Estuaries and tidal rivers
This article sets out to introduce the reader to the concept of an estuary and the relationship with tidal rivers. Estuaries are highly complex and dynamic ecosystems . At their simplest estuaries can be defined in relation to their form and function . The definition of an estuary used here encompasses both these aspects.
"Estuaries" is one of the habitat sub-categories within the section dealing with biodiversity of coastal and marine habitats and ecosystems. This forms part of the ENCORA Theme 7.
Estuaries, or transitional waters, represent the transition between freshwater and marine biomes and as such are influenced by both aquatic realms. Salinity level marks the position within the mixing zones of an estuary. The uppermost limit of an estuary is referred to as its head, while the southernmost limit is called the mouth of the estuary. Between the freshwater head and the saline mouth of the estuary lie a number of distinctive zones marked by intermediate salinity values, each with distinct characteristics pertaining to the water clarity and type of substratum, thus hosting different communities of organisms.
The physical form provides a basis for discussing the importance of an individual estuary, including:
1 Drowned river valleys. They develop where the land was inundated by the sea when sea levels rose rapidly (about 20mm per year) towards the end of the last glaciation between 15,000 and 7,000 years ago . At its height around 8,000 years ago, when a catastrophic melting of the ice cap took place in northwest Europe, it may have been as much as 75mm per year . The ‘pulse’ of sediment mobilised and moved landward by these events has helped create the extensive low-lying coastal plains where sediment movement plays an important part in the development of the estuary structure.
2 Bar-built estuaries. Similar to (1) above but where tidal influence and hence sediment penetration is restricted. This may be caused by the nature of the hinterland but usually stems from the presence of spits, bars or barrier islands which surround the river mouth and help create embayments. Because of the restricted opening to the sea, some of these may more closely resemble lagoons.
3 Rias. Tidal inlets which are closely related to drowned river valleys though here the steep-sided nature of the land results in a much narrower profile. Depending on the elevation of the adjacent land, these estuaries may reach some distance inland with a series of side arms and are developed on rocky coasts.
4 Fjords and fjards which occur in glaciated, rocky coastal areas. Fjords with very steep walls occur in areas of high relief. Fjards occur in areas of low relief and may include shallow marine waters and small islands (skerries). For both the hard rock nature of the land results in a limited sediment supply.
Some of these estuary types may also provide a link with deltas and lagoons and it is sometimes difficult to distinguish between them. They appear to occur mostly on meso-tidal coasts where wave energy is relatively high (Pethick 1984) and in micro-tidal zones, such as the deltas of the Mediterranean.
- About estuaries US EPA National Estuary Program
- Discovery of Estuarine Environments: Estuarine Science, Policy and Management US EPA, Narragansett Bay Program & Univ. of Rhode Island Office of Marine Programs
- Classification of estuaries US EPA, Narragansett Bay Program & Univ. of Rhode Island Office of Marine Programs
- Estuaries: Science and Technology Focus US ONR
- The Estuary Guide UK Environment Agency and Defra
- Estuary management issues WFD Information Centre UK
- Pethick, J., 1984. An Introduction to Coastal Geomorphology. Edward Arnold.
- Healy, T.R., 2005. Estuaries. In: Encyclopedia of Coastal Science, ed., M. Schwartz, Springer, 436-439.
- Warrick, R.A., 1993. Climate and sea level change: a synthesis. In: Climate and Sea level Change - Observations, Projections and Implications, eds., R.A., Warrick, E.M. Barrow & T.M.L. Wigley, University Press, Cambridge, 3-24.
- Tooley, M.J., 1993. Long term changes in eustatic sea level. In: Climate and Sea Level Change - Observations, Projections and Implications, eds., R.A., Warrick, E.M. Barrow & T.M.L. Wigley, University Press, Cambridge, 81-110.
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