Estuary

Estuary is the most common term used in the environmental science and earth science branch of the science. In this article, we have covered all the important topics about the estuary like estuary meaning , its type, history, circulation, and human impact on it. Let us discuss the estuary meaning first, An estuary is a brackish water body that is partly enclosed along the shore, with one or more rivers or streams flowing into it and a free access to the open sea. Estuaries serve as an ecotone, or transition region, between river and marine environments. Tides, waves, and the influx of saline water all have an effect on estuaries, as do riverine effects including freshwater flows and sediment. Estuaries are among the most active natural ecosystems on the planet. This is because the mixing of seawater and freshwater produces high levels of nutrients both in the water column and in the sediment.

History of Estuary

Most current estuaries were created during the Holocene period, when sea levels started to rise around 10,000–12,000 years ago, flooding river-eroded or glacially scoured valleys. Estuaries are usually categorised on the basis of geomorphological characteristics or water circulation patterns. Although some of the water bodies do not specifically follow the above mentioned concept of an estuary and may be entirely freshwater, they may be referred to as bays, harbours, lagoons, inlets, or sounds. You may note that estuaries and lagoons are often confused with each other, but they have some key differences in their property and structure.

Types of Estuaries

Drowned River Valley

Coastal plain estuaries are also known as drowned river valleys. Sea water gradually penetrates river valleys in areas where the sea level is rising compared to the surface, and the topography of the estuary resembles that of a river valley. In temperate climates (climate between polar and tropical), this is the most common form of estuary. The Severn Estuary in the United Kingdom and the Ems Dollard on the Dutch-German border are two well known estuaries.

The width-to-depth ratio of these estuaries is normally high, with the inner portion being wedge-shaped, appearing in cross-section, and broadening and deepening as it moves outward. The depth of the estuarine water rarely exceeds 30 metres (nearly 100 feet). The Hudson River, Chesapeake Bay, and Delaware Bay on the Mid-Atlantic coast, and Galveston Bay and Tampa Bay on the Gulf Coast, are examples of this form of estuary.

Lagoon Type or Bar Built

Bar-built estuaries occur where sediment accumulation has kept pace with rising sea levels, resulting in shallow estuaries isolated from the sea by sand spits or barrier islands. In tropical and subtropical areas, they are fairly common.

Barrier beaches keep these estuaries semi-isolated from ocean waters. Therefore, known as barrier islands or barrier spits. The estuary is partly encircled by barrier islands, with only small inlets providing access to the ocean waters. Bar-built estuaries usually form on gently sloping plains near tectonically stable continent edges and marginal sea coasts. They run the length of the Atlantic and Gulf coasts of the United States, where there is active coastal sediment deposition and tidal ranges are less than 4 m (nearly 13 feet). Barrier beaches that surround bar-built estuaries have been established in a variety of ways, including:

• reworking of sediment drainage from rivers onto beaches, overwash flats, and dunes through wave action, in which sand from the seafloor is deposited in elongated bars parallel to the shoreline.

• Construction of offshore bars by wave action, in which sand from the seafloor is deposited in elongated bars parallel to the shoreline.

• Due to sea level rise, engulfment of mainland beach ridges (ridges formed by the erosion of coastal plain sediments about 5000 years ago), resulting in ridge breaching and flooding of the coastal lowlands, creating shallow lagoons.

• Barrier spits are elongating as a result of headland erosion caused by longshore currents, with the spits rising in the direction of littoral drift.

Fjord Type

Fjords were created when Pleistocene glaciers enlarged and deepened existing river valleys, forming U-shaped cross-sections. They usually occur in the form of rocks, barriers, or sills of glacial deposits at their mouths, which have the effect of altering the estuarine circulation.

Fjord-like estuaries are created by glaciers in deeply eroded valleys. Steep sides, rock bottoms, and underwater sills contoured by glacial movement characterise these U-shaped estuaries. The estuary's mouth is the shallowest, with sills formed by terminal glacial moraines or rock bars that limit water flow. The depth of the estuary can reach 300 metres in the upper reaches. In general, the width-to-depth ratio is small. Tidal oscillations only impact the water up to the depth of the sill in estuaries with very shallow sills, and the waters deeper than that can stay stagnant for a long time, resulting in only an occasional exchange of the estuary's deep water with the ocean. Water distribution is less limited when the sill depth is deep, and there is a slow but steady exchange of water between the estuary and the ocean. Along the coasts of Alaska, the Puget Sound area of western Washington state, British Columbia, eastern Canada, Greenland, Iceland, New Zealand, and Norway, fjord-type estuaries can be found also known as estuarine coast.

Tectonically Produced

Land movement associated with faulting, volcanoes, and landslides creates estuaries, which are created by subsidence or land cut off from the ocean by land movement. The construction of these estuaries has also been aided by eustatic sea-level rise during the Holocene Epoch. There are only a few tectonically formed estuaries in the world. The San Francisco Bay, for example, was created by the San Andreas fault system's crustal motions, which inundated the Sacramento and San Joaquin rivers lower reaches.

