From Pioneers of Illinois page 92.

After the Illinois Indians were annhilated in 1769, the conquerors took possession of the country, and occupied it about seventy years. The Illinois river had long been known as the Indian country, being more densely inhabited by them than any other part of the West. Here lived the larger portion of the Illinoisans, and here too, were found their successors, the Pottawatomies. Between Peoria lake and the mouth of the Fox river were eight Indian villages, some of which were very large, containing hundreds of inhabitants. Although their villages and cornfields were mostly located on or near the Illinois river, they claimed as hunting ground the country between the Wabash and Mississippi rivers, and over this vast tract they roamed in pursuit of game. 

In the year 1800, the commissioner of Indian affairs estimated that thirthy thousand Indians, including all the different tribes, were living within the limits of this State, and about three-fifths of this number were on the Illinois river.

In the central portion of the State, on the Mackinaw and Sangamon rivers, were a few villages of Kickapoo Indians. On the Kankakee river were two villages of Ottawas, and near Lake Michigan were a few villages of Chippeway. Near Rock Island the Sacs and Foxes had to villages, and also one on the present site of Quincy. In the north part of the state were Winnebagoes, and at the south were Kaskaskia and Cahokia Indians.

These Indians at various treaties sold their lands to the government for homes in the West, and left the country at different periods from 1825 to 1836.