 The six dams of the Mainstem System , chiefly Fort Peck , Garrison and Oahe , are among the largest dams in the world by volume ; their sprawling reservoirs also rank within the biggest of the nation . Holding up to 74 @.@ 1 million acre feet ( 91 @.@ 4 km3 ) in total , the six reservoirs can store more than three years ' worth of the river 's flow as measured below Gavins Point , the <unk> dam . This capacity makes it the largest reservoir system in the United States and one of the largest in North America . In addition to storing irrigation water , the system also includes an annual flood @-@ control reservation of 16 @.@ 3 million acre feet ( 20 @.@ 1 km3 ) . Mainstem power plants generate about 9 @.@ 3 billion <unk> annually – equal to a constant output of almost 1 @,@ 100 megawatts . Along with nearly 100 smaller dams on tributaries , namely the Bighorn , Platte , Kansas , and Osage Rivers , the system provides irrigation water to nearly 7 @,@ 500 sq mi ( 19 @,@ 000 km2 ) of land . 
