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A headwater is the place from which the water in the river or stream originates. It is also called the source of the river or stream
A stream or river called that flows into a larger stream or river is called a tributary.
The difference between a river and a stream is the name applied to it by local residents, and relative size. In the same region, something called a river is usually larger than something called a stream. However, something that is called a river in one place could be the same size as something called a stream in another place.
stream, creek or either a lake or a ocean
The point at which a river forms is called a river source. The source may be from a spring, melted snow from a mountain top, or a lake with a stream flowing out from it.
Tributary is the answer you are looking for, I believe.
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.
A shallow place in a river is called a ford if it is used as a place to wade across to the other side. A shallow place can also be called a sandbar if the water does not cover all of the sand in a river or stream.
Are you crazy? A stream or a small river is called a TRIBUTARY.
A tributary.
A stream?