That's a delta...
in a place called Redcar. it has an estuary which goes in to the north sea.
A river
It is the River Amazon.
The Mediterranean sea is the sea that the Nile river goes through!
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.
Ebro
it changes as it goes on its journey to the sea
Ultimately - it flows into a sea, or an ocean.