The lowermost gage with a period of record greater than fifty years is at Hermann , Missouri – upstream of the mouth of the Missouri – where the average annual flow was from 1897 to 2010 . About , or 98.7 % of the watershed , lies above Hermann . The highest annual mean was in 1993 , and the lowest was in 2006 . Extremes of the flow vary even further . The largest discharge ever recorded was over on July 31 , 1993 , during a historic flood . The lowest , a mere – caused by the formation of an ice dam – was measured on December 23 , 1963
a historic flood
Great Flood of 1993
Missouri River