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Estuary facts for kids

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Estuary-mouth
Estuaries

An estuary is where a river meets the sea. There, saltwater mixes with freshwater to become brackish water. The river becomes wider and wider and flows slowly to the ocean.

Bays, marshes, swamps, and inlets can all have estuaries. A view of an estuary from the air is usually an interesting sight: many estuaries meander (curve and bend) to find their way to the sea. Estuaries come in all sizes and shapes, each according to its location and climate. Where rivers meet the sea and fresh water mixes with salt water, the mixture is called brackish water.

Some estuaries are very large. They may be large ocean bays that have more than one river flowing into them. For example, Chesapeake Bay is a large estuary, and several different rivers meet the Atlantic Ocean there.

Definition

Exe estuary from balloon
River Exe estuary
Estuary mouth
Estuary mouth located in Darwin, Northern Territory, Australia
Thekkumbhagam Estuary, Paravur
A crowded estuary mouth in Paravur near the city of Kollam, India
Estuary-mouth
Estuary mouth
Yachats River estuary mouth
Estuary mouth of the Yachats River in Yachats, Oregon
Mouths of amazon geocover 1990
Amazon estuary

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 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 effects of tides on estuaries can show nonlinear effects on the movement of water which can have important impacts on the ecosystem and waterflow. 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.

Habitat

Estuaries are usually filled with shallow waters, and sunlight reaches all levels of the water. Marsh grasses, algae, and other kinds of plants live in estuaries and provide food for a variety of fish, crabs, oysters, and shrimp. Estuaries are especially important since they act as nurseries for many different types of young fish and other animals before they head out toward the open ocean. Many sea birds also nest in estuaries.

The United States government has a program to study and protect the natural environment in many different estuaries, called the National Estuarine Research Reserve System. One reason that the natural environment in so many estuaries is in danger is because they are also good places for people to live and build cities. Unfortunately, many of world's largest cities are at or near estuaries.

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.

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 islands and barrier spits).

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 currents, with the spits growing in the direction of the littoral drift.

Fjord-type

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.

They are found in cold climates.

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.

Tectonically produced

These estuaries are formed by land cut off from the ocean by land movement associated with faulting, volcanoes, and landslides.

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

Salt wedge

In this type of estuary, river output is greater than the seawater coming in. 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 speed difference develops between the two layers, its generates internal waves, mixing the seawater upward with the freshwater.

An example of a salt wedge estuary is the Mississippi River.

Partially mixed

As tidal forcing increases, river output becomes less than the seawater input. Examples include the Chesapeake Bay and Narragansett Bay.

Well-mixed

Tidal mixing forces exceed river output, resulting in a well-mixed water column.

The freshwater-seawater boundary is eliminated due to the intense turbulent mixing effects.

The lower reaches of Delaware Bay and the Raritan River in New Jersey are examples of these estuaries.

Inverse

Inverse estuaries occur in dry climates where evaporation greatly exceeds the inflow of freshwater. A salinity maximum zone is formed, and both the river and seawater 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.

Intermittent

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.

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 a 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 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 flow.

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.

Human impact

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.

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