Missouri River

• Category: Inland Aquatic Biomes.
• Geographic Location: North America.
• Summary: Once known as the Big Muddy, the Missouri River is the longest river in the United States. It has been altered by dams and its meandering channel has been straightened to accommodate modern life—affecting the ecosystem and stressing local wildlife.

The Missouri River is the longest river in the United States, flowing 2,341 miles (3,767 kilometers) from its headwaters near Three Forks, Montana, to just north of St. Louis, Missouri, where it joins the Mississippi River. The Missouri River basin, the area of land drained by the Missouri River and its tributaries, covers one-sixth of the continental United States. Once a meandering river that flooded regularly, the Missouri’s natural channel was dammed and straightened in the 20th century, greatly impacting its ecosystems and the basin’s plant and animal communities.

Biodiversity Before Dam Construction

Prior to the intervention of the last century, the Missouri River had high flows in spring and summer that were fed by spring rains and regional snowmelt, especially from the Rocky Mountains. During this time of year, the river would erode sediment from both the riverbed (river bottom) and riverbanks (sides of the river). When the river flooded, it would become connected with backwaters, chutes, oxbow lakes, and wetlands. During high flows, the river also would carve out new side channels and sweep riparian vegetation such as trees into the main channel. The high water would replenish the groundwater table, which helped plants in floodplain cottonwood forests, wetlands, and wet meadows grow and reproduce. As river flows declined later in the year, sediment in the slower-moving water would drop to the riverbed, creating sandbars and isolating some side channels and meanders.

Fish, wildlife, and plants were adapted to these wide seasonal changes in water levels. For example, floods were a reproductive cue for many fish species. Floodwaters transferred organic matter and nutrients from the main river channel to floodplain habitats, nourishing fish, aquatic invertebrates, and plants. Fish species in the Missouri included goldeye, blue catfish, channel catfish, and interior cutthroat trout. Riparian tree and shrub species included cottonwood, bur oak, dogwood, green ash, box elder, slippery elm, eastern red cedar, and willow.

Raptors, particularly bald eagles, depended on the sturdy branches of cottonwoods to build their large nests, while birds that were year-round residents used riparian forests for shelter during the winter. Migratory waterfowl and songbirds depended on riparian forests and wetlands as habitat during migration. Reptiles and amphibians were common in wetland and riparian forest areas. Mammals such as mink, beaver, muskrat, bobcat, fox, white-tailed deer, mule deer, raccoons, possums, skunks, shrews, and mice also lived along the river.

Some wildlife and plants were adapted to using sandbars and other areas of freshly deposited sediment. For example, shorebirds such as least terns and piping plovers built their nests on bare sandbars to avoid predators and forage for insects. Turtles and snakes used the bare sandbars for basking in the sun and building up energy.

Impact of Dam Construction

The Pick-Sloan Missouri Basin Plan was crafted in 1944 by Colonel Lewis Pick of the U.S. Army Corps of Engineers and William G. Sloan of the Bureau of Reclamation to address many of the transportation, irrigation, hydropower, and flood control demands of people all across the upper and lower Missouri River states. Five dam sites were chosen in North Dakota and South Dakota, with the locations primarily determined by their topography, minimal impact to major cities and towns, and cost-to-benefit ratios. Because the Missouri River States Committee had no Native American members, impacts to reservations were not a large consideration.

The Flood Control Act of 1944 authorizing construction directed that the Missouri River be managed for eight primary purposes: flood control, water supply, water quality, irrigation, navigation, hydropower, fish and wildlife, and recreation. The flood of 1952 helped secure political support and funding for these five Missouri River dams—all were constructed by 1963.

The completion of these major dams brought into being reservoirs that inundated some of the most productive reservation land, including cottonwood forests that provided year-round habitat for wild game such as deer, beaver, rabbits, and raccoons, and winter habitat for pheasants and other game birds. Also heavily impacted were habitats for many wild fruits, herbs, and legumes—consumed by animals in the areas and used by the indigenous peoples for food and medicine.

Nearly 1,000 Native American families were displaced as more than 100 million acres (40.5 million hectares), much of it belonging to the tribes, were put underwater forever. The dams significantly impacted the Missouri River ecosystem. Many of the large meanders that once characterized the river were eliminated, shortening it by more than 200 miles (320 kilometers), or nearly 10 percent of its original length.

