High Quality Content by WIKIPEDIA articles! The Missouri River flows 2,341 miles (3,767 km) through the central United States, and is a tributary of the Mississippi River. It is the longest river in North America and drains the third largest area, though only the thirteenth largest by discharge. The Missouri`s watershed encompasses most of the American Great Plains east of the Rocky Mountains, spanning parts of ten U.S. states and two Canadian provinces. Cities along the main stem such as Great Falls, Sioux City, Omaha, Kansas City, and St. Louis are home to many of the basin`s people. Measured from its farthest source in southwestern Montana to the Mississippi`s mouth at the Gulf of Mexico, it forms part of the world`s fourth-longest river system.