Estuaries form a transition zone between river environments and maritime environments. They are subject both to marine influences—such as tides, waves, and the influx of saline water—and to riverine influences—such as flows of fresh water and sediment. The mixing of seawater and fresh water provide high levels of nutrients both in the water column and in sediment, making estuaries among the most productive natural habitats in the world.
Most existing estuaries formed during the Holocene epoch with the flooding of river-eroded or glacially scoured valleys when the sea level began to rise about 10,000–12,000 years ago. Estuaries are typically classified according to their geomorphological features or to water-circulation patterns. They can have many different names, such as bays, harbors, lagoons, inlets, or sounds, although some of these water bodies do not strictly meet the above definition of an estuary and may be fully saline.
The banks of many estuaries are amongst the most heavily populated areas of the world, with about 60% of the world's population living along estuaries and the coast. As a result, many estuaries suffer degradation from a variety of 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, polychlorinated biphenyls, radionuclides and hydrocarbons from sewage inputs; and diking or damming for flood control or water diversion.
The word "estuary" is derived from the Latin word aestuarium meaning tidal inlet of the sea, which in itself is derived from the term aestus, meaning tide. There have been many definitions proposed to describe an estuary. The most widely accepted definition is: "a semi-enclosed coastal body of water, which has a free connection with the open sea, and within which sea water is measurably diluted with freshwater derived from land drainage". However, this definition excludes a number of coastal water bodies such as coastal lagoons and brackish seas. A more comprehensive definition of an estuary is "a semi-enclosed body of water connected to the sea as far as the tidal limit or the salt intrusion limit and receiving freshwater runoff; however the freshwater inflow may not be perennial, the connection to the sea may be closed for part of the year and tidal influence may be negligible". This broad definition also includes fjords, lagoons, river mouths, and tidal creeks. An estuary is a dynamic ecosystem having a connection to the open sea through which the sea water enters with the rhythm of the tides. The sea water entering the estuary is diluted by the fresh water flowing from rivers and streams. The pattern of dilution varies between different estuaries and depends on the volume of fresh water, the tidal range, and the extent of evaporation of the water in the estuary.
Drowned river valleys are also known as coastal plain estuaries. In places where the sea level is rising relative to the land, seawater progressively penetrates into river valleys and the topography of the estuary remains similar to that of a river valley. This is the most common type of estuary in temperate climates. Well-studied estuaries include the Severn Estuary in the United Kingdom and the Ems Dollard along the Dutch-German border.
The width-to-depth ratio of these estuaries is typically large, appearing wedge-shaped (in cross-section) in the inner part and broadening and deepening seaward. Water depths rarely exceed 30 m (100 ft). Examples of this type of estuary in the U.S. are the Hudson River, Chesapeake Bay, and Delaware Bay along the Mid-Atlantic coast, and Galveston Bay and Tampa Bay along the Gulf Coast.
Bar-built estuaries are found in place where the deposition of sediment has kept pace with rising sea level so that the estuaries are shallow and separated from the sea by sand spits or barrier islands. They are relatively common in tropical and subtropical locations.
These estuaries are semi-isolated from ocean waters by barrier beaches (barrier islands and barrier spits). Formation of barrier beaches partially encloses the estuary, with only narrow inlets allowing contact with the ocean waters. Bar-built estuaries typically develop on gently sloping plains located along tectonically stable edges of continents and marginal sea coasts. They are extensive along the Atlantic and Gulf coasts of the U.S. in areas with active coastal deposition of sediments and where tidal ranges are less than 4 m (13 ft). The barrier beaches that enclose bar-built estuaries have been developed in several ways:
Barrier beaches form in shallow water and are generally parallel to the shoreline, resulting in long, narrow estuaries. The average water depth is usually less than 5 m (16 ft), and rarely exceeds 10 m (33 ft). Examples of bar-built estuaries are Barnegat Bay, New Jersey; Laguna Madre, Texas; and Pamlico Sound, North Carolina.
Fjords were formed where pleistocene glaciers deepened and widened existing river valleys so that they become U-shaped in cross-sections. At their mouths there are typically rocks, bars or sills of glacial deposits, which have the effects of modifying the estuarine circulation.
