo, 956 feet apart

Charles Mound in Jo Daviess County reaches 1,235 feet, and it stands in a farmyard whose owners open the driveway to visitors on a handful of weekends a year. The low point is 279 feet at Cairo, where the Ohio pours into the Mississippi behind a ring of levees. That gives Illinois 956 feet of total relief, among the flattest profiles of any state.

Cairo also sits farther south than Richmond, Virginia. The state stretches through more than five degrees of latitude, which is why the far south grows cypress in standing water while the northern counties average subzero January nights.

The divide that engineers moved

A low ridge across northeastern Illinois once separated water flowing to the Great Lakes and the Atlantic from water flowing to the Mississippi and the Gulf. Voyageurs dragged canoes across it at the Chicago Portage. In 1900 the Sanitary District of Chicago finished the Chicago Sanitary and Ship Canal and reversed the Chicago River so that it flowed away from Lake Michigan and down toward the Illinois River, carrying the city's sewage with it.

The result is a continental drainage divide relocated by excavation. The Illinois Waterway now links Lake Michigan to the Mississippi as a continuous navigable channel, the only such connection in the country, and it runs 336 miles from Chicago to Grafton. The Illinois River itself forms where the Kankakee and Des Plaines meet near Channahon and travels 273 miles to its mouth.

Where the glaciers stopped, and where they never arrived

Most of Illinois is a till plain, flattened and then buried in fine ground rock by successive ice sheets. The Illinoian glaciation, named for this state, pushed farther south than the later Wisconsin ice, reaching almost to the Shawnee Hills.

Two corners escaped. The northwest tip around Jo Daviess County belongs to the Driftless Area shared with Wisconsin, Iowa and Minnesota, which is why it has bluffs and ravines while the counties east of it are table flat. The far south, below the glacial limit, keeps hills, sandstone bluffs and the Garden of the Gods rock formations inside Shawnee National Forest, the state's only national forest.

Illinois spans four physiographic provinces in total: the Central Lowland across most of its area, the Interior Low Plateaus in the Shawnee Hills, a corner of the Ozark Plateaus in the southwest, and the Coastal Plain at the very bottom where the Mississippi Embayment reaches north.

Twenty two million acres of grass, almost all gone

Before settlement roughly 60 percent of the state, about 22 million acres, was tallgrass prairie. Grass roots pushed ten feet down and left behind mollisols so productive that Drummer silty clay loam, the official state soil, ranks among the best cropland on Earth. Under 2,300 acres of high quality original prairie survive, preserved in fragments at places like Goose Lake Prairie and Nachusa Grasslands.

The whole state sits in a single time zone, Central, and it has no split like its eastern neighbour. Weather extremes come from the open exposure to both Gulf moisture and Canadian air: 117 degrees at East St. Louis in July 1954, and 38 below zero at Mount Carroll in January 1919, a mark the state climatologist office confirmed as the record in 2021. The southern counties also sit close enough to the New Madrid Seismic Zone that the 1811 and 1812 earthquakes rang church bells hundreds of miles away.

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