ESTUARY: CHARACTERISTICS, LOCATION, FLORA, FAUNA, CLIMATE, EXAMPLES - GEOGRAPHY - 2025

An estuary is a geomorphological accident that forms an ecosystem located at the mouth of a large river to the sea. It is identified by the existence of a single outlet to the sea with a strong influence of the tides that prevent sedimentation.

What defines the estuary is the dynamic that is established between the fresh water of the river and the salty water of the sea. The resulting ecosystem has characteristics of both riparian (river) and marine ecosystems.

Estuary of the Rio de La Plata. Source: Earth Sciences and Image Analysis Laboratory, NASA Johnson Space Center / Public domain

Estuaries are found on all the coasts of the world where a great river flows under strong tides. It is considered a biome of great importance, due to the diversity of ecosystems it generates.

In those estuaries located in tropical and subtropical zones, mangroves and underwater grasslands develop. The fauna is home to a wide variety of fish, aquatic turtles, crustaceans, mollusks and marine mammals, such as river dolphins.

The climate of the estuaries is variable, depending on the latitude where they are, and can occur in tropical, temperate or cold climates. However, due to its coastal condition, its climate is tempered by the influence of the oceanic mass.

Productivity in the estuary is high due to the different aquatic environments it houses and the nutrient load provided by the river. In them are some of the most important fishing areas.

Some examples of estuaries are the Rio de La Plata between Argentina and Uruguay and the Guadalquivir estuary in Spain.

Characteristics of the estuary

The estuary is a transition zone between a river of great flow and depth and the sea, where the tides determine the hydrological dynamics. The clear, open shape, with a single mouth channel that characterizes the estuary, is due to the tides.

In this area, the tides are strong and when they rise, they retain the river's water, letting it come out suddenly when retreating, thus preventing sedimentation. This differentiates it from the deltas where the river accumulates sediments that form the characteristic channels with multiple mouths.

The tides

The tide is the upward and downward movement experienced by marine waters under the gravitational pull of the Sun and the Moon. This flow (ascent) and ebb (descent) of the marine waters occurs every 6 hours, that is to say twice daily.

High tide and low tide

The highest point of height reached by the tide in a given area is called high tide, as well as the duration of this phenomenon. While the low tide is the lowest point of the sea level reached at the ebb of the tide.

Dynamics in the estuary

During high tide, the rise of the marine waters towards the continent exerts pressure against the waters of the river that advance to empty. This force of the marine waters retains the waters of the river, so the channel at the mouth widens.

Similarly, it causes overflow, forming marshes (salt water swamps). Then when the marine waters retreat at low tide, the river waters move strongly towards the sea.

The flow that the river reaches, the force of the tide as well as the direction and speed of the winds also affect this dynamic. For example, if the river flow is low, the tidal action dominates and the salinity of the water will be maximum.

Salinity gradient

In the estuary there is a dynamic salinity gradient, with areas where there is a greater amount of fresh water and others where salt water dominates. This is the product of the interaction of fresh river water with sea water.

Zoning and Stratification

The salinity gradient occurs both horizontally and vertically. Therefore, there is fresh water towards the entrance of the river and salt water in the sea, with an intermediate estuarine zone of medium salinity.

On the other hand, given the difference in density between fresh and salt water, there are different layers. As fresh water is less dense, it tends to occupy the upper layer while salt water forms the lower layer of the estuary.

These layers move in opposite directions, where fresh water heads to the sea and salt water tends to penetrate inland.

Productivity

Due to the contribution of nutrients carried by the river, estuaries are very productive, attracting a large number of marine, river and terrestrial species. In some estuaