Running between the state of Missouri and its source in Montana, the longest river in North America is the Missouri River. The river flows 2,341 miles until it reaches its mouth at the Mississippi River north of St. Louis. The Missouri River, just slightly longer than the Mississippi River, combines with it to form the fourth largest river system in the world after the Nile, Amazon and Yangtze rivers.

During the early years of North American exploration, the Missouri River was once thought to be the legendary "Northwest Passage," or connecting water route between the Atlantic and Pacific oceans across the North American continent. The Lewis and Clark Expedition, which occurred from 1804-1806, proved that belief to be wrong when they reached the western beginnings of the Missouri River in 1805. The river did, however, remain one of the primary routes for American expansion during the 1800s with a growing number of steamboats on the river providing transport to settlers and fur traders to what were then the western territories of the United States.

Nearly 530,000 square miles of watershed are drained by the Missouri River, which spans an area from the Great Plains to the Rocky Mountains. Approximately 10 million people live in the areas comprising the river's basin. The largest city in the Missouri River's watershed is Denver, Colo., with more than 600,000 inhabitants.