Physical Geography

Illinois is predominantly flat, lying within the Central Lowlands region of the United States. The surface slopes gradually downward from the northeast to the southwest and reaches its lowest point, 279 feet (85 m) above sea level, in the extreme southwest at the junction of the Mississippi and Ohio rivers. The state's average elevation is 600 feet (183 m) above sea level.

During the last Ice Age, glaciers covered the state as far south as Carbondale; only the extreme south and a small section of the northwest were free from glaciation. In most of the extreme south low, eroded plateaus extend into the state from Kentucky and Missouri. In the northwest section is the state's highest point, Charles Mound, reaching 1,235 feet (376 m). As the glaciers receded they left behind deep layers of debris.

Illinois is bordered by two of the nation's great rivers—the Mississippi and the Ohio. The Mississippi forms the state's western boundary. The rivers flowing southwesterly into it drain much of the state. The chief Mississippi tributary in Illinois is the Illinois River, flowing across the central part of the state. Tributaries of the Illinois include the Kankakee, Des Plaines, Fox, and Sangamon. Other rivers that join the Mississippi include the Rock, Kaskaskia, and Big Muddy. The Ohio River forms the Illinois-Kentucky border. Its main tributary in Illinois is the Wabash River, which forms part of the Illinois-Indiana border. Illinois rivers that flow into the Wabash include the Little Wabash and Embarras rivers.

Illinois has few large lakes. In northeastern Illinois are a number of small glacial lakes. Fox Lake, the largest, is part of the Chain O'Lakes, a group connected by the Fox River. Artificial lakes, formed by dams, are found in many parts of the state; among the large ones are Lake Shelbyville, Carlyle Lake, and Rend Lake. Many communities in northeastern Illinois get their water from Lake Michigan.

Illinois has a continental type of climate, with cold winters and hot summers. Because of the state's great length, temperature differences from north to south are often pronounced. Average January temperatures are 36° F. (2° C.) in Cairo in the extreme south, 20° F. (-7° C.) in Rockford in the north; average July temperatures are 81° F. (27° C.) in Cairo, 73° F. (23° C.) in Rockford. Illinois is subject to occasional severe storms, including tornadoes, violent thunderstorms, and blizzards.

Annual precipitation increases from an average 34 inches (860 mm) in the north to 43 inches (1,090 mm) in the south. Annual snowfall however, decreases north to south from an average of 30 inches (760 mm) to 10 inches (250 mm). Flooding often occurs in the early spring, especially when there are heavy rainfalls while snow and river ice are melting.

In the early 1800's, when rapid settlement began, tall prairie grasses covered most of northern and central Illinois. Forests covered the southern fourth of the state and extended as narrow strips along rivers far into the grasslands. By the early 20th century, most of the forests and all but tiny remnants of the grasslands had been turned into farmland.

Forests presently cover only about 10 per cent of the state. Virtually all the trees are second-growth hardwoods of commercial quality, mainly oaks. Most of the forests are privately owned and consist of scattered parcels on farms.