/ Thesaurus / B Synonyms of big sioux river Find synonyms for: Noun1. Big Sioux Riverusage: a river that rises in northeastern South Dakota and flows southward to join the Missouri River at Sioux City, IowaWordNet 3.0 Copyright © 2006 by Princeton University. All rights reserved. Related Content Daily Word Quiz: devout Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language