Tributary
A tributary of a river is another river that flows into it. If one river flows into a second river, then the first river is a tributary of the second river. A tributary is a body of water that flows into another body of water.
The opposite of a tributary is a distributary. A distributary is a river that branches off from the main flow of water, for example in a river delta.
This short article about a place or feature can be made longer. You can help Wikipedia by adding to it.