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 known as ecotone
. Estuaries are subject both to marine influences such as tides
, waves, and the influx of saline water and to riverine influences such as flows of freshwater and sediment. The mixing of seawater and freshwater provides 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 bay
s, or sound
s, although some of these water bodies do not strictly meet the above definition of an estuary and could be fully saline.
Many estuaries suffer degeneration
from a variety of factors including soil erosion
and the filling of wetlands. Eutrophication
may lead to excessive nutrients from sewage and animal wastes; pollutants including heavy metals
, polychlorinated biphenyl
s and hydrocarbon
s 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 seawater 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 fjord
s, river mouth
s, and tidal creek
s. An estuary is a dynamic ecosystem
having a connection to the open sea through which the sea water
enters with the rhythm of the tide
s. The seawater 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 freshwater, the tidal range, and the extent of evaporation of the water in the estuary.
Classification based on geomorphology
Drowned river valleys
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 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 . 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
Lagoon-type or bar-built
Bar-built estuaries are found in a place where the deposition of sediment has kept pace with rising sea levels 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 island
s 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 . 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 seafloor is deposited in elongated bars parallel to the shoreline,
* reworking of sediment discharge from rivers by a 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 current
s, with the spits growing in the direction of the littoral drift.
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.
-type estuaries are formed in deeply eroded valleys formed by glacier
s. 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 moraine
s or rock bars form sills that restrict water flow. In the upper reaches of the estuary, the depth can exceed . 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
, New Zealand, and Norway.
These estuaries are formed by subsidence or land cut off from the ocean by land movement associated with faulting
es, and landslide
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
Classification based on water circulation
In this type of estuary, river output greatly exceeds marine input and tidal effects have 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 freshwater. 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.
[Gostin, V. & Hall, S.M. (2014): Spencer Gulf: Geological setting and evolution. In: ''Natural History of Spencer Gulf.'' Royal Society of South Australia Inc. p. 21. ]
Estuary type varies dramatically depending on freshwater input, and is capable of changing from a wholly marine embayment
to any of the other estuary types.
The most important variable characteristics of estuary water are the concentration of dissolved oxygen, salinity
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
zones can develop.
Implications for marine life
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
s 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
. Many species of fish
s have various methods to control or conform to the shifts in salt concentrations and are termed osmoconformer
s and osmoregulator
s. Many animals also burrow
to avoid predation
and to live in a more stable sedimental environment. However, large numbers of bacteria are found within the sediment which has 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.
are key primary producers in estuaries. They move with the water bodies and can be flushed in and out with the tide
s. Their productivity is largely dependent upon the turbidity
of the water. The main phytoplankton present is diatoms
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
. 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
s 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
* Lake St Lucia
* Gulf of Ob
* Yenisei Gulf
* Puerto Princesa Underground River
* Hàn River
* Kraburi River
* Waeru River Estuary of Chanthaburi Province
* Dawei River
* Naf River
* Meghna River
* The Gironde
* Golden Horn
* The Humber
* Severn Estuary
* Shannon Estuary
* Thames Estuary
* The Wash
* Western Scheldt
* Albemarle Sound
including Outer Banks
of North Carolina
* Chesapeake Bay
including Hampton Roads
* Columbia River Estuary
* Delaware Bay
* Drake's Estero
* East River
* Estuary of Saint Lawrence
* Fraser River
* Galveston Bay
* Great Bay
* Indian River Lagoon
* Laguna Madre
* Lake Borgne
* Lake Merritt
* Lake Pontchartrain
* Long Island Sound
* Mobile Bay
* Narragansett Bay
* Newport Back Bay
* New York-New Jersey Harbor
* Puget Sound
* Pamlico Sound
including the Outer Banks
of North Carolina
* San Francisco Bay
* Tampa Bay
* Gippsland Lakes
* Port Jackson
* Spencer Gulf
* Amazon River
* Iguape-Cananéia-Paranaguá estuary lagoon complex
* Lagoa dos Patos
and Lagoon Mirim
* Rio de la Plata
Animated documentary on Chesapeake BayNOAA
The Estuary Guide (Based on experience and R&D within the UK)
Category:Fishing and the environment
Category:Coastal and oceanic landforms
Category:Bodies of water