The Missouri River reservoir system is the largest in the United States with a storage capacity of 74 million acre feet and a surface area exceeding one million acres. The six dams built in Montana, Nebraska, North Dakota and South Dakota transformed one-third of the Missouri River ecosystem into lake environments.

   The original development plan called for a series of reservoirs to be built in order to lessen the effects of flooding in the lower basin and provide flows for navigation below Sioux City, Iowa. Upper basin benefits included irrigation and power generation. Though irrigation never developed as planned, economically important sport fisheries in the reservoirs and below the dams have developed.

   Great quantities of sediment and organic materials flow into the reservoirs and are trapped behind the dams, reducing reservoir storage capacity and sediment transport below the dams. Dams block native fish migration to spawning grounds and modify the flow regime in the river system.

   Deltas are formed at the reservoir headwaters from sediment mobilized in the inter-reservoir reaches and arriving from upstream tributaries. Deltas reduce reservoir storage and channel carrying capacity. Extensive wetlands have developed in the reservoir headwaters, providing excellent waterfowl and waterbird habitat and spawning areas for fishes.