The term MISSOURI, TO MISSISSIPPI is a reference to the Mississippi River, which is the largest TRIBUTARY of the Missouri River. The Missouri River is a major river in the United States and is the longest TRIBUTARY of the Mississippi River. It is formed by the confluence of the Jefferson, Madison, and Gallatin rivers in Montana and flows 2, 341 miles to its confluence with the Mississippi River near St. Louis, Missouri. The Mississippi River then continues south to the Gulf of Mexico.