From its source the Illinois River flows almost due westward for 63 miles across Grundy and La Salle counties to the Great Bend at Hennepin in Putnam County. Here the stream bends sharply to the southward. After crossing Putnam County its course is southwestward to the northern edge of Pike County. The stream then flows almost due south to the southern part of Calhoun County, where another sharp bend gives the last few miles of its source an easterly direction to its confluence with the Mississippi River. The Illinois River joins the Mississippi at Grafton, 24 miles above the mouth of the Missouri, 215 miles from the Great Bend, and 278 miles from the confluence of the Des Plaines and Kankakee rivers.
The Des Plaines River rises in Racine County, Wisconsin. Its length is about 110 miles, 20 of which are in Wisconsin. Joliet is the largest city on the Des Plaines. The Kankakee River rises near South Bend, Indiana. It has a length of 135 miles, more than half of which is in Indiana. lklomence and Kankakee are located on this stream.
The total stream length along which water may flow within the Illinois River Basin is more than 400 miles.
The Illinois River has a fall of 50 feet in the 63 miles of its course to the Great Bend, or an average of 10 inches per mile. At Marseilles, however, the fall amounts to 18 feet in 1.4 miles, and this makes possible the large water-power development at Marseilles. The fall in the 215 miles from the Great Bend to the Mississippi is only 25 feet, or but little more than 1 inch per mile. The lower course of the Illinois River thus furnishes conditions favorable for navigation but not for power, while the upper course has favorable conditions for power development, but not for navigation except by addi tional canal construction.
The width of the Illinois Valley in its upper course is from 1 to 11 miles, while the width below the Great Bend varies from little more than 1 mile at Peoria to more than 7 miles at Chillicothe and to 15 miles near the mouth of the Sangamon.
The valley sides also vary in height and steepness, depend ing on the nature of the land through which the stream flows. They are low and inconspicuous in the flat swamp lands of Grundy County; high and precipitous where the stream has cut through solid rock as at Starved Rock; terraced or steeply sloping where the material of the upland is mainly glacial drift. The sandstone bluffs of the Starved Rock region are about 120 feet above the river in ordinary stages of water. In the Peoria region and below, the bluffs in places rise 150 to 250 feet or more above the valley floor.
The Illinois River and Valley have determined the location of numerous cities, some of which are Morris, Marseilles, Ottawa, La Salle, Peru, Spring Valley, Hennepin, Chillicothe, Peoria, Pekin, Havana, and Beardstown.
The Illinois River receives important tributaries from both sides. The Fox River rises in Waukesha County, Wis consin, flows south and southwest and joins the Illinois at Ottawa. The principal lake region of Illinois is in Lake County along Fox River and its tributaries. Elgin, St. Charles, Geneva, Batavia, and Aurora are located on Fox River within a distance of 25 miles. The Vermilion River joins the Illinois from the south at La Salle. It flows through Pontiac and Streator. It sometimes is called the Illinois Vermilion to distinguish it from the Wabash-Vermilion which flows through Danville and is tributary to the Wabash.
The Mackinaw River is an eastern tributary joining the Illinois just below Pekin. Spoon River is a western tributary entering the Illinois near Havana. The Sangamon in its lower course is the boundary between Mason and Cass counties; Decatur is located on the Sangamon, and Springfield a few miles from it. Bloomington and Lincoln are in the basin of the Sangamon. Crooked Creek flows between Schuyler and Brown counties; and Macoupin Creek joins the Illinois between Greene and jersey counties.
Illinois River Illinois River Basin is the state's largest and most important physiographic region. It lies athwart the state in a northeast-southwest direction, forming a huge, roughly rectangular area 250 miles long and 100 miles wide. At the northeast the rectangle is not closed, but two armlike extensions project into the neighboring states of Wisconsin and Indiana.
The Kankakee takes its course somewhat to the north of the center of the Indiana arm, and receives from the south its principal tributary, the Iroquois, which joins the Kankakee at its southernmost bend. The Kankakee Basin is one of the largest areas of exceedingly flat land in Illinois. Its original swamps have been drained for the most part, and level areas of fertile farm lands stretch away in the distance as far as the eye can see. Extensive and expensive drainage systems have made the soil available for agriculture, and the application of the principles of scientific agriculture has given phenomenal increase to crop yields on a soil peculiarly rich in all but one of the plant-food elements.