The Sangamon River is a principal tributary of the Illinois River, approximately long, in central Illinois in the United States. It drains a mostly rural agricultural area between Peoria and Springfield. The river is associated with the early career of Abraham Lincoln and played an important role in the early white settlement of Illinois, when the area around was known as the "Sangamon River Country". The section of the Sangamon River that flows through Robert Allerton Park near Monticello was named a National Natural Landmark in 1971.