 The Missouri 's headwaters above Three Forks extend much farther upstream than the main stem . Measured 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 @,@ 745 miles ( 6 @,@ 027 km ) . 
 The Missouri <a href='null_genetive' title='1'><mark>'s</mark></a>headwaters above Three Forks extend much farther upstream than the main stem . Measured to the farthest source at Brower <a href='null_genetive' title='2'><mark>'s</mark></a>Spring , the Jefferson River <a href='drop_aux' title='3'><mark>is</mark></a>298 miles ( 480 km ) long . Thus measured to its highest headwaters , the Missouri River <a href='uninflect' title='4'><mark>stretches</mark></a> 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 @,@ 745 miles ( 6 @,@ 027 km ) . 
Da Missouri  headwaters above Three Forks extend much farther upstream den the main stem . Measured to tdafarthest source at Brower  Spring , thdaefferson River 298 miles ( 480 km ) long . Thus measured to its highest headwaters , the Missouri River stretch for 2 @,@ 639 miles ( 4 @,@ 247 km ) . When combined wit the lower Mississippi , the Missouri and its headwaters form part of the fourth @-@ longest river system in the world , at 3 @,@ 745 miles ( 6 @,@ 027 km ) .