﻿ Estuaries
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Thame estuary

The Thames Barrier

The barrier is an impressive construction and is the worlds largest movable flood barrier, spanning 520 metres across the Thames at Woolwich. The barrier completed in 1982, is valued at over £1000 million and took 8 years to build 

The barrier was designed to protect the capital from flooding until the year 2030, although whether global warming will invalidate the original calculations remains to be seen. It was the results of the 1953 floods in which 300 people were drowned and 160,000 acres of farmland flooded, that finally prompted an urgent enquiry into solving the threat. The design was selected from 41 proposals and succeeded because it minimises interference with the natural flow of the river, does not restrict headroom for shipping, and is meant to be aesthetically pleasing.

The four main gates comprising the barrier are each as high as a five storey building and as wide as the as the opening of Tower Bridge (61 metres). Each gate is operated by its own electrically powered 'hydraulic power packs', which are housed immediately below the stainless steel roofs. When fully open the semicircular gates are set flush into the river bed in concrete supporting sills, so as not to provide an obstruction. They can also be raised clear of the water for maintenance purposes.

Since its completion the barrier has been raised over 64 times as a precaution against flooding, they are also raised monthly  for a test period at low tide.

Seven Bore

The River Severn Bore is a natural example of a self-reinforcing solitary wave or solution.

A curious phenomenon associated with the lower reaches of the Severn is the tidal phenomenon known as the Severn Bore . The river's estuary , part of the Bristol Channel , has the second largest tidal range in the world which is approximately 15 metres , exceeded only (couple of feet) by the Bay of Fundy in Canada - and at certain combinations of the tides, the rising water is funneled up the estuary into a wave that travels rapidly upstream against the river current



According to some sources, the name " Severn " is derived from the name Sabrina (or "Sabern"), based on the mythical story of the drowning of a nymph in the river . Sabrina is also the goddess of the River Severn in Brythonic mythology .

Winter migration - A Winters Tale

Every winter Britain plays host to a great variety of migrating birds. Whooper swans escape the harsh Icelandic weather, barnacle geese fly in from Greenland, and all manner of wading birds grace our estuaries and mud flats. Breathtaking images taken from the BBC's Natural World.







You are here: Home > Marine Science > Marine Ecology > Marine Habitats > Estuaries


Estuaries have for long been important to mankind, either as places of navigation, or as locations on their banks for towns and cities. Nowadays they are under pressure, either as repositories for the effluent of industrial processes and domestic waste, or as prime sites for land-claim to create sites for industry or urban development. Against this background the biologist has been attracted to other functions of estuaries: Vital feeding areas for many species of birds, especially waders and wildfowl, like the locations of coastal fisheries or as fascinating areas present challenges to our understanding of how animals and plants adapt to their environment.

An estuary is a partially enclosed body of water formed where freshwater from rivers and streams flows into the oceans, mixing with the seawater. Estuaries and the lands surrounding them are places of transition from
land to sea, and from fresh to saltwater. Although influenced by tides, estuaries are protected from the full force of the ocean waves, winds and storms by reefs, barrier islands or fingers of land, mud, and sand that
define an estuary’s coastal boundary. Estuaries come in all shapes and sizes and can be called by many different names—bays, lagoons, harbors, and inlets or sounds. When considering any estuarine habitat
worldwide there are many generalizations that can be perceived, for example, common features being the gradient of conditions from the open sea into the sheltered estuary, and on to the freshwater river. Along this gradient there are clear changes in salinity ranging from full strength seawater decreasing to freshwater. Associated changes in sedimentary conditions from coarse sediment (sand or gravel) without the estuaries to fine sediments (muds) within the estuaries are invariably found.

