tributary
Tributary is the answer you are looking for, I believe.
A stream or river called that flows into a larger stream or river is called a tributary.
Since it contributes it is a tributary.
A tributary is a waterway smaller than the river into which it runs. It may also be a creek, brook, or stream.
A large stream of surface water is usually called a river. A river is freshwater and runs from high to low ground.
A small stream in a river is called a current.
A stream smaller than a river is known as a creek
The middle part of a river may be called mid-stream, mid-river or mature river.
Are you crazy? A stream or a small river is called a TRIBUTARY.
A tributary.
River
A stream?