An estuary is a partially enclosed coastal body of brackish water with
one or more rivers or streams flowing into it, and with a free
connection to the open sea.
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 sea water 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.
2 Classification based on geomorphology
2.1 Drowned river valleys
2.2 Lagoon-type or bar-built
2.4 Tectonically produced
3 Classification based on water circulation
3.1 Salt wedge
3.2 Partially mixed
4 Physiochemical variation
5 Implications for marine life
6 Human impact
7.4 North America
7.6 South America
8 See also
10 External links
New Jersey Harbor Estuary
River Exe estuary
Estuary mouth located in Darwin, Northern Territory, Australia
A crowded estuary mouth in Paravur near the city of Kollam, India
Río de la Plata
Río de la Plata estuary
Estuary mouth of the
Yachats River in Yachats, Oregon
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.
Classification based on geomorphology
Drowned river valleys
Main article: Ria
Drowned river valleys are also known as coastal plain estuaries. In
places where the sea level is rising relative to the land, sea water
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
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
Galveston Bay and
Tampa Bay along the Gulf Coast.
Lagoon-type or bar-built
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:
building up of offshore bars by wave action, in which sand from the
sea floor is deposited in elongated bars parallel to the shoreline,
reworking of sediment discharge from rivers by wave, current, and wind
action into beaches, overwash flats, and dunes,
engulfment of mainland beach ridges (ridges developed from the erosion
of coastal plain sediments around 5000 years ago) due to sea level
rise and resulting in the breaching of the ridges and flooding of the
coastal lowlands, forming shallow lagoons, and
elongation of barrier spits from the erosion of headlands due to the
action of longshore currents, with the spits growing in the direction
of the littoral drift.
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
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
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
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
San Andreas fault system causing the
inundation of the lower reaches of the Sacramento and San Joaquin
Classification based on water circulation
See also: Estuarine water circulation
In this type of estuary, river output greatly exceeds marine input and
tidal effects have a minor importance.
Fresh water 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
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
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
Raritan River in
New Jersey are examples of vertically
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.
Estuary type varies dramatically depending on freshwater input, and is
capable of changing from a wholly marine embayment to any of the other
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.
settles in intertidal mudflats which are extremely difficult to
colonize. No points of attachment exist for algae, so vegetation based
habitat is not established.[clarification needed]
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
Implications for marine life
Estuaries provide habitats for a large number of organisms and support
very high productivity. Estuaries provide habitats for many fish
nurseries, depending upon their locations in the world, such as salmon
and sea trout. Also, migratory bird populations, such as the
black-tailed godwit, make essential use of 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
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.
It is important to remember that a primary source of food for many
organisms on estuaries, including bacteria, is detritus from the
settlement of the sedimentation.
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.
For example, Chinese and Russian industrial pollution, such as phenols
and heavy metals, has devastated fish stocks in the
Amur River 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. 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.
Orange River Estuary
Gulf of Ob
Puerto Princesa Underground River
Chesapeake Bay including Hampton Roads
Columbia River Estuary
Gulf of Saint Lawrence
Indian River Lagoon
New Jersey Harbor
San Francisco Bay
Port Jackson (Sydney Harbour)
Iguape-Cananéia-Paranaguá estuary lagoon complex
Lagoa dos Patos
Lagoa dos Patos and
Rio de la Plata
Coastal and Estuarine Research Federation
List of estuaries of England
List of estuaries of South Africa
List of waterways
National Estuarine Research Reserve
Region of freshwater influence
Shell growth in estuaries
^ a b Pritchard, D. W. (1967). "What is an estuary: physical
viewpoint". In Lauf, G. H. Estuaries. A.A.A.S. Publ. 83. Washington,
DC. pp. 3–5.
^ a b McLusky, D. S.; Elliott, M. (2004). The Estuarine Ecosystem:
Ecology, Threats and Management. New York: Oxford University Press.
^ a b c d Wolanski, E. (2007). Estuarine Ecohydrology. Amsterdam:
Elsevier. ISBN 978-0-444-53066-0.
^ Silva, S., Lowry, M., Macaya-Solis, C., Byatt, B., & Lucas, M.
C. (2017). Can navigation locks be used to help migratory fishes with
poor swimming performance pass tidal barrages? A test with lampreys.
Ecological Engineering, 102, 291-302.
^ Kunneke, J. T.; Palik, T. F. (1984). "
Tampa Bay environmental atlas"
Fish Wildl. Serv. Biol. Rep. 85 (15): 3. Retrieved January
^ a b c d Kennish, M. J. (1986). Ecology of Estuaries. Volume I:
Physical and Chemical Aspects. Boca Raton, FL: CRC Press.
^ Wolanski, E. (1986). "An evaporation-driven salinity maximum zone in
Australian tropical estuaries". Estuarine, Coastal, and Shelf Science.
22 (4): 415–424. Bibcode:1986ECSS...22..415W.
^ Tomczak, M. (2000). "Oceanography Notes Ch. 12: Estuaries".
Retrieved 30 November 2006.
^ Day, J. H. (1981). Estuarine Ecology. Rotterdam: A. A. Balkema.
^ Kaiser; et al. (2005). Marine Ecology. Processes, Systems and
Impacts. New York: Oxford University Press.
^ Gillanders, Bronwyn M. (2003). "Evidence of connectivity between
juvenile and adult habitats for mobile marine fauna: an important
component of nurseries" (PDF). Marine Ecology Progress Series.
^ Gill, Jennifer A. (2001). "The buffer effect and large-scale
population regulation in migratory birds". Nature. 412 (6845):
436–438. doi:10.1038/35086568. PMID 11473317.
^ Ross, D. A. (1995). Introduction to Oceanography. New York: Harper
Collins College Publishers. ISBN 978-0-673-46938-0.
^ Branch, G. (1999). "Estuarine vulnerability and ecological impacts:
Estuaries of South Africa, edited by Brian R. Allanson and Dan Baird".
Trends in Ecology & Evolution. 14 (12): 499.
^ J. García-Alonso, D. Lercari, B.F. Araujo, M.G. Almeida, C.E.
Rezende (5 March 2017). "Total and extractable elemental composition
of the intertidal estuarine biofilm of the Río de la Plata:
Disentangling natural and anthropogenic influences". Estuarine,
Coastal and Shelf Science. 187: 53–61.
doi:10.1016/j.ecss.2016.12.018. CS1 maint: Multiple names:
authors list (link)
^ "Indigenous Peoples of the Russian North, Siberia and Far East:
Nivkh" by Arctic Network for the Support of the Indigenous Peoples of
the Russian Arctic
^ Gerlach, Sebastian A. (1981). Marine Pollution: Diagnosis and
Therapy. Berlin: Springer. ISBN 0387109404.
Oyster Reefs: Ecological importance". US National Oceanic and
Atmospheric Administration. Archived from the original on October 3,
2008. Retrieved 2008-01-16.
Wikimedia Commons has media related to Estuaries.
Look up estuary in Wiktionary, the free dictionary.
Animated documentary on
Chesapeake Bay NOAA.
"Habitats: Estuaries – Characteristics". www.onr.navy.mil. Archived
from the original on 2009-05-17. Retrieved 2009-11-17.
Estuary Guide (Based on experience and R&D within the UK)
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