sliver
A stream or river called that flows into a larger stream or river is called a tributary.
Stream.
The stream is a tributary of the river.
A stream or brook can get bigger and become a river.
Ohio River
A small stream in a river is called a current.
The middle part of a river may be called mid-stream, mid-river or mature river.
No, a stream is actually smaller than a river.
Brook.
Tributary is the answer you are looking for, I believe.
The oxygen content of a river or stream is likely to be highest at the source. The source of the river or stream is where it originates.