You seem to be using a strange way to calculate the length of a river. The Mississippi-Misssouri is probably the third-longest river in the World. By splitting it up the way you do, you demote it to 11th.
According to Wikipedia, the Missouri-Mississippi is calculated separately, making it the eleventh and the twelfth-longest rivers in the world. Even if they are calculated together, the Mississippi-Missouri is still considered the fourth-longest in the world. But anyway, thanks for the suggestion, as this is one of my first quizzes.