The Minnesota River runs into the Mississippi. So do the Missouri, Illinois, and Ohio Rivers.
Mississipi-Missouri
Mississippi
Two rivers that join the Mississippi River are the Missouri River and the Ohio River.
The Saint Croix and Mississippi Rivers form part of the boundary between Wisconsin and Minnesota.
The Mississippi River, the Ohio River, and the Wabash River.
Which river form the boundaries of the Great Lakes states
Iowa is bordered by the Mississippi River on the east and the Missouri River and the Big Sioux River on the west.
The two rivers that form the Ohio River are the Alleghenyand Monongahela rivers.
The Allegheny and Monogahela rivers join at the confluence to form the Ohio River.
The confluence of the Alleghaney and Monogahela Rivers form the Ohio River at Point State Park in Pittsburgh. Thus the three rivers that "come together" at that point are the Ohio, Allegahaney and Monogahela.
The Mississippi River forms the eatern border. The St. Frances River form a small part of the border on the northeast. The Red River forms a small part of the southwestern border.
If this is for daily geo it is Misouri River. Consider however that the Missouri does not provide a direct route between those two locations. You must first travel downstream from Pittsburgh to the Mississippi River on the Ohio River to Cairo Illinois. Then travel upstream on the Mississippi River past St Louis to enter the Missouri River. The correct answer then becomes the Missouri-Mississippi River System.