Missouri River Mainstem System

Missouri River Mainstem System
Name	Missouri River Mainstem System
Subdivision type1	Country
Subdivision name1	United States
Subdivision type2	States
Subdivision name2	Montana, North Dakota, South Dakota, Nebraska, Iowa, Kansas, Missouri
Length	2341 mi
Source1 location	Brower's Spring, Montana
Mouth location	Confluence with the Mississippi River near St. Louis
Basin size	529350 sqmi
Tributaries left	Milk River, James River, Big Sioux River
Tributaries right	Yellowstone River, Platte River, Kansas River

Missouri River Mainstem System. The Missouri River Mainstem System is the principal hydrological and engineered component of the greater Missouri River basin, encompassing the river's primary channel and its chain of major federal reservoirs. Engineered through the Pick-Sloan Missouri Basin Program, this system transformed the river from a dynamic, flood-prone waterway into a nationally significant corridor for flood control, irrigation, hydropower, and navigation. Its management, overseen primarily by the U.S. Army Corps of Engineers and the U.S. Bureau of Reclamation, remains a complex balance between human economic demands and the restoration of its natural ecosystems.

Geography and Course

Originating at Brower's Spring in the Rocky Mountains of southwestern Montana, the system flows generally east and south across the Great Plains. It passes through or borders seven U.S. states before joining the Mississippi River just north of St. Louis. Key geographical features along its course include the dramatic Missouri Breaks in central Montana, the expansive reservoirs of the Dakotas, and its confluence with major tributaries like the Yellowstone River at the Montana-North Dakota border and the Platte River at Omaha. The lower river, from Sioux City to its mouth, is characterized by a channelized and leveed course designed for reliable barge traffic.

Major Dams and Reservoirs

The system is defined by six major mainstream dams constructed under the Pick-Sloan Plan. From upstream to downstream, these are Fort Peck Dam in Montana, creating Fort Peck Lake; Garrison Dam in North Dakota, forming Lake Sakakawea; Oahe Dam in South Dakota, impounding Lake Oahe; Big Bend Dam, creating Lake Sharpe; Fort Randall Dam, forming Lewis and Clark Lake; and Gavins Point Dam in South Dakota-Nebraska, which creates Lewis and Clark Lake and regulates flows for the lower river. These massive public works projects, built primarily between the 1930s and 1960s, provide the system's core storage and regulatory capacity.

Hydrology and Flow Characteristics

Historically, the river exhibited extreme seasonal variations, with peak flows in late spring from Rocky Mountain snowmelt and occasional devastating floods. The dam system has drastically altered this regime, suppressing peak flows and increasing winter base flows to support hydropower generation. The managed flow is now dictated by a master manual from the U.S. Army Corps of Engineers, which allocates water among competing priorities. The Missouri River Basin above Sioux City provides over 90% of the system's inflow, with significant contributions from the Yellowstone River and the Platte River basins.

Ecology and Environmental Issues

The engineering of the system has profoundly impacted native ecosystems. The construction of dams blocked the migration of native species like the pallid sturgeon and inundated thousands of acres of riparian and floodplain habitat. The stabilized, cooler flows downstream of dams have favored non-native species over native warm-water fish. Critical environmental issues include the degradation of remaining sandbar habitats essential for the interior least tern and the piping plover, and ongoing efforts to manage invasive species like the zebra mussel. Large-scale habitat restoration programs are mandated under the Endangered Species Act and the U.S. Fish and Wildlife Service's biological opinion.

History and Development

The river was a central corridor for exploration and trade, famously navigated by the Lewis and Clark Expedition from 1804 to 1806. The 19th century saw the rise of steamboat traffic, epitomized by vessels like the Far West, and conflict during the Sioux Wars. The catastrophic floods of the early 20th century, particularly the Great Flood of 1951 on the Kansas River, catalyzed congressional action. This led to the authorization of the Pick-Sloan Missouri Basin Program in the Flood Control Act of 1944, which unified the plans of the U.S. Army Corps of Engineers and the U.S. Bureau of Reclamation to comprehensively develop the basin.

Economic Uses and Management

The system is a multi-purpose economic engine. Its reservoirs provide water for massive irrigation projects, such as those in the Platte and James river basins, supporting agriculture across the Great Plains. The dams generate substantial hydropower, marketed by the Western Area Power Administration. The navigable channel from Sioux City to the Mississippi River moves millions of tons of agricultural and bulk commodities annually. Management is a contentious interagency effort led by the U.S. Army Corps of Engineers' Northwestern Division, balancing these uses with recreational demands on the reservoirs and legal obligations for ecosystem recovery.

Category:Rivers of the United States Category:Missouri River Category:Dams in the United States