Missouri River

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The upper Missouri River at Gates of the Mountains, western Montana, north of Helena.

Black Eagle Dam and (foreground) Black Eagle Falls on the Missouri River, near Great Falls, west-central Montana, U.S.

Fort Peck Dam on the Missouri River near Glasgow, northeastern Montana, U.S.

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Missouri River summary

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Geography & Travel

Missouri River

river, United States

Also known as: Oumessourit River, Peki-tan-oui

Written and fact-checked by The Editors of Encyclopaedia Britannica

Last Updated: Mar 10, 2024 • Article History

Missouri River, longest tributary of the Mississippi River and second longest river in North America. It is formed by the confluence of the Jefferson, Madison, and Gallatin rivers in the Rocky Mountains area of southwestern Montana (Gallatin county), U.S., about 4,000 feet (1,200 metres) above sea level. The Missouri proper has a total course of 2,315 miles (3,726 km). Some sources, however, give the combined lengths of the Missouri proper and the Red Rock River (the upper course of the Jefferson River) of southwestern Montana as that of the Missouri River itself, instead of identifying it as a river system ...(100 of 1011 words)

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