Inland Fisheries

5.1.3 Geo-morphological classification of estuaries

Estuaries are divided into five types as below:

1. Saltwedge estuary

It is formed when a river flows into a tideless sea. As the freshwater is less dense than salt water, it tends to flow over the surface of the salt water. The seawater rests on the bottom as motionless saltwedge. Hence, this is called saltwedge estuary.

2. Partially mixed estuary

It is formed when a river flows into the sea with appreciable tidal movements. The turbulence caused at the meeting place, not only mixes the saltwater upwards into the freshwater, but also mixes the freshwater downwards. This dilutes the salt water near the bed and produces a salinity gradient towards the head of the estuary.

3. Well mixed estuary

It is shallow with a high tidal range, and probably with intertidal mudflats or banks. The mixing is more intense. As it is shallow, there is no vertical circulation. There is a lateral variation in salinity. The freshwater tends to flow down on one side of the estuary and the saltier water enters the other side. Consequently, a horizontal residual circulation is established.

4. Fjord

It is the deepest estuary. It can be considered as saltwedge estuary with an infinite layer. The salinity of the bottom layer does not vary significantly. The effect of tidal flow in the mixing is negligible. They often have a shallow bar of rock or silt near their mouths. The tidal flow over this can cause turbulent mixing within a limited area. There is no fjord in India.

5. Bar-built estuary

It is found when offshore barrier, sand islands and sand spits are deposited about sea level. Such islands or spits extend between headlands in a chain broken by one or more inlets. The area enclosed by the barrier beach is generally elongate and parallel to the coastline. Although these estuaries have a small drainage area, they generally receive more than one river. They are shallow and wind provides the important mixing mechanism. These estuaries remain open during monsoon seasons. They remain land locked when there is no water flow in the river. Hence, they are also called temporary estuaries.