Most modern-day estuaries were formed during the Holocene epoch by the flooding of river-eroded or glacially-scoured valleys when sea level began to rise about 10,000-12,000 years ago.  Estuaries are amongst the most heavily populated areas throughout the world, with about 60% of the world’s population living along estuaries and the coast. As a result, estuaries are suffering degradation by many factors, including sedimentation from soil erosion from deforestation; overgrazing and other poor farming practices; overfishing; drainage and filling of wetlands; eutrophication due to excessive nutrients from sewage and animal wastes; pollutants including heavy metals, PCBs, radionuclides and hydrocarbons from sewage inputs; and diking or damming for flood control or water diversion.

The estuarine environment is characterized by having a constantly changing mixture of salt and freshwater, and by being dominated by fine sedimentary material carried into the estuary from the sea and from rivers, which accumulates in the estuary to form mudflats. The mixtures of salt and freshwater present challenges to the physiology of the animals, which few are able to adapt to. Estuaries have been claimed to be amongst the most productive natural habitats in the world, and we shall attempt to explain why they are so productive, and how the energy produced is utilized by succeeding trophic levels. Estuaries are transition zones between rivers and the sea, which differ from both in abiotic and biotic conditions. Temperature, salinity, and turbidity fluctuate on a daily basis and reach more extremes in estuarine waters than they do at sea or in rivers. From a biotic viewpoint, estuaries are highly productive ecosystems ranking at the same level as coral reefs and mangrove swamps. An elevated productivity is maintained because of high nutrient levels in both sediment and water column.

Parts of an estuarine ecosystem

Within an estuary ecosystem certain characteristic zones emerge, each with typical sediments and salinity. In particular estuaries one zone may occupy a large proportion of the area, and the other zones may be compressed, but a clear sequence can always be seen:

1. Head. Where freshwater enters the estuary, and river currents predominate. Tidal but very limited salt penetration. FSI salinity 5. Sediments become finer downstream.

2. Upper reaches. Mixing of fresh and saltwater. Minimal currents, especially at high tide, leading to turbidity maxima. Mud deposition. Salinity 5–18.

3. Middle reaches. Currents due to tides. Principally mud deposits, but sandier where currents faster. Salinity 18–25.

4. Lower reaches. Faster currents due to tides. Principally sand deposits, but muddier where currents weaken. Salinity 25–30.

5. Mouth. Strong tidal currents. Clean sand or rocky shores. Salinity similar to adjacent sea (30).

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Types of estuaries

Fjords. Drowned glacial troughs. Fjord-type estuaries occur where valleys have been deeply eroded by glaciation. Characterized by deep inner basins linked to the sea by shallow entrance sills, for example, Sea lochs in West of Scotland, Fjords in Norway, Sweden, Alaska, British Columbia, New Zealand, such as the coasts of Norway, Western Scotland, Alaska, and New Zealand.

Fjards. Typical of glaciated lowland coasts. More complex than fjords, with a more open and irregular coastline, for example, Solway Firth England/Scotland, eastern Canada, and New England.

Rias. Drowned river valleys, formed by subsidence of land and/or a rise in sea level. Deep, narrow channels with a strong marine influence, for example, Estuaries of Cornwall, England and Brittany, France.

Coastal plain estuaries. Formed by the flooding of pre-existing valleys. Unlike Rias, these estuaries are often very shallow and filled with sediment so that extensive mudflats and saltmarshes occur. Commonest type of estuary in United Kingdom, for example, Severn, Dee, Humber, Thames, England. Chesapeake Bay, Charleston Harbor, Delaware Bay, USA.

Bar-built estuaries. Also drowned river valleys, but recent sedimentation has kept pace with their drowning so that they have a characteristic bar across their mouths, for example, Alde, England; Ythan, Scotland. Barnegat Bay, New Jersey, Laguna Madre, Texas, Albufeira, Portugal, and most estuaries of North Carolina—Florida coast. In many estuaries in South Africa and Australia the bar may seasonally close the estuary, creating closed or blind estuaries.

Complex estuaries. Drowned river valleys of complex origin, typically a mixture of glaciation, river erosion, and sea level rise, for example, Scottish Firths: Solway, Moray, Dornoch, Tay, and Forth. San Francisco Bay is a complex estuary created by tectonic activity (land movement due to faulting).

