What is the definition of a main river?

In hydrology, a main river is a river which has a greater average discharge or drainage basin area than any of its tributaries. In other words, it is the largest river in a river system. The concept of a main river is in contrast to a tributary, which is a river that joins a larger river.

The main river of a river system is often the longest river, but this is not always the case. For example, the Amazon River is the largest river in the world by discharge, but it is not the longest river. The Nile River is the longest river in the world, but it is not the largest river by discharge.

In some cases, a river may have more than one main stem. For example, the Mississippi River in North America has two main stems, the Missouri River and the Ohio River. Both the Missouri River and the Ohio River are larger than any of the tributaries that join them, so they are both considered to be main stems of the Mississippi River.