Fjord-type estuaries are formed in deeply eroded valleys formed by glaciers. These U-shaped estuaries typically have steep sides, rock bottoms, and underwater sills contoured by glacial movement. The estuary is shallowest at its mouth, where terminal glacial moraines or rock bars form sills that restrict water flow. In the upper reaches of the estuary, the depth can exceed 300 m (1,000 ft). The width-to-depth ratio is generally small. In estuaries with very shallow sills, tidal oscillations only affect the water down to the depth of the sill, and the waters deeper than that may remain stagnant for a very long time, so there is only an occasional exchange of the deep water of the estuary with the ocean. If the sill depth is deep, water circulation is less restricted, and there is a slow but steady exchange of water between the estuary and the ocean. Fjord-type estuaries can be found along the coasts of Alaska, the Puget Sound region of western Washington state, British Columbia, eastern Canada, Greenland, Iceland, New Zealand, and Norway.
These estuaries are formed by subsidence or land cut off from the ocean by land movement associated with faulting, volcanoes, and landslides. Inundation from eustatic sea level rise during the Holocene Epoch has also contributed to the formation of these estuaries. There are only a small number of tectonically produced estuaries; one example is the San Francisco Bay, which was formed by the crustal movements of the San Andreas fault system causing the inundation of the lower reaches of the Sacramento and San Joaquin rivers.
In this type of estuary, river output greatly exceeds marine input and tidal effects have a minor importance. Freshwater floats on top of the seawater in a layer that gradually thins as it moves seaward. The denser seawater moves landward along the bottom of the estuary, forming a wedge-shaped layer that is thinner as it approaches land. As a velocity difference develops between the two layers, shear forces generate internal waves at the interface, mixing the seawater upward with the freshwater. An example of a salt wedge estuary is the Mississippi River.
As tidal forcing increases, river output becomes less than the marine input. Here, current induced turbulence causes mixing of the whole water column such that salinity varies more longitudinally rather than vertically, leading to a moderately stratified condition. Examples include the Chesapeake Bay and Narragansett Bay.
Tidal mixing forces exceed river output, resulting in a well-mixed water column and the disappearance of the vertical salinity gradient. The freshwater-seawater boundary is eliminated due to the intense turbulent mixing and eddy effects. The lower reaches of Delaware Bay and the Raritan River in New Jersey are examples of vertically homogenous estuaries.
Inverse estuaries occur in dry climates where evaporation greatly exceeds the inflow of fresh water. A salinity maximum zone is formed, and both riverine and oceanic water flow close to the surface towards this zone. This water is pushed downward and spreads along the bottom in both the seaward and landward direction. An example of an inverse estuary is Spencer Gulf, South Australia.
The most important variable characteristics of estuary water are the concentration of dissolved oxygen, salinity and sediment load. There is extreme spatial variability in salinity, with a range of near zero at the tidal limit of tributary rivers to 3.4% at the estuary mouth. At any one point the salinity will vary considerably over time and seasons, making it a harsh environment for organisms. Sediment often settles in intertidal mudflats which are extremely difficult to colonize. No points of attachment exist for algae, so vegetation based habitat is not established. Sediment can also clog feeding and respiratory structures of species, and special adaptations exist within mudflat species to cope with this problem. Lastly, dissolved oxygen variation can cause problems for life forms. Nutrient-rich sediment from man-made sources can promote primary production life cycles, perhaps leading to eventual decay removing the dissolved oxygen from the water; thus hypoxic or anoxic zones can develop.
Estuaries are incredibly dynamic systems, where temperature, salinity, turbidity, depth and flow all change daily in response to the tides. This dynamism makes estuaries highly productive habitats, but also make it difficult for many species to survive year-round. As a result, estuaries large and small experience strong seasonal variation in their fish communities. In winter, the fish community is dominated by hardy marine residents, and in summer a variety of marine and anadromous fishes move into and out of estuaries, capitalizing on their high productivity. Estuaries provide critical habitat to a variety of species that rely on estuaries for life-cycle completion. Pacific Herring (Clupea pallasii) are known to lay their eggs in estuaries and bays, surfperch give birth in estuaries, juvenile flatfish and rockfish migrate to estuaries to rear, and anadromous salmonids and lampreys use estuaries as migration corridors. Also, migratory bird populations, such as the black-tailed godwit, rely on estuaries.
