That's a delta...
in a place called Redcar. it has an estuary which goes in to the north sea.
A river
The Mediterranean sea is the sea that the Nile river goes through!
It is the River Amazon.
a river mouth
it goes into the sea
Salmon definitely go from the sea and up river to spawn (lay eggs)
It is called the mouth of the river.
it changes as it goes on its journey to the sea
Ultimately - it flows into a sea, or an ocean.
Ebro