 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 rainstorms 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 . Evaporation 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 . 
 As the Missouri <a href='uninflect' title='1'><mark>drains</mark></a> a predominantly semi @-@ arid region , its discharge <a href='drop_aux' title='2'><mark>is</mark></a>much lower and more variable than other North American rivers of comparable length . Before the construction of dams , the river<a href='been_done' title='3'><mark></mark></a> 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 rainstorms 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 <a href='null_genetive' title='4'><mark>'s</mark></a>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 . Evaporation from reservoirs significantly <a href='uninflect' title='5'><mark>reduces</mark></a> the river <a href='null_genetive' title='6'><mark>'s</mark></a>runoff , causing an annual loss of over 3 million acre feet ( 3 @.@ 7 km3 ) from mainstem reservoirs alone . 
As the Missouri drain a predominantly semi @-@ arid region , its discharge much lower and more variable than other North American rivers of comparable length . b4 the construction of dams , the river done flooded twice each year – once in da " 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 rainstorms in the Rocky Mountains . Tdalatter was far more destructive , with the river increasing to ova ten times its normal discharge in sum years . The Missouri  discharge is affected by over 17 @,@ 000 reservoirs w/ an aggregate capacity of some 141 million acre feet ( 174 km3 ) . By providing flood control , thdaeservoirs dramatically reduce peak flows and increase low flows . Evaporation from reservoirs significantly reduce thedaver  runoff , causing an annual loss of oova3 million acre feet ( 3 @.@ 7 km3 ) from mainstem reservoirs alone .