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 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.
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
It is known as the river's estuary.
A Delta
A delta
Estuary
Estuary
Estuary