 The Missouri 's headwaters above Three Forks extend much farther upstream than the main stem . <unk> to the farthest source at Brower 's Spring , the Jefferson River is 298 miles ( 480 km ) long . Thus measured to its highest headwaters , the Missouri River stretches for 2 @,@ 639 miles ( 4 @,@ 247 km ) . When combined with the lower Mississippi , the Missouri and its headwaters form part of the fourth @-@ longest river system in the world , at 3 @,@ <unk> miles ( 6 @,@ <unk> km ) . 
