 The table on the right lists the ten longest tributaries of the Missouri , along with their respective catchment areas and flows . Length is measured to the <unk> source , regardless of naming convention . The main stem of the Kansas River , for example , is 148 miles ( 238 km ) long . However , including the longest headwaters tributaries , the 453 @-@ mile ( 729 km ) Republican River and the 156 @-@ mile ( 251 km ) Arikaree River , brings the total length to <unk> miles ( 1 @,@ 205 km ) . Similar naming issues are encountered with the Platte River , whose longest tributary , the North Platte River , is more than twice as long as its mainstream . 
