A Delta
the mouth
I think that you mean where a river meets the ocean? Since the ocean and the sea are the same thing. It is called an estuary.
Please rephrase - the question is unclear.
It Is Where the River Meets The Sea
it is the mouth of the river; where it meets the sea
The River Darent
At the mouth of the river, when it abruptly widens and meets the sea.
Mississippi
Estuary.
It is known as the river's estuary.
A delta
Estuary