Last edited 21 Jul 2021



Pictured above is the estuary of the River Teifi (Afon Teifi).

There are many different variations of an estuary, but in most basic terms it is defined as the tidal mouth of a river where the tide and stream meet. It is semi-enclosed and is connected to the sea, so estuaries generally contain seawater (or coastal brackish water) mixed with freshwater that has drained from the land. However, some estuaries - particularly those along freshwater lakes before connecting to the sea and so may not mix with seawater.

Estuaries often have an identifiable ecosystems and act as natural filters for runoff to support plant, bird, fish and other animal communities by providing food, breeding grounds and places to rest during migration. They can be susceptible to degradation based on conditions such as erosion, overdevelopment and pollution.

As transition zones between freshwater and seawater, estuaries display oceanic properties, experiencing rising tides and waves and registering saltwater content. They also mimic the properties of rivers by registering freshwater flow and producing sediment.

The four most common types of estuaries classified by their geology are:

  • Drowned river valleys.
  • Lagoon-type or bar-built.
  • Tectonically produced.
  • Fjord-type.

Estuaries can also be classified by their water circulation patterns. The most common types are salt wedge, partially mixed, well-mixed, inverse and intermittent.

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