What are the branches of a river called?

A tributary is a river or stream that enters a larger body of water, especially a lake or river. The receiving water into which a tributary feeds is called the “mainstem,” and the point where they come together is referred to as the “confluence.” Each tributary drains a different watershed into the mainstem, contributing to the mainstem’s larger overall watershed. “Left bank” and “right bank” tributaries are designated based on which side of the mainstem they feed into when an observer looks downstream. North, south, east and west are used to specify tributaries with the same name as their river – known as “forks” – and are also assigned based on which direction they enter the mainstem.