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.
The side of a stream is usually called its bank.
A small stream is called a creek.
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.
this is called a meandering stream.
a tributary
a waterway is a large stream
a stream