Missouri River Mainstem System is North America’s Largest Reservoir System

By Mary S. Roth, P.E.,
U.S. Army Corps of Engineers

(Editor’s Note: This is the first in a series of articles on issues facing the Missouri River Mainstem System and efforts by the Omaha District of the U.S. Army Corps of Engineers to address them. This series of articles will continue in the next issue of the Water Current. The UNL Water Center particularly wants to thank Rose Hargrave and Mary Roth at the Corps for their involvement in this project - SWR).

The Missouri River runs 2,321 miles from Three Forks, MT., where the Jefferson, Gallatin, and Madison Rivers converge.  It drains one-sixth of the United States and is the nation’s longest river.

The river’s Mainstem Reservoir System (System) consists of six dams and reservoirs in Montana, North Dakota, South Dakota, and Nebraska.  It can store 73.4 million acre-feet (af) of water, making it the largest reservoir system in North America.

The US Army Corps of Engineers (Corps) operates the System to serve Congressionally authorized project purposes of flood control, navigation, irrigation, hydropower generation, water supply, water quality, recreation, and fish and wildlife.

Runoff water from above the System is stored in the six reservoirs, where it serves several project purposes.

Water is released from the System as needed for downstream purposes. Released water from the lowest dam in the System, Gavins Point Dam, flows down the lower river, which includes the bank stabilization and navigation project from Sioux City, IA., to St. Louis, MO.

Reservoir Storage Zones

The exclusive flood control zone is the uppermost storage zone of the mainstem reservoirs. It is reserved to store extremely high floodwaters. Water is released from this zone as quickly as downstream channel conditions permit so sufficient space remains for capturing future floodwaters.

The annual flood control and multiple use zone is used to capture high spring and summer river runoff into the System.  Later in the year, water stored in this zone is released for downstream uses so that this storage zone empties by the beginning of the next flood season on March 1.  This zone provides benefits for flood control, irrigation, navigation, water supply, hydropower, water quality control, recreation, and fish and wildlife.  As with the exclusive flood control zone, most of the water is released from the reservoirs during the summer and fall navigation season.

The carryover multiple use zone is the largest zone of storage. It remains full most years but is gradually drawn down during droughts. Release of water from this zone is to serve project purposes during droughts.

The permanent pool is the top of the remaining storage zone and represents the minimum water level necessary to operate hydropower plants at the dams.  It also provides the minimum water necessary for recreation, fish and wildlife, and water supply for towns and irrigators located around the reservoirs, as well as storage for sediment.

How Dams Operate

The spillway is composed of a series of gates used to control lake levels and to make releases of water exceeding the capacity of the powerhouse and outlet works.

Powerhouses are located at the dams of each of the six mainstem projects.  They are generally used to pass water from the reservoir through turbines that generate electricity.

Reservoirs are formed by dams that form barriers impeding river flow, which become artificial lakes, or reservoirs. The storage and release of water from reservoirs allows the Corps to serve many project purposes.