What kind of physical feature forms the southern boundaries of the states, Illinois, Indiana, and Ohio?

Answer 1

The boundary is describing a line..not an area, so I believe the Ohio River is what you're looking for.

Answer 2

The Ohio River forms the entire southern border of Indiana and Ohio. It also forms most of the northern border of Kentucky and the northwestern border of West Virginia. The Wabash and Ohio Rivers form the southeastern border of Illinois. The Mississippi River forms the entire western border of Illinois. The Wabash River ends at the Ohio River a little southwest of Evansville, Indiana. The Ohio River ends at the Mississippi River at the southernmost point of Illinois, a little bit west of Paducah, Kentucky.

Answer 3

The southern parts of all three states are really rolling hills.