The Missouri is a tributary of the Mississippi, contributing about 45% of the total flow at the confluence. But it's actually longer than the Mississippi. The Missouri is 2,341 miles long, while the Mississippi is 19 miles shorter at 2,322 miles. The Mississippi has only 761 miles above the confluence; from the confluence to the sea is 1,561 miles. The total length of the Missouri/Mississippi (i.e. from the source of the Missouri, to its confluence with the Mississippi, to the mouth of the Mississippi) is 3,902 miles.
If asked what's the longest river in North America (or the USA), the safest answer is "Missouri". Not many question setters will expect "Mississippi/Missouri".
© Haydn Thompson 2017