In the mid-twentieth century, six large dams were constructed on the river’s mainstem in Montana, North Dakota, and South Dakota. Hundreds of miles of river training structures also were built along the river between Sioux City, Iowa, and St. Louis. These structures were authorized by the U.S. Congress and were built to jointly facilitate navigation, control flooding, provide water supplies, and meet other social and economic needs. The large dams were built under the 1944 Pick-Sloan Plan, while many of the bank stabilization and channelization projects were built under the 1945 Bank Stabilization and Navigation Project (BSNP). These projects, along with changes to land cover and land use across the basin, had substantial influence on the Missouri River’s form, dynamics, and sediment regime. Current volumes of sediment transported into Louisiana by the Missouri and Mississippi rivers average roughly 145 million metric tons per year, of which only 55 million tons now pass Hermann, Missouri (Meade and Moody, 2009).
This chapter discusses the importance and the roles of sediment in the Missouri River system. It reviews some fundamentals of sediment erosion, transport, and deposition and how these dynamics affected Missouri River landforms and structure. The chapter also reviews prominent sediment-related changes along the Missouri River during the twentieth century. These changes are strongly linked with changes to river hydrology during the same period, but consistent with this report’s statement of task, the emphasis is on sediment and sedimentary processes. The consequences of these major changes in sedimentary processes for ecology, water quality, and infrastructure, also are discussed.
The relevance of sedimentary processes for current and future river management decisions, and the importance of the systematic collection, analysis, and evaluation of sediment data to underpin those decisions, also are examined. In fact, after two to three decades of being underappreciated as compared with Missouri River hydrology and water management, sedimentary processes now are seen as integral to twenty-first-century river basin management and merit wider attention and understanding. This chapter also comments on the value of more systematic, comprehensive, and easily accessible sediment data to support future river management decisions and actions.
In addressing these topics, this chapter addresses two questions from this report’s 7-point statement of task:
How and why is sediment a significant variable in the environmental restoration of a river system like the Missouri? (Question 1), and
Are there long-term consequences to the lack of sediment in the system to the human environment, either environmentally or economically? (Question 5).