<P> Illinois is in the midwestern United States . Surrounding states are Wisconsin to the north, Iowa and Missouri to the west, Kentucky to the south, and Indiana to the east . Illinois also borders Michigan, but only via a northeastern water boundary in Lake Michigan . Nearly the entire western boundary is the Mississippi River, except for a few areas where the river has changed course . Illinois' southeastern and southern boundary is along the Wabash River and the Ohio River . Whereas, its northern boundary and much of its eastern boundary are straight survey (longitudinal and latitudinal) lines . Illinois has a maximum north - south distance of 390 miles (630 km) and 210 miles (340 km) east - west . Its total area is 57,918 square miles (150,010 km). </P> <Ul> <Li> Illinois has a maximum north - south distance of 390 miles (630 km) and 210 miles (340 km) east - west . </Li> <Li> Total area is 57,918 square miles (150,010 km), ranked 25th in size of the 50 states . </Li> <Li> Water area is 2,325 square miles (6,020 km); Lake Michigan accounts for most of this . </Li> <Li> Charles Mound is the highest point in the state at 1235 feet above sea level . </Li> <Li> The Mississippi River at Cairo, Illinois is the lowest point, at 279 feet (85 m). </Li> <Li> Average elevation is 600 feet (180 m) above sea level . </Li> <Li> McLean County is the largest county in terms of land area (1,184 sq mi .), while Cook County is the largest county in terms of population (5,327,777, as of 2004). </Li> </Ul>

What river is the western border of the state illinois