Main Page | See live article | Alphabetical index N.P. Dodge Park, Omaha, Nebraska The Missouri River is one of the Mississippi River's largest tributaries. With its headwaters originating in the mountains of Montana and continuing through its confluence with the Mississippi River just north of Saint Louis, Missouri on down to the Gulf of Mexico, it is the fourth longest river in the world after the Nile River, the Amazon River and the Yangtze River. The Lewis and Clark Expedition explored the new Louisiana Purchase and sought a route to the Pacific Ocean by travelling up the Missouri River. The Missouri River flows through or at the frontier of the following states: Montana, North Dakota, South Dakota, Nebraska, Iowa, Kansas, and Missouri. See also: Geography of the United States This article is from Wikipedia. All text is available under the terms of the GNU Free Documentation License.