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.
Estuaries are not always associated with drainage from land; but our understanding of estuarine dynamics was formed by pioneering research efforts in the lower reaches of rivers, and the definition of the term estuary is still often based on the assumption that the water of estuaries has to be diluted by freshwater.
In contrast, the circulation in an estuary is maintained by the large density differences produced by the salinity contrast between freshwater and oceanic water (or, in the case of the inverse estuary, by the salinity contrast between the hyper saline estuary and the open ocean).
The characteristics of the inverse estuary are: The salinity in the upper layer decreases towards the sea; the salinity in the lower layer increases towards the estuary head.
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