Two of the main challenges of estuarine life are the variability in salinity and sedimentation. Many species of fish and invertebrates have various methods to control or conform to the shifts in salt concentrations and are termed osmoconformers and osmoregulators. Many animals also burrow to avoid predation and to live in the more stable sedimental environment. However, large numbers of bacteria are found within the sediment which have a very high oxygen demand. This reduces the levels of oxygen within the sediment often resulting in partially anoxic conditions, which can be further exacerbated by limited water flux.
Phytoplankton are key primary producers in estuaries. They move with the water bodies and can be flushed in and out with the tides. Their productivity is largely dependent upon the turbidity of the water. The main phytoplankton present are diatoms and dinoflagellates which are abundant in the sediment.
Of the thirty-two largest cities in the world in the early 1990s, twenty-two were located on 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. The elemental composition of biofilm reflect areas of the estuary impacted by human activities, and over time may shift the basic composition of the ecosystem, and the reversible or irreversible changes in the abiotic and biotic parts of the systems from the bottom up.
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. This can result in reductions in water quality, fish, and other animal populations. Overfishing also occurs. Chesapeake Bay once had a flourishing oyster population that has been almost wiped out by overfishing. Oysters filter these pollutants, and either eat them or shape them into small packets that are deposited on the bottom where they are harmless. 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.
A bay is a recessed, coastal body of water that directly connects to a larger main body of water, such as an ocean, a lake, or another bay. A large bay is usually called a gulf, sea, sound, or bight. A cove is a type of smaller bay with a circular inlet and narrow entrance. A fjord is a particularly steep bay shaped by glacial activity.
A bay can be the estuary of a river, such as the Chesapeake Bay, an estuary of the Susquehanna River. Bays may also be nested within each other; for example, James Bay is an arm of Hudson Bay in northeastern Canada. Some large bays, such as the Bay of Bengal and Hudson Bay, have varied marine geology.
The land surrounding a bay often reduces the strength of winds and blocks waves. Bays were significant in the history of human settlement because they provided safe places for fishing. Later they were important in the development of sea trade as the safe anchorage they provide encouraged their selection as ports.Ecology of the San Francisco Estuary
The San Francisco Estuary together with the Sacramento–San Joaquin River Delta represents a highly altered ecosystem. The region has been heavily re-engineered to accommodate the needs of water delivery, shipping, agriculture, and most recently, suburban development. These needs have wrought direct changes in the movement of water and the nature of the landscape, and indirect changes from the introduction of non-native species. New species have altered the architecture of the food web as surely as levees have altered the landscape of islands and channels that form the complex system known as the Delta.This article deals particularly with the ecology of the low salinity zone (LSZ) of the estuary. Reconstructing a historic food web for the LSZ is difficult for a number of reasons. First, there is no clear record of the species that historically have occupied the estuary. Second, the San Francisco Estuary and Delta have been in geologic and hydrologic transition for most of their 10,000 year history, and so describing the "natural" condition of the estuary is much like "hitting a moving target". Climate change, hydrologic engineering, shifting water needs, and newly introduced species will continue to alter the food web configuration of the estuary. This model provides a snapshot of the current state, with notes about recent changes or species introductions that have altered the configuration of the food web. Understanding the dynamics of the current food web may prove useful for restoration efforts to improve the functioning and species diversity of the estuary.Estuary English
Estuary English is an English accent associated with the area along the River Thames and its estuary. Phonetician John C. Wells proposed a definition of Estuary English as "Standard English spoken with the accent of the southeast of England". Estuary English may be compared with Cockney, and there is some debate among linguists as to where Cockney speech ends and Estuary English begins. In October 1984, David Rosewarne in the Times Educational Supplement suggested that it may eventually replace Received Pronunciation in the southeast.Firth of Forth
The Firth of Forth (Scottish Gaelic: Linne Foirthe) is the estuary (firth) of several Scottish rivers including the River Forth. It meets the North Sea with Fife on the north coast and Lothian on the south. It was known as Bodotria in Roman times. In the Norse sagas it was known as the Myrkvifiörd. An early Welsh name is Merin Iodeo, or the "Sea of Iudeu".Humber
The Humber is a large tidal estuary on the east coast of Northern England. It is formed at Trent Falls, Faxfleet, by the confluence of the tidal rivers Ouse and Trent. From there to the North Sea, it forms part of the boundary between the East Riding of Yorkshire on the north bank and North Lincolnshire on the south bank. Although the Humber is an estuary from the point at which it is formed, many maps show it as the River Humber.Below Trent Falls, the Humber passes the junction with the Market Weighton Canal on the north shore, the confluence of the River Ancholme on the south shore; between North Ferriby and South Ferriby and under the Humber Bridge; between Barton-upon-Humber on the south bank and Kingston upon Hull on the north bank (where the River Hull joins), then meets the North Sea between Cleethorpes on the Lincolnshire side and the long and thin headland of Spurn Head to the north.
