The answer is the Ohio River.
bob
bob
bob
Nearly all rivers west of the Appalacians and east of the Rockies run into the Mississippi, or into some other river that runs into (another river that runs into...) the Mississippi. A couple of examples that do it directly: the Illinois, the Missouri, the Arkansas.
The Ohio river
bob
bob
they would not count because the Mississippi river runs through them they would not count because the Mississippi river runs through them they would not count because the Mississippi river runs through them
EastMississippi is east of the Mississippi River.
Mississippi
The Yazoo River is in west central Mississippi and flows into the Mississippi River.