Head is the source and mouth is where it ends.
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.
A small stream is called a creek.
The side of a stream is usually called its bank.
It is occasionally called a sinking stream or a disappearing stream, but geographers and geologists normally call it a stream, just like another stream.
increases
Tributary is the answer you are looking for, I believe.
a waterway is a large stream
a tributary
a stream
The source of a stream is called a headwater or headstream. This is where the stream initially begins, often from a spring or melting snow.