Ports on the Humber include the Port of Hull, Port of Grimsby, Port of Immingham, as well as lesser ports at New Holland and North Killingholme Haven. The estuary is navigable for the largest of deep-sea vessels. Inland connections for smaller craft are extensive but handle only a quarter of the goods traffic handled in the Thames.Merseyside
Merseyside ( MUR-zee-syde) is a metropolitan county in North West England, with a population of 1.38 million. It encompasses the metropolitan area centred on both banks of the lower reaches of the Mersey Estuary and comprises five metropolitan boroughs: Knowsley, St Helens, Sefton, Wirral, and the city of Liverpool. Merseyside, which was created on 1 April 1974 as a result of the Local Government Act 1972, takes its name from the River Mersey.
Merseyside spans 249 square miles (645 km2) of land which border Lancashire (to the north-east), Greater Manchester (to the east), Cheshire (to the south and south-east) and the Irish Sea to the west. North Wales is across the Dee Estuary. There is a mix of high density urban areas, suburbs, semi-rural and rural locations in Merseyside, but overwhelmingly the land use is urban. It has a focused central business district, formed by Liverpool City Centre, but Merseyside is also a polycentric county with five metropolitan districts, each of which has at least one major town centre and outlying suburbs. The Liverpool Urban Area is the fifth most populous conurbation in England, and dominates the geographic centre of the county, while the smaller Birkenhead Urban Area dominates the Wirral Peninsula in the south.
For the 12 years following 1974 the county had a two-tier system of local government; district councils shared power with the Merseyside County Council. The county council was abolished in 1986, and so its districts (the metropolitan boroughs) are now effectively unitary authority areas. However, the metropolitan county continues to exist in law and as a geographic frame of reference, and several county-wide services are co-ordinated by authorities and joint-boards, such as Merseytravel (for public transport), Merseyside Fire and Rescue Service and the Merseyside Police (for law-enforcement); as a ceremonial county, Merseyside has a Lord Lieutenant and a High Sheriff. The boroughs of Merseyside are joined by the neighbouring borough of Halton in Cheshire to form the Liverpool City Region, which is a local enterprise partnership and combined authority area.
Merseyside is an amalgamation of 22 former local government districts from the former administrative counties of Lancashire, Cheshire and six autonomous county boroughs centred on Birkenhead, Bootle, Liverpool, Southport, St Helens, and Wallasey.Oakland Estuary
The Oakland Estuary is the strait in the San Francisco Bay Area, California, separating the cities of Oakland and Alameda and the Alameda Island from the East Bay mainland. On its western end, it connects to San Francisco Bay, while its eastern end connects to San Leandro Bay.River Camel
The River Camel (Cornish: Dowr Kammel, meaning crooked river) is a river in Cornwall, England, United Kingdom. It rises on the edge of Bodmin Moor and with its tributaries its catchment area covers much of North Cornwall. The river flows into the eastern Celtic Sea between Stepper Point and Pentire Point having covered about 30 miles. The river is tidal upstream to Egloshayle and is popular for sailing, birdwatching and fishing. The name Camel comes from the Cornish language for 'the crooked one', a reference to its winding course. Historically the river was divided into three named stretches. Heyl (Cornish: Heyl, meaning estuary) was the name for the estuary up to Egloshayle, the River Allen (Cornish: Dowr Alen, meaning shining river) was the stretch between Egloshayle and Trecarne, whilst the Camel was reserved for the stretch of river between its source and Trecarne.River Fal
The River Fal (Cornish: Dowr Fala) flows through Cornwall, England, rising at Pentevale on Goss Moor (between St. Columb and Roche) and reaching the English Channel at Falmouth. On or near the banks of the Fal are the castles of Pendennis and St Mawes as well as Trelissick Garden. The River Fal separates the Roseland peninsula from the rest of Cornwall. Like most of its kind on the south coast of Cornwall and Devon, the Fal estuary is a classic ria, or drowned river valley. The Fal estuary from Tregony to the Truro River was originally called Hafaraell (Cornish: Havarel, meaning fallow place).River Fowey
The River Fowey ( (listen) FOY; Cornish: Fowi) is a river in Cornwall, England, United Kingdom.