Barrier beaches. Open coast system where a bar or barrier develops offshore, and an estuary is thereby created behind the barrier. For example, North Norfolk Coast, Lindisfarne, England.

Linear shore sites. Formed where the shore is sheltered, for example, by barrier islands. Usually considered as a subdivision of a complex estuary. For example Essex and North Kent coast, England.

Embayments. Large natural areas formed between rocky headlands that naturally fill with soft sediments. For example, Carmathen bay, Wales, Morecambe bay, The wash, England.

Estuary Wildlife

Estuaries are a hostile environment for the animals that live there, the changing tide requires less mobile species to be specially adapted to survive under water at high tide and at low tide when exposed. Birds use different parts of the estuary depending on the position of the tide; feeding on the estuary mudflats at low water and roosting on higher ground (such as Bowling Green) at high-water.

The following are examples of species that depend on coastal and floodplain grazing marsh, some of which are of conservation concern:

Mammals: Common & grey seal, Otter
Birds: avocet, black tailed godwit, Brent goose, common sandpiper, Cormorant, curlew, Dunlin, greenshank, green sandpiper, Lapwing, little egret, spotted redshank , teal, Shelduck, Oystercatcher, wigeon .
Fish: Bass, allis shad, twaite shad, smelt, salmon, Couch's goby .
Molluscs: horse lip mussel, lagoon snail, spiny cockle, brown venus, flask shell.
Anenomes: trumpet anemone, ginger tiny and glaucous pimplet.
Worms: tentacled lagoon-worm (l), Ophelia bicornis (l), Armanda cirrhosa (l).
Vascular Plants: sea lavender (m), eelgrass Zostera marina (l), Z. noltii.
Algae: maerl, red algae

There is more to an estuaries wildlife than first meets the eye. The vast mudflats are home to an abundance of invertebrate species making the estuary as rich in biodiversity as a tropical rainforest. A cross section through the sand and mud reveals the wealth of life that thrives below and on the surface of this rich habitat.

All life on an estuary starts with organisms that are too small for us to even see. Microscopic bacteria and algae, which get their energy from the sun or non living materials, are fed upon or filtered from the water by species of invertebrates such as snails, worms and clams.

The reason why so many wading birds flock there in the winter is that the invertebrates are a vital food source. Each cubic metre of estuary mud contains the same calorific value as 14 Mars bars.

Threats to Estuaries


As ecosystems, estuaries are under threat from human activities such as pollution and overfishing. They are also threatened by sewage, coastal settlement, land clearance and much more. Estuaries are affected by events far upstream, and concentrate materials such as pollutants and sediments. Land run-off and industrial, agricultural, and domestic waste enter rivers and are discharged into estuaries. Contaminants can be introduced which do not disintegrate rapidly in the marine environment, such as plastics, pesticides, furans, dioxins,phenols and heavy metals.

Such toxins can accumulate in the tissues of many species of aquatic life in a process called bioaccumulation. They also accumulate in benthic environments, such as estuaries and bay muds: a geological record of human activities of the last century.

For example, Chinese and Russian industrial pollution, such as phenols and heavy metals, in the Amur River have devastated fish stocks and damaged its estuary soil.

Estuaries tend to be naturally eutrophic because land runoff discharges nutrients into estuaries. With human activities, land run-off also now includes the many chemicals used as fertilizers in agriculture as well as waste from livestock and humans. Excess oxygen depleting chemicals in the water can lead to hypoxia and the creation of dead zones. It can result in reductions in water quality, fish, and other animal populations.

Overfishing also occurs. Chesapeake Bay once had a flourishing oyster population which has been almost wiped out by overfishing. Historically the oysters filtered the estuary's entire water volume of excess nutrients every three or four days. Today that process takes almost a year, and sediment, nutrients, and algae can cause problems in local waters. Oysters filter these pollutants, and either eat them or shape them into small packets that are deposited on the bottom where they are harmless.

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