I belive you refer to either an Estuary(such as the East river) or a Tidal basin. in this zone there would be a mixture of fresh and salt water, naturally the latter retarding the formation of ice, a natural anti-freeze as salt water has a lower freezing point.That is why the Hudson rarely freezes and even pack ice is rare-of course shiptraffic churns things up.
It is a mouth. Two types: an estuary - with a bay; a delta - when the river divides into several parts before reaching the sea.
The mouth of a river. Some rivers terminate at the sea.
estuary, firth, river mouth
intertidal zone
Source
It Is Where the River Meets The Sea
The mouth of a river is the open space at the end of a river allowing passage. The mouth of a river, sometimes called a delta, is the point at which the river ends and the water flows into 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
It is known as the river's estuary.
Estuary
A Delta
A delta
Estuary
the mouth