It rises at Fowey Well (originally Cornish: Fenten Fowi, meaning spring of the river Fowey) about 1-mile (1.6 km) north-west of Brown Willy on Bodmin Moor, not far from one of its tributaries rising at Dozmary Pool and Colliford Lake, passes Lanhydrock House, Restormel Castle and Lostwithiel, then broadens at Milltown before joining the English Channel at Fowey. The estuary is called Uzell (Cornish: Usel, meaning howling place). It is only navigable by larger craft for the last 7 miles (11 km). There is a ferry between Fowey and Bodinnick. The first road crossing going upstream is in Lostwithiel. The river has seven tributaries, the largest being the River Lerryn. The section of the Fowey Valley between Doublebois and Bodmin Parkway railway station is known as the Glynn Valley (Cornish: Glyn, meaning deep wooded valley). The valley is the route of both the A38 trunk road and the railway line (built by the Cornwall Railway in 1859). The railway line is carried on eight stone viaducts along this stretch (see Cornwall Railway viaducts).River Hayle
The River Hayle (Cornish: Heyl, meaning estuary) is a small river in West Cornwall, England, United Kingdom which issues into St Ives Bay at Hayle on Cornwall's Atlantic coast.The River Hayle is approx 12 miles (19 kilometres) long and it rises south-west of Crowan village. Its course is west for approx 5 miles (which brings the river to within 3 miles of the south coast at Marazion on Mount's Bay). It then flows through a steep wooded valley north of the granite high ground at Trescowe Common, formerly a mining area, before turning abruptly north near the hamlet of Relubbus. It then follows a northerly course for the remaining six miles to the estuary, passing St Erth.
The Hayle Estuary encompasses a disused port on the east bank and a substantial area of salt marsh named Lelant Saltings to the west. The port was once of considerable importance to Hayle's industry (see main article Hayle). Lelant saltings is an important habitat for birds and the Royal Society for the Protection of Birds runs a nature reserve there. Areas around the estuary are designated as sites of special scientific interest (see also Hayle SSSI).River Menalhyl
The River Menalhyl (Cornish: Dowr Melynheyl, meaning river of the estuary mill) is a river in Cornwall, England, that flows through the civil parishes of St Columb Major and Mawgan-in-Pydar. Its length is about 12 miles and it flows in a generally north-west direction. The name comes from the Cornish words melyn meaning mill and heyl meaning estuary - estuary mills. The name was recorded as Mellynheyl in the 19th century, but it had been known as Glyvion.River Mersey
The River Mersey () is a river in the North West of England. Its name is derived from the Anglo-Saxon language and translates as "boundary river". The river may have been the border between the ancient kingdoms of Mercia and Northumbria and for centuries it formed part of the boundary between the historic counties of Lancashire and Cheshire.The start of the Mersey is at the confluence of the River Tame and River Goyt in Stockport. It flows westwards through the suburban areas of south Manchester, then into the Manchester Ship Canal at Irlam, becoming a part of the canal and maintaining the canal's water levels. After 4 miles (6.4 km) the river exits the canal, flowing towards Warrington where the river widens. It then narrows as it passes between the towns of Runcorn and Widnes. From Runcorn the river widens into a large estuary, which is 3 miles (4.8 km) across at its widest point near Ellesmere Port. The course of the river then turns north as the estuary narrows between Liverpool and Birkenhead on the Wirral Peninsula to the west, and empties into Liverpool Bay. In total the river flows 70.33 miles (113 km).
