The water continues to go downhill and spreads out creating a delta, a series of islands made of eroded material droped off by the river.
A tributary, it contributes to the flow of the main river. kool-Aid grape slurpy
The mouth of a river is the outlet.
river
Tributary
A river mouth is a portion of a stream where it merges with another stream. Note that here the definition of stream is not "a small river", but "any flowing current within well defined banks."
up stream
The flow of water in a river
A stream is a flow of rain water that is smaller than a river.
The base level of a river is the lowest place where it can flow, generally it's mouth. Therefore if a river or stream flows into another river, then the first river's base level is the point where it joins the other river.
Upstream or up river
Upstream or up river
Head is the source and mouth is where it ends.