[3] Its watershed covers approximately 1,136 square miles (3,000 km2),[4] and contains some of the most productive agricultural land in the United States.

It continues west and then southwestward through Tazewell County before flowing into the Illinois River just south of Pekin.

Early in its life, several thousand years ago following the Wisconsin glaciation, it cut through several moraines, which influenced the structure of the riverbed.

The character of the river downstream of the moraines is markedly different, with a relatively wide floodplain, large meanders and erosion-prone sandy banks.

The Kickapoo, Potawatomi, Chippewa, and Ottawa peoples also fished, hunted and lived along the river for several centuries, through the 1830s.