Types of Estuary Depending Upon the Water Circulation

Salt Wedge

River production far outnumbers marine input in this form of estuary, and tidal effects are negligible. Freshwater floats on top of seawater in a thin layer that thins out as it goes out to sea. The denser seawater flows landward along the estuary's floor, creating a wedge-shaped layer that becomes thinner as it gets closer to the shore. Shear forces produce internal waves at the interface as a velocity differential arises between the two layers, mixing the seawater upward with the freshwater. The Mississippi River is an example of a salt wedge river estuary.

Partially Mixed

River production becomes less than marine input as tidal forcing increases. In this case, current-induced turbulence mixes the entire water column, causing salinity to vary more longitudinally than vertically, resulting in a moderately stratified state. The Chesapeake Bay and Narragansett Bay are two examples.

Well Mixed

As tidal mixing forces surpass river output, the water column becomes well-mixed and the vertical salinity gradient vanishes. Because of the extreme turbulent mixing and eddy effects, the freshwater-seawater boundary is removed. Vertically homogeneous estuaries include the lower reaches of Delaware Bay and the Raritan River in New Jersey.

Inverse

In dry climates, where evaporation far outnumbers freshwater inflow, inverse estuaries form. A salinity maximum zone forms, and both riverine and oceanic water flow into it close to the sea. This water is pulled downward and spreads out along the bottom, both seaward and landward. Spencer Gulf estuary located in South Australia is an example of an inverse estuary.

Intermittent

The type of estuary changes significantly depending on the amount of freshwater input, and it can change from a completely marine embayment to any of the other estuary forms.

Impact of Human on Estuary

Estuaries are threatened as habitats by human activities such as deforestation and overfishing. Sewage, coastal settlement, land clearing, and other factors all pose a threat to them. Events far upstream have an effect on estuaries, which concentrate materials including toxins and sediments. Land runoff, mining, agricultural, and residential waste all join waterways and end up in estuaries. Plastics, pesticides, furans, dioxins, phenols, and heavy metals are examples of contaminants that do not degrade quickly in the marine setting.

Toxins can build up (bioaccumulation) in the tissues of many aquatic organisms, a process known as bioaccumulation. They also build up in benthic ecosystems including estuaries and bay muds, providing a geological record of human activities from the previous century. The abiotic and biotic components of biofilm reflect areas of the estuary impacted by human activities, and over time can shift the basic composition of the ecosystem, as well as the reversible or permanent changes in the abiotic and biotic sections of the systems from the bottom up.

For example, industrial contamination from China and Russia, such as phenols and heavy metals, has decimated fish populations in the Amur River and harmed the soil in its estuary.

Since nutrients are discharged into estuaries by land runoff, estuaries are naturally eutrophic. Land run-off now contains many chemicals used as fertilisers in agriculture, as well as waste from animals and people, due to human activities. Hypoxia and the formation of dead zones will occur when there are too many oxygen-depleting chemicals in the water. Water quality, fish, and other animal populations can suffer as a result. Overfishing is also an issue. Overfishing has nearly wiped out a once-thriving oyster community in the Chesapeake Bay. These contaminants are filtered by oysters, which either eat them or shape them into small packets that float to the bottom, where they are harmless. The oysters used to filter the entire volume of water in the estuary of excess nutrients every three or four days. Today, the process takes nearly a year, and sediment, nutrients, and algae can wreak havoc on local waters.

Estuarine Circulation

Microtidal, mesotidal, and macrotidal tidal ranges, as well as topography, such as submerged river valleys and fjords, and morphological classification, can all be used to classify stuaries. Estuaries are classified by oceanographers according to their salinity composition, or the ratio of freshwater input to seawater mixed by the tides (Dyer, 1997).

The conflicting powers of freshwater from riverine inflow and the influx of seawater through tidal waves and turbulent mixing decide the circulation of an estuary, with local winds and bathymetry playing a minor role. The salinity variations along the estuary are typically so high that the density differences that result dictate circulation. The circulation is normally dominated by along-channel flow, while across-channel flow may be an order of magnitude smaller. As a result, a two-dimensional velocity regime is often used to define circulation.

The ratio of freshwater input to mixed seawater, which specifies the properties and salinity distribution of each estuary, can be used to classify them. Finally, the properties that define estuarine circulation would have an effect on the estuary's connectivity. The most critical forms of estuaries are: 

• Salt wedge estuary

• Highly stratified estuary

• Slightly stratified estuary

• Vertically mixed estuary

• Inverse estuary, and 

• Intermittent estuary. 

We can measure this by comparing the amount of freshwater entering the river during one tidal time to the volume of water carried into the estuary and withdrawn during one tidal cycle (Tomczak and Godfrey, 1994). Estuaries can be of any size and still belong to the same type since the ratio of freshwater to tidal volume is significant.

Did you know that?

• Estuaries are among the world's most active habitats.

• Estuaries are very fragile habitats.

• Positive, inverse, and low-inflow estuaries are the three types of estuaries classified by water balance.