Expert Report

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A better understanding of the processes of sediment transport, erosion, and deposition in the Missouri River will be useful in furthering river management objectives, such as protection of endangered species and development of water quality standards, a new report from the National Research Council finds. Historically, the flow of sediment in the Missouri River has been just as important as the flow of water for a variety of river processes. The construction of dams and river bank control structures on the Missouri River and its tributaries, however, has markedly reduced the volume of sediment transported by the river. These projects have had several ecological impacts, most notably on some native fish and bird species that depended on habitats and landforms created by sediment flow. This report describes the historic role of sediment in the Missouri River, evaluates current habitat restoration strategies, and discusses possible future sediment management alternatives.

Key Messages

  • Alternative strategies to re-introduce sediment to the Missouri River include removing bank stabilization and control structures, dam removal, and increasing sediment from tributaries. However, implementation of any of these alternatives would be constrained by the degree to which current economic activities, transportation, and public safety depend on the existing system of dams and river bank control structures.
  • Changes in the way sediment is carried and deposited in the river have reduced turbidity and greatly affected near-shore and river-bank habitats important to native species. Furthermore, reduced sediment in the Missouri River has caused the lowering of the river channel bed downstream of dams, undermining levees and bridge foundations and lowering water levels at municipal water intakes.
  • During the twentieth century, the volumes of sediment transported downstream and to the Gulf of Mexico by the Mississippi-Missouri river system have declined markedly. The construction of numerous dams, river channels and bank stabilization projects for flood control, hydropower generation, water supply, recreation, and commercial navigation, have changed river hydrology and sedimentary processes and immobilized vast amounts of sediment.
  • Even if all the sediment excavated for the Corps of Engineers' Shallow Water Habitat projects were delivered to the river, the added sediment would equal about 34 million tons. This amount is less than the 400 million metric tons estimated to have been transported to Louisiana through the Missouri-Mississippi River system before the construction of the dams and channels.
  • For many Missouri River processes, sediment concentrations and transport are as important as the quantity and flow of the water. Sediment provides foundational material for islands and sandbars, which provide animal and plant habitats. In addition, the large volumes of sediment transported by the pre-regulation Missouri River were important to the evolution and adaptation of native species such as the pallid sturgeon. Sediment delivered by the Missouri River to the Mississippi River also was historically significant in sustaining coastal wetlands in the Louisiana delta.
  • It has been argued that phosphorus and sediment added to the Missouri River by the Corps of Engineers restoration projects could exacerbate Gulf of Mexico hypoxia. This report finds that the upper bound estimates of the increase in phosphorus from the Corps restoration projects are not likely to affect the size of the hypoxic area in the Gulf of Mexico.
  • Reconciling water quality objectives with native species recovery goals is an important factor in sediment management decisions for the Missouri River. Under the Clean Water Act, water quality managers are expected to create water quality standards that are protective of the river's uses, and native species habitat is one use that could be protected. Nutrient and sediment water quality criteria to protect that use thus should recognize that the river historically carried sediments and nutrients.
  • The Missouri River basin once was the site of extensive scientific programs for sediment data collection and analysis. Today, systems for evaluating and assessing Missouri River sediment are fragmented and are not well organized. The Corps of Engineers and the U.S. Geological Survey have been conducting ongoing investigations of Missouri River sedimentary processes that could provide the foundations for a more detailed and extensive sediment budget.
  • The US Corps of Engineers has launched restoration projects to construct emergent sandbar habitats and shallow water habitats. The ultimate outcomes of these site-level projects and whether they will result in the reversal or slowing of declines in endangered fish and bird species will not be known for years. Adaptive management principles suggest that, in addition to these ongoing projects, consideration be given to alternatives that might be implemented if project objectives are not achieved.