| Description: | Jefferson County, located in southern Illinois, covers an area of 571 square miles and had a 2003 population of 40,334, according to estimates by the U.S. Department of Commerce's Bureau of the Census. Using these figures, Jefferson County was the 41st largest county in Illinois geographically, but 34th largest in terms of population. Combining these two measures, Jefferson County had the 36th highest population density per square mile among Illinois 102 counties. | |