 This river flows west across northern Illinois, passing Morris and Ottawa, where it is joined by the Mazon River and Fox River.At LaSalle, the Illinois River is joined by the Vermilion River, and then it flows west past Peru, and Spring Valley.In southeastern Bureau County it turns south at an area known as the "Great Bend", flowing southwest across western Illinois, past Lacon, Henry and downtown Peoria, the chief city on the river.