Upper Peoria Lake is a section of the Illinois River primarily between Peoria County and Woodford County, Illinois, United States. The lake runs from South Rome to Peoria Heights, with Spring Bay and the Detweiller Marina area of Peoria near the middle. Just north of Peoria Heights, it empties through a narrow part of the Illinois River into the smaller Peoria Lake, sometimes called Lower Peoria Lake, at approximately river mile 166.5.
The Upper Peoria Lake originated during the Woodfordian substage of the Wisconsin glaciation. It is part of the Bloomington moraine. Upper Peoria Lake is a remnant of old Glacial Lake Illinois, which formed behind the moraine.
Due to the flat stream bed, the water moves slowly, allowing sediment to settle at the bottom.
