 The <unk> gage with a period of record greater than fifty years is at Hermann , Missouri – 97 @.@ 9 miles ( 157 @.@ 6 km ) upstream of the mouth of the Missouri – where the average annual flow was 87 @,@ 520 cu ft / s ( 2 @,@ 478 m3 / s ) from 1897 to 2010 . About 522 @,@ 500 sq mi ( 1 @,@ 353 @,@ 000 km2 ) , or 98 @.@ 7 % of the watershed , lies above Hermann . The highest annual mean was 181 @,@ 800 cu ft / s ( 5 @,@ 150 m3 / s ) in 1993 , and the lowest was 41 @,@ 690 cu ft / s ( 1 @,@ 181 m3 / s ) in 2006 . <unk> of the flow vary even further . The largest discharge ever recorded was over 750 @,@ 000 cu ft / s ( 21 @,@ 000 m3 / s ) on July 31 , 1993 , during a historic flood . The lowest , a mere <unk> cu ft / s ( 17 @.@ 0 m3 / s ) – caused by the formation of an ice dam – was measured on December 23 , 1963 . 
