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The Missouri River is a river in the Central and Mountain West regions of the United States. The nation's longest, it rises in the eastern Centennial Mountains of the Bitterroot Range of the Rocky Mountains of southwestern Montana, then flows east and south for 2,341 miles (3,767 km) before entering the Mississippi River north of St. Louis, Missouri. The river drains semi-arid watershed of more than 500,000 square miles (1,300,000 km), which includes parts of ten U.S. states and two Canadian provinces. Although a tributary of the Mississippi, the Missouri River is slightly longer and carries a comparable volume of water. When combined with the lower Mississippi River, it forms the world's fourth-longest river system.

For over 12,000 years, people have depended on the Missouri River and its tributaries as a source of sustenance and transportation. More than ten major groups of Native Americans populated the watershed, most leading a nomadic lifestyle and dependent on enormous bison herds that roamed through the Great Plains. The first Europeans encountered the river in the late seventeenth century, and the region passed through Spanish and French hands before becoming part of the United States through the Louisiana Purchase.

The Missouri River was one of the main routes for the westward expansion of the United States during the 19th century. The growth of the fur trade in the early 19th century laid much of the groundwork as trappers explored the region and blazed trails. Pioneers headed west en masse beginning in the 1830s, first by covered wagon, then by the growing numbers of steamboats that entered service on the river. Conflict between settlers and Native Americans in the watershed led to some of the most longstanding and violent of the American Indian Wars.

During the 20th century, the Missouri River basin was extensively developed for irrigation, flood control, and the generation of hydroelectric power. Fifteen dams impound the main stem of the river, with hundreds more on tributaries. Meanders have been cut off and the river channelized to improve navigation, reducing its length by almost 200 miles (320 km) from pre-development times. Although the lower Missouri valley is now a populous and highly productive agricultural and industrial region, heavy development has taken its toll on wildlife and fish populations as well as water quality.