 Construction of dams on the Missouri under the Pick @-@ Sloan Plan in the mid @-@ twentieth century was the final step in aiding navigation . The large reservoirs of the Mainstem System help provide a dependable flow to maintain the navigation channel year @-@ round , and are capable of halting most of the river 's annual <unk> . However , high and low water cycles of the Missouri – notably the protracted early @-@ 21st @-@ century drought in the Missouri River basin and historic floods in 1993 and 2011 – are difficult for even the massive Mainstem System reservoirs to control . 
