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Illinois is flat. Lying wholly within the Central Plains, the state exhibits a natural topographic monotony relieved mainly by hills in the northwest (an extension of Wisconsin's Driftless Area) and throughout the southern third of the state, on the fringes of the Ozark Plateau. The highest natural point, Charles Mound, tucked into the far northwest corner, is only 1,235 ft (377 m) above sea level—far lower than Chicago's towering skyscrapers. The low point, at the extreme southern tip along the Mississippi River, is 279 ft (85 m) above sea level. The average elevation is about 600 ft (183 m).

Although some 2,000 rivers and streams totaling 9,000 mi (14,500 km) crisscross the land, pioneers in central Illinois confronted very poor drainage. The installation of elaborate and expensive networks of ditches and tiled drains was necessary before commercial agriculture became feasible. Most of the 2,000 lakes of 6 acres (2.4 ha) or more were created by dams. The most important rivers are the Wabash and the Ohio, forming the southeastern and southern border; the Mississippi, forming the western border; and the Illinois, flowing northeast–southwest across the central region and meeting the Mississippi at Grafton, just northwest of the junction between the Mississippi and the Missouri rivers. The artificial Lake Carlyle (41 sq mi/106 sq km) is the largest body of inland water. Illinois also has jurisdiction over 1,526 sq mi (3,952 sq km) of Lake Michigan.