An estuary is a semi-enclosed coastal body of water which has a free connection with the open sea and within which sea water mixes with fresh water. The key feature of an estuary is that it is an interface between sea water and fresh water and there is an influence of the ocean tide creating a dynamic relationship between the two waters.
An estuary is typically the mouth of a river, and esturies are often characterised by sedimentation of silt from the river, which provides a unique habitat, for example, for wadingbirds. Estuaries are more likely to occur on submerged coastlines, where the sea level has risen in relation to the land, as this process floods valleys to form rias, fjords etc, which can be classified as estuaries.
The estuaries are sometimes confused with fjords. The Lim bay in Istria, Croatia, is often called "Lim fjord" although it's not actually caused by glaciation but by land erosion by the river Pazinčica.
Structurally the estuary may represent the long-continued action of river erosion and tidal erosion confined to a narrow channel, most effective where most concentrated, or an estuary may be the drowned portion of the lower part of a river-valley.
In a map of Britain showing sea-depths it will be observed that under the Severnestuary the sea deepens in a number of steps descending by concentric V's that become blunter towards deep water until the last is a mere indentation pointing towards the long narrow termination of the present estuary.
In this and in similar cases the progress of the estuary is indicated upon what is now the continental shelf.
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