A semi-enclosed embayment of the coast in which fresh river water entering at its head mixes with saline water entering from the ocean.
This is a usual definition given by Cameron and Pritchard (1963) 
However, this definition excludes the tidal river upstream of the seawater intrusion limit, where tidal motion can still have important consequences for ecosystem, water quality and morphology – for example: import of marine sediments and organisms, generation of a high turbidity zone, sedimentation/erosion of intertidal wetlands.
From a morphological/sedimentary point of view an estuary can be defined as
Tidal inlet zone where river flow is modulated by tides.
Fully developed estuarine systems consist of complex dynamic interactions between channels, tidal flats, middle ground shoals and salt marshes.
Estuaries are of particular ecological value and significance because they provide important natural values concerning, for example, fish and wildlife habitat, flood protection, and maintenance of water quality.
Articles on estuaries
- Estuarine ecosystems
- Estuaries and tidal rivers
- Seawater intrusion and mixing in estuaries
- Estuarine circulation
- Salt wedge estuaries
- Estuarine dispersion: dye experiments in the Eastern Scheldt scale model
- Morphology of estuaries
- Estuarine morphological modelling
- Physical processes and morphology of synchronous estuaries
- Estuary forums in the United Kingdom, Severn estuary case study
- US National Estuary Program
- Heavy metal content of mussels in the Western Scheldt estuary
- Cameron, W. M. and D. W. Pritchard (1963) Estuaries. In M. N. Hill (editor), The Sea, Vol. 2. John Wiley & Sons, New York, 306– 324.