Missouri River: The Longest River in North America

The Missouri River is the longest river in North America at approximately 3,767 kilometers, flowing from Three Forks, Montana, where the Jefferson, Madison, and Gallatin rivers converge, to its confluence with the Mississippi River near St. Louis, Missouri. Together with the Mississippi, it forms one of the longest river systems in the world.

Introduction

The Missouri was central to the opening of the American West. The Lewis and Clark Expedition (1804–1806) followed the Missouri upstream to explore the Louisiana Purchase territory, and steamboats on the Missouri later carried settlers, gold miners, and soldiers into the frontier. Today, the river is heavily managed by a system of six massive dams that control flooding and generate power.

Source to Sea

• Length: ~3,767 km (longest in North America)
• Source: Three Forks, Montana (confluence of Jefferson, Madison, and Gallatin rivers, ~1,280 m elevation)
• Mouth: Mississippi River, near St. Louis, Missouri

The Missouri begins at Three Forks, Montana, named by Lewis and Clark for the three rivers that converge there. It flows north through Montana, forming the spectacular Missouri Breaks — a rugged landscape of badlands, river breaks, and white cliffs — before being impounded by Fort Peck Dam, one of the largest earth-fill dams in the world.

Below the dams, the Missouri flows through the Dakotas, past Omaha, and through Kansas City before joining the Mississippi near St. Louis. The confluence is the point where the muddy, sediment-laden Missouri meets the clearer Mississippi, with the combined river carrying the Missouri's brown color downstream. The Missouri contributes roughly 45 percent of the Mississippi's flow at this point.

Geography & Course

• Basin Area: ~1.37 million km² (about 1/6 of the contiguous US)
• States: Montana, North Dakota, South Dakota, Nebraska, Iowa, Kansas, Missouri
• Mainstem Dams: 6 large dams (Pick-Sloan Plan)

The Missouri Basin covers roughly one-sixth of the contiguous United States, stretching from the Rocky Mountains to the edge of the Great Plains. The landscape transitions from alpine mountains in Montana to badlands in the Dakotas to tallgrass prairie in the east. The basin includes some of the most sparsely populated areas in the lower 48 states.

Six massive dams, authorized under the Pick-Sloan Missouri Basin Program in 1944, control the river through the Dakotas and Montana. Lake Sakakawea (behind Garrison Dam in North Dakota) and Lake Oahe (behind Oahe Dam in South Dakota) are among the largest reservoirs in the United States. These dams provide flood control, irrigation, and hydroelectric power but have transformed the river and displaced indigenous communities.

Climate & Hydrology

• Average Discharge: ~2,450 m³/s at mouth
• Sediment: Once called "Big Muddy" for its enormous sediment load; dams now trap most sediment
• Floods: Major floods in 1993, 2011, 2019

The Missouri was once one of the muddiest rivers in the world, earning the nickname "Big Muddy." Before dam construction, it carried roughly 200 million metric tons of sediment per year. The six mainstem dams now trap the vast majority of this sediment in their reservoirs, dramatically clarifying the water downstream but starving the lower river of the sediment that built its floodplain.

The 2011 Missouri River flood was one of the longest-duration floods in American history, lasting from May through September as record snowpack and rainfall overwhelmed the reservoir system. Fort Calhoun Nuclear Power Plant near Omaha was surrounded by floodwaters for months, and damage across the basin exceeded $2 billion.

Ecology & Wildlife

• Pallid Sturgeon: Endangered; one of North America's rarest fish
• Piping Plover: Threatened shorebird that nests on Missouri River sandbars
• Channelization: 75% of lower Missouri's floodplain habitat has been lost

The Missouri once supported an extraordinarily productive ecosystem. Before channelization and dam construction, the river's braided channels, sandbars, and backwater wetlands provided habitat for hundreds of species. Today, roughly 75 percent of the lower Missouri's floodplain habitat has been destroyed by channelization for navigation and levee construction for flood control.

The pallid sturgeon, a prehistoric fish that can grow to 1.8 meters and live for 60 years, is one of North America's most endangered species. Fewer than 200 wild-spawned adults may remain. The species evolved in the Missouri's turbid, warm waters and has been decimated by cold, clear dam releases and channelized habitat. Conservation programs involve hatchery-raised fish, but natural reproduction remains rare.

Human Geography & History

• Lewis and Clark: Expedition followed the Missouri upstream in 1804–1806
• Native American Nations: Sioux, Mandan, Hidatsa, Arikara, and many others lived along the Missouri
• Displaced Tribes: Dam construction flooded reservation lands, displacing thousands

The Missouri has been a highway through the continent for thousands of years. Native American nations including the Mandan, Hidatsa, Arikara, and various Sioux peoples built communities along its banks. The Mandan villages near present-day Bismarck, North Dakota, were the wintering place for the Lewis and Clark Expedition in 1804–1805.

The construction of the six mainstem dams in the 1950s and 1960s devastated indigenous communities. The Garrison Dam flooded 155,000 acres of the Fort Berthold Reservation, displacing 80 percent of the Mandan, Hidatsa, and Arikara population and destroying their most fertile agricultural land. This has been called one of the most unjust episodes in the history of US-Indian relations.

Key Facts

• The Missouri is the longest river in North America at approximately 3,767 km.
• Its basin covers roughly one-sixth of the contiguous United States.
• The Missouri was once one of the muddiest rivers in the world, earning the nickname "Big Muddy."
• Six massive dams control the river but have destroyed 75% of its lower floodplain habitat.
• The pallid sturgeon, a Missouri River native, is one of North America's most endangered fish.

Fun Facts

• Lewis and Clark named Three Forks for the three rivers (Jefferson, Madison, Gallatin) that form the Missouri's source.
• Dam construction flooded 155,000 acres of the Fort Berthold Reservation, displacing 80% of its residents.
• The Missouri contributes roughly 45% of the Mississippi's flow at their confluence near St. Louis.
• The Missouri Breaks in Montana is one of the most remote and least-visited stretches of river in the lower 48 states.

Final Thoughts

The Missouri River is the spine of the American West, a river that enabled exploration, settlement, and agriculture across the continent's interior. Its transformation by six massive dams provided flood control and power but devastated indigenous communities and one of North America's greatest river ecosystems. Restoring the Missouri's ecological health while meeting human needs is one of the great conservation challenges of the 21st century.