Estuaries as complex systems

Estuaries provide a complex of habitats of which tidal waters, sand and mud flats and saltmarshes are the primary habitats. Sand and shingle structures can be associated with them and in some cases provide the shelter necessary for the development of the primary habitats. Estuaries are usually defined as:

• "a semi-enclosed coastal body of water which has a free connection to the open sea and within which sea water is measurably diluted with fresh water derived from land drainage" (Pritchard 1952) or;
• "an inlet of the sea reaching into a river valley as far as the upper limit of tidal rise" (Fairbridge 1980).
  

Estuaries are most extensive in coastal areas with macro-meso tidal range and/or an abundant supply of sediment, in Europe these are especially significant around the southern North Sea.

Although not considered as a separate section in the context of this guide, the individual habitats, transitions between them and the plants and animals associated with them are key concerns. This is especially true where managed realignment is being considered as there is often a trade off as one habitat (such as coastal grazing marsh) is replaced by another (saltmarsh). This has important knock-on effects on the habitats and species at a particular site, which may affect their conservation status.

References

Fairbridge, R.W. 1980.

The estuary its definition and geodynamic cycle

Pritchard, D. 1952.

Estuarine hydrology