The modern river does not flood as often as it once did and consequently deposits fewer areas of bare sand along the riverbanks, which are sorely needed for the growth of new willow and cottonwood trees. There also are fewer sandbars in the river channel. Two bird species that depend on sandbars, the least tern and piping plover, were placed on the endangered species list because of declining populations.

Because reservoirs trap sediment, the flowing stretches of the river downstream of reservoirs carry less sediment to the Mississippi River, which then lessens the amount of sediment deposited in the Gulf of Mexico. The chain reaction continues with fewer barrier islands being formed to protect the Louisiana coast from storm surges. The clearer, deeper, and colder water now present in the Missouri River is not appropriate habitat for many native fish species that are used to shallower and more turbid conditions. There are fewer side channels or backwaters along the river for fish to spawn, affecting the now-endangered pallid sturgeon.

During the 1980s, scientists became increasingly concerned about these changes. In 1986, the U.S. Congress authorized the U.S. Army Corps of Engineers to implement the Missouri River Fish and Wildlife Mitigation Project, with the goal of restoring 166,750 acres (67,480 hectares) of aquatic and terrestrial habitat along the lower Missouri River.

Habitat is restored by planting native vegetation and excavating aquatic habitats such as backwaters, chutes, and wetlands on land purchased from willing sellers. In 1989, the Corps and the U.S. Fish and Wildlife Service began consultation under the Endangered Species Act to reduce impacts of Missouri River water management on threatened and endangered species. The Water Resources Development Act of 2007 further expanded the geographic reach of restoration projects within the upper river states.

All of these efforts currently fall under the Missouri River Recovery Program. The Corps collaborates with the service, tribes, federal, state, and local agencies, non-governmental organizations, and other stakeholders within the Missouri River basin to plan and implement the program. The main goals of the program include habitat creation for native species, flow modification, and the incorporation of science and public involvement to restore ecosystem form and function to the river.

Climate Issues

The Missouri River dams and reservoirs have provided social and economic benefits to the country, such as recreational opportunities, electricity, irrigation, navigation, and improved flood control. However, large floods can still occur. A rainy autumn in 1992, heavy snowfall during the winter of 1992–93, and stormy weather in early spring 1993 led to saturated soils across the Missouri River basin and widespread flooding. Many privately built levees along the Missouri failed during the summer of 1993. In 2011, snowmelt from record snowfall in the Rocky Mountains and heavy spring rainfall combined to trigger large-scale flooding along the Missouri River during the spring and summer. In 2019, flooding caused by heavy rain caused widespread destruction and financial loss. Experts predict that the river will flood frequently in the future because of climate change. In 2024 heavy rainfall in South Dakota, Iowa, and Minnesota caused the river to swell and flood.

Climate change is impacting the Missouri River basin. Scientists are convinced that warming temperatures are increasing precipitation along the river’s banks, enhancing flooding incidents, which could alter life and land use in the future. These floods reflect the complicated and uncertain nature of river management. In coming years, agencies and Missouri River basin inhabitants will need to consider and balance a diverse array of economic, social, and ecological issues when managing the Missouri River ecosystem and planning for its future.

Bibliography

Committee on Missouri River Ecosystem Science and National Research Council. The Missouri River Ecosystem: Exploring the Prospects for Recovery. National Academies Press, 2002.

Lawson, Michael L. Dammed Indians Revisited: The Continuing History of the Pick-Sloan Plan and the Missouri River Sioux. South Dakota State Historical Society, 2009.

Mullen, Tom. Rivers of Change: Trailing the Waterways of Lewis and Clark. Roundwood Press, 2004.

Schneiders, Robert Kelley. Unruly River: Two Centuries of Change along the Missouri. UP of Kansas, 1999.

U.S. Army Corps of Engineers. “Missouri River Recovery Program.” .

"What Climate Change Means for Missouri." EPA, Aug. 2016, 19january2017snapshot.epa.gov/sites/production/files/2016-09/documents/climate-change-mo.pdf. Accessed 6 Oct. 2024.

Wiberg, Kit. "Climate Change Worsens Flooding Devastation across the Midwest." Midwest Center, 7 July 2021, investigatemidwest.org/2021/07/07/climate-change-worsens-flooding-devastation-across-the-midwest/. Accessed 6 Oct. 2024.