A stream or river called that flows into a larger stream or river is called a tributary.
A stream
Tributary is the answer you are looking for, I believe.
A small stream in a river is called a current.
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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.
A stream changes by joining larger bodies of water as it makes it way to the ocean. It begins at the headwaters and then into a tributary river, a river delta, and then to the ocean.
River
A stream?
A stream.