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Definition of estuary:
A semi-enclosed embayment of the coast in which fresh run-off water mixes with saline water entering from the ocean.
This is the common definition for estuary, other definitions can be discussed in the article

This is the definition given by Cameron and Pritchard (1963) [1]. However, it 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.

Estuaries are characterized by the presence of tides and a prominent system of channels and shoals (open bays and enclosed coastal seas such as the Bohai Sea or North Sea therefore do not qualify as estuaries). According to this characteristic, estuaries can also be defined as

Definition of estuary:
Semi-enclosed inlet system where the tide propagates along channels.
The part of the estuary upstream of the seawater intrusion limit, but subject to tidal motion, is usually referred to as 'tidal river'.
This is the common definition for estuary, other definitions can be discussed in the article

According to the first, more common definition, the tidal river is excluded from the estuary, but included according to the second definition. The second definition, which is closer to the etymological origin of the word estuary, includes tidal inlet systems without significant fresh water inflow, for example:

  • drowned coastal valleys;
  • tidal lagoons – drowned coastal plains semi-closed by a barrier system;
  • inverse estuaries – estuaries where evaporation exceeds fresh water inflow.

From a morphological or ecological viewpoint, such inlet systems can be very similar to estuaries with small fresh water inflow according to the first definition.

Mudflat in the Deben estuary, UK. Photo credit: Bescoe

Estuaries are widely diverse and can be classified according to different criteria:
tidal range

microtidal: tidal range < 2 m;
mesotidal: tidal range 2-4 m;
macrotidal: tidal range > 4 m.

salinity [math]\delta = \Delta s / s[/math] (ratio of bottom-surface salinity difference and sea salinity)

well-mixed: [math]\delta \lt 0.05[/math] (see Seawater intrusion and mixing in estuaries)
partially mixed: [math]0.05 \lt \delta \lt 0.25[/math] (see Estuarine circulation)
stratified: [math]\delta \gt 0.25[/math] (see Salt wedge estuaries)

morphology (see Morphology of estuaries)

river-dominated: delta-shoal system;
tide-dominated: funnel-shape channel system;
wave-dominated: coastal barrier system.

Estuaries generally offer a great diversity of habitats consisting of subtidal and intertidal areas with different bed structures, different sediment composition and different salinity regimes. Typical estuarine habitats are intertidal mudflats and salt marshes. They host unique ecosystems with essential spawning and nursery functions for adjacent marine and fluvial ecosystems. They also provide important natural ecosystem services such as living resources, water quality regulation and flood protection (see also Estuarine ecosystems).

Articles on estuaries


  1. Cameron, W. M. and D. W. Pritchard (1963) Estuaries. In M. N. Hill (editor), The Sea, Vol. 2. John Wiley & Sons, New York, pp. 306–324

The main author of this article is Job Dronkers
Please note that others may also have edited the contents of this article.

Citation: Job Dronkers (2021): Estuary. Available from [accessed on 20-06-2021]