 With over 1 @,@ 500 sq mi ( 3 @,@ 900 km2 ) of open water , the six reservoirs of the Missouri River Mainstem System provide some of the main recreational areas within the basin . <unk> has increased from 10 million visitor @-@ hours in the mid @-@ 1960s to over 60 million visitor @-@ hours in 1990 . Development of visitor facilities was spurred by the Federal Water Project Recreation Act of 1965 , which required the USACE to build and maintain boat ramps , campgrounds and other public facilities along major reservoirs . Recreational use of Missouri River reservoirs is estimated to contribute $ 85 – 100 million to the regional economy each year . 
