No, a stream is actually smaller than a river.
brook
The middle part of a river may be called mid-stream, mid-river or mature river.
A large natural stream, which may be a waterway.
It is called a "meander". They are the curved parts in a stream.
Tributary is the answer you are looking for, I believe.
A Tributary
STREAM
A tributary.
The mouth of a river is the outlet.
In the Cayuga (Gayogoho:no') language, the common word for a creek, river or stream is gihęde.
* Brook * Stream * River * Creek
In geographical terms, the word mouth means an opening into a place such as a cave or volcano. It also means a place where a stream meets with another stream, such as the mouth of a river.