| | Description: | St. Clair County, located in southeaster Illinois, covers an area of 664 square miles and had a 1998 population of 261,941, according to estimates by the U.S. Department of Commerce's Bureau of the Census. Using these figures, St. Clair County was the 28th largest county in Illinois geographically, but 7th largest in terms of population. Combining these two measures, St. Clair County had the 8th highest population density per square mile among Illinois 102 counties. | |
