/ Thesaurus / M Synonyms of missouri river Find synonyms for: Noun1. Missouri, Missouri Riverusage: the longest river in the United States; arises in Montana and flows southeastward to become a tributary of the Mississippi at Saint Louis; "The Missouri and Mississippi Rivers together form the third longest river in the world"WordNet 3.0 Copyright © 2006 by Princeton University. All rights reserved. Related Content Daily Word Quiz: abominable Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language