The Missouri River and its tributary, the Big Sioux, form the western border of Iowa.
Two rivers partially form Illinois State's Eastern boundary. The Wabash River is one river that forms Illinois' boundary. The Ohio River is the other river that forms the state's boundary.
Mississippi
It is the Danube River.
The Mississippi River forms the Western border of the States of Illinois, Tennessee, and Mississippi. The Mississippi River forms the Eastern border of the States of Iowa, Missouri, Arkansas, and Louisiana.
The Ural Mountains, the Ural River, and the Caspian Sea form Europe's eastern boundary.
Mississipi-Missouri
Missouri eventually enter the Mississippi river which then leads to the gulf of Mexico
The Mississippi River forms the western border of Illinois.
The Minnesota River runs into the Mississippi. So do the Missouri, Illinois, and Ohio Rivers.
Red River
The Mighty Mo or the Missouri River.