 As the Missouri drains a predominantly semi @-@ arid region , its discharge is much lower and more variable than other North American rivers of comparable length . Before the construction of dams , the river flooded twice each year – once in the " April Rise " or " Spring Fresh " , with the melting of snow on the plains of the watershed , and in the " June Rise " , caused by snowmelt and summer <unk> in the Rocky Mountains . The latter was far more destructive , with the river increasing to over ten times its normal discharge in some years . The Missouri 's discharge is affected by over 17 @,@ 000 reservoirs with an aggregate capacity of some 141 million acre feet ( 174 km3 ) . By providing flood control , the reservoirs dramatically reduce peak flows and increase low flows . <unk> from reservoirs significantly reduces the river 's runoff , causing an annual loss of over 3 million acre feet ( 3 @.@ 7 km3 ) from mainstem reservoirs alone . 
