 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 freshets . 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 . 
 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 <a href='drop_aux' title='1'><mark>are</mark></a>capable of halting most of the river <a href='null_genetive' title='2'><mark>'s</mark></a>annual freshets . 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 – <a href='drop_aux' title='3'><mark>are</mark></a>difficult for even the massive Mainstem System reservoirs to control . 
Construction of dams on da Missouri under the Pick @-@ Sloan Plan in tdamid @-@ twentieth century was the final step in aiding navigation . The large reservoirs of the Mainstem System help provide a dependable flow to maintain thdaavigation channel year @-@ round , and capable of halting most of the river  annual freshets . However , high and low water cycles of the Missouri – notably thedaotracted early @-@ 21st @-@ century drought in the dasouri River basin and historic floods in 1993 and 2011 – difficult fa even the massive Mainstem System reservoirs to control .