Illinois Geography: Prairie, Rivers, and the Great Lakes Shore

Illinois is overwhelmingly flat — one of the most glaciated states in the US, with a topographic relief of only 1,235 feet between its highest and lowest points.

The Illinois Plains
Most of Illinois is the Illinois Till Plain — a former prairie landscape of glacial till deposits left by the Wisconsin Glacier (retreated ~12,000 years ago). The deep, dark loam soils (Drummer silty clay loam is the official state soil) are among the most productive agricultural soils on Earth. This is the heart of the Corn Belt: Illinois produces 15% of US corn and 14% of US soybeans.

Rivers
The Mississippi River forms the entire western border (579 miles). The Illinois River runs diagonally from the northeast to the southwest, connecting the Chicago region to the Mississippi. The Ohio River forms the southern border. The Chicago River, reversed by an engineering project in 1900 (the Chicago Sanitary and Ship Canal), flows away from Lake Michigan rather than into it — one of the most significant hydrological engineering projects in US history.

Lake Michigan Shore
Illinois has 63 miles of Lake Michigan shoreline — concentrated in the Chicago metro. The lakefront is publicly accessible (the "forever open, clear, and free" principle established by Montgomery Ward's 1890s litigation) and hosts parks, beaches, and the Museum Campus along the entire Chicago waterfront.

Shawnee Hills (Southern Illinois)
The only significant topography in Illinois — the Shawnee Hills in the state's southern tip (near the confluence of the Ohio and Mississippi) are sandstone hills reaching 1,064 feet (Charles Mound, the state high point, is in the far northwest at 1,235 feet). The Garden of the Gods in Shawnee National Forest has remarkable sandstone formations.

Climate
Illinois has a continental climate with hot, humid summers and cold winters. Chicago's lakefront moderates temperatures somewhat; the northern tier can receive 40+ inches of snow per year. The southern tip (near Cairo) has a climate approaching the Upper South — longer growing season and milder winters.