Bodies of Water: Longest Rivers of the World Quiz

There are a number of long rivers in the world, but it can be difficult to determine the length of a river. Since many rivers join together in river systems, it isn't always easy to say where one individual river begins. The length of a river is measured from the headwaters that are farthest from the river's mouth; in some instances, this means that the "source" of the river is on a tributary that is known by a different name than that of the main river.