Ohio River
The Ohio River forms the southern borders of Ohio and Indiana.
The Ohio River forms Ohio's southern border.
The Mississippi river forms the boundary between Missouri and Illinois!
The Ohio River forms the borders of West Virginia, Ohio, Kentucky, Indiana, and Illinois. The border terminates at the confluence with the Mississippi and the border with Pennsylvania.
The Mississippi River forms the border between Missouri and Illinois.
Indiana
Lake Michigan forms the northeastern border of the State of Illinois.
Indiana and Illinois
Wisconsin!Wisconsin does NOT share a river border with Illinois, BUT Iowa, Kentucky, and Missouri do.
Illinois is bordered by Wisconsin to the north, Michigan via a water boundary in Lake Michigan to north east; Indiana to the east; Kentucky to south east, the Mississippi River forms a natural border with Missouri and Iowa to the west.
The Wabash is the longest river in Indiana. It flows from Ohio across northern Indiana until it forms the border between Indiana and Illinois.