A railway tunnel between Birkenhead and Liverpool as part of the Mersey Railway opened in 1886. Two road tunnels pass under the estuary from Liverpool: the Queensway Tunnel opened in 1934 connecting the city to Birkenhead, and the Kingsway Tunnel, opened in 1971, to Wallasey. A road bridge, completed in 1961 and later named the Silver Jubilee Bridge, crosses between Runcorn and Widnes, adjacent to the Runcorn Railway Bridge which opened in 1868. A second road bridge, the Mersey Gateway, opened in October 2017, carrying a six-lane road connecting Runcorn's Central Expressway with Speke Road and Queensway in Widnes. The Mersey Ferry operates between Pier Head in Liverpool and Woodside in Birkenhead and Seacombe, and has become a tourist attraction offering cruises that provide an overview of the river and surrounding areas.
Water quality in the Mersey was severely affected by industrialisation, and in 1985, the Mersey Basin Campaign was established to improve water quality and encourage waterside regeneration. In 2009 it was announced that the river is "cleaner than at any time since the industrial revolution" and is "now considered one of the cleanest in the UK". The Mersey Valley Countryside Warden Service manages local nature reserves such as Chorlton Ees and Sale Water Park. The river gave its name to Merseybeat, developed by bands from Liverpool, notably the Beatles. In 1965 it was the subject of the top-ten hit single "Ferry Cross the Mersey" by Gerry and the Pacemakers.River Severn
The River Severn (Welsh: Afon Hafren) is the longest river in Great Britain at a length of 220 miles (354 km), and the second longest in the British Isles after the River Shannon in Ireland. It rises at an altitude of 2,001 feet (610 m) on Plynlimon, close to the Ceredigion/Powys border near Llanidloes, in the Cambrian Mountains of mid Wales. It then flows through Shropshire, Worcestershire and Gloucestershire, with the county towns of Shrewsbury, Worcester and Gloucester on its banks. With an average discharge of 107 m3/s (3,800 cu ft/s) at Apperley, Gloucestershire, the Severn is by far the greatest river in terms of water flow in England and Wales.
The river is usually considered to become the Severn Estuary after the Second Severn Crossing between Severn Beach, South Gloucestershire and Sudbrook, Monmouthshire. The river then discharges into the Bristol Channel which in turn discharges into the Celtic Sea and the wider Atlantic Ocean. The Severn's drainage basin area is 4,409 square miles (11,419 km2), excluding the River Wye and Bristol Avon which flow into the Severn Estuary. The major tributaries to the Severn are the Vyrnwy, Clywedog, Teme, Avon and Stour.River Truro
The Truro River is a river in the city of Truro in Cornwall, England, UK. It is the product of the convergence of the two rivers named Kenwyn and Allen which run under the city: the Truro River (named after the city) flows into the River Fal, estuarial waters where wildlife is abundant, and then out into the Carrick Roads. The river is navigable up to Truro.
The river valleys form a bowl surrounding the city on the north, east and west and open to the Truro River in the south. The fairly steep-sided bowl in which Truro is located, along with high precipitation swelling the rivers and a spring tide in the River Fal, were major causes of flooding in 1988 which caused large amounts of damage to the city centre. Since then, flood defences have been constructed around the city, including an emergency dam at New Mill on the River Kenwyn and a tidal barrier on the Truro River, to prevent future problems. The valley of the Tresillian River is between the valleys of the Truro River and the Fal; the Tresillian River flows into the Truro River just upstream of where the latter joins the Fal. Early records give the Tresillian River the name "Seugar" (1297) or "Sowgar" (1530); the meaning of this name is unknown.Río de la Plata
The Río de la Plata (Spanish pronunciation: [ˈri.o ðe la ˈplata], lit. "river of silver")—rendered River Plate in British English and the Commonwealth and La Plata River (occasionally Plata River) in other English-speaking countries—is the estuary formed by the confluence of the Uruguay and the Paraná rivers. It empties into the Atlantic Ocean, forming a funnel-shaped indentation on the southeastern coastline of South America. Depending on the geographer, the Río de la Plata may be considered a river, an estuary, a gulf or a marginal sea. For those who consider it a river, it is the widest river in the world, with a maximum width of about 220 kilometres (140 mi).
The river is about 290 kilometres (180 mi) long, and it widens from about 2 kilometres (1.2 mi) at its source to about 220 kilometres (140 mi) at its mouth. It forms part of the border between Argentina and Uruguay, with the major ports and capital cities of Buenos Aires and Montevideo on its western and northern shores, respectively. The coasts of La Plata are the most densely populated areas of Argentina and Uruguay.Severn Estuary
The Severn Estuary (Welsh: Môr Hafren) is the estuary of the River Severn, the longest river in Great Britain. It is the confluence of four major rivers, being the Severn, Wye, Usk and Avon, and other smaller rivers. Its high tidal range, approximately 50 feet (15 m), means that it has been at the centre of discussions in the UK regarding renewable energy.Tagus
The Tagus (; Spanish: Tajo, [ˈtaxo]; Portuguese: Tejo, [ˈtɛʒu]) is the longest river in the Iberian Peninsula. It is 1,007 km (626 mi) long, 716 km (445 mi) in Spain, 47 km (29 mi) along the border between Portugal and Spain and 275 km (171 mi) in Portugal, where it empties into the Atlantic Ocean near Lisbon. It drains an area of 80,100 square kilometers (30,927 sq mi) (the second largest in the Iberian peninsula after the Douro). The Tagus is highly utilized for most of its course. Several dams and diversions supply drinking water to places of central Spain and Portugal, while dozens of hydroelectric stations create power. Between dams it follows a very constricted course, but after Almourol it enters a wide alluvial valley, prone to flooding. Its mouth is a large estuary near the port city of Lisbon.
The source of the Tagus is the Fuente de García, in the Frías de Albarracín municipal term, Montes Universales, Sistema Ibérico, Sierra de Albarracín Comarca. All its major tributaries enter the Tagus from the right (north) bank. The main cities it passes through are Aranjuez, Toledo, Talavera de la Reina and Alcántara in Spain, and Abrantes, Santarém, Almada and Lisbon in Portugal.Thames Estuary
The Thames Estuary is where the River Thames meets the waters of the North Sea, in the south-east of Great Britain.
The limits of the estuary have been defined in several ways:
Although physically the head of Sea Reach or the Kent / Essex Strait, south of Canvey Island on the northern (Essex) shore presents a western boundary, the Tideway itself can be considered estuarine; it starts in south-west London at Teddington/Ham.
The Yantlet Line between the Crowstone in Chalkwell and the London Stone on the Isle of Grain.
The Nore sandbank between Havengore Creek, Essex, and Warden Point, Kent.
The eastern boundary of the estuary suggested in a Hydrological Survey of 1882-9 is a line drawn from North Foreland, Margate, Kent via the Kentish Knock lighthouse to Harwich in Essex. It is to this line that the typical estuarine sandbanks extend. The estuary downstream of the Tideway has a tidal movement of 4 metres, moving at a speed of up to 2.6 knots (4.8 km/h; 3.0 mph).
A line from Warden Point on the Isle of Sheppey Kent via Sea Reach No. 1 buoys to Havengore Head Essex.The estuary is one of the largest of 170 such inlets on the coast of Great Britain. It constitutes a major shipping route: its thousands of movements each year include large oil tankers, container ships, bulk carriers and roll-on/roll-off (ro-ro) ferries entering the estuary for the Port of London and the Medway Ports of Sheerness, Chatham and Thamesport.
The traditional Thames sailing barge worked in this area, designed to be suitable for the shallow waters in the smaller ports. More recently one of the largest wind farms in the UK has been developed in the estuary, located 8.5 km north of Herne Bay, Kent. The farm contains 30 wind turbines generating a total of 82.4MW of electricity. The much larger 630 MW London Array was inaugurated in 2013.
This area has had several proposed sites for the building of a new airport to supplement, or even to replace Heathrow/Gatwick. In the 1960s Maplin Sands was a contender; in 2002 it was to be at Cliffe, Kent. The new airport would be built on a man-made island in the estuary north of Minster-in-Sheppey
There is also some discussion about the need for a Lower Thames Crossing in order to alleviate traffic congestion at Dartford.
The Thames Estuary is the focal part of the 21st-century toponym, the "Thames Gateway", designated as one of the principal development areas in Southern England.