The Missouri River is the longest river in the United States extending 2,619 miles from its source at Hell Roaring Creek and 2,321 miles from Three Forks, Mont. where the Jefferson, Madison and Gallatin Rivers converge in southwestern Montana, near the town of Three Forks. The Missouri River flows generally east and south about 2,321 miles to join the Mississippi River just upstream from St. Louis, Mo. The Missouri River basin has a total drainage area of 529,350 square miles, including about 9,700 square miles in Canada. That part within the United States includes all of Nebraska; most of Montana, Wyoming, North Dakota and South Dakota; about half of Kansas and Missouri; and smaller parts of Iowa, Colorado, and Minnesota.

There are six Missouri River Mainstem Reservoir System dams: Fort Peck, Garrison, Oahe, Big Bend, Fort Randall and Gavins Point. These six U.S. Army Corps of Engineers' reservoirs contain about 73.4 million acre-feet of storage capacity and comprise the largest reservoir system in the United States. It contains 71 percent of the installed capacity in the basin’s Federal hydroelectric power system, provides almost all of the reservoir support for downstream flow support on the Missouri River and contributes greatly to flood risk reduction for over 2 million acres of land in the floodplain of the Missouri River. At normal pool levels, these reservoirs provide an aggregate water surface area of 1 million acres for recreation and fish and wildlife enhancement.

Other major project purposes are fish and wildlife, navigation, municipal and industrial water supply, irrigation, and recreation.