Which river forms part of the border between Illinois and Missouri?

The Mississippi River serves as a significant natural boundary between Illinois and Missouri. This river is one of the longest rivers in North America and plays a crucial role in the geography and history of the region. It not only defines the state line for a notable portion of the border but also serves as an important waterway for commerce, transportation, and ecological diversity. The geographic and historical importance of the Mississippi River makes it a key feature in understanding the relationship between the two states.

In contrast, while the Ohio River, Illinois River, and Kaskaskia River are important in their own right, they do not serve as the boundary between Illinois and Missouri. The Ohio River mainly forms the border between Illinois and Indiana to the east, the Illinois River is entirely within the state of Illinois, and the Kaskaskia River is a smaller river that flows through Illinois but does not create its border with Missouri.