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
The River Darent
it is the mouth of the river; where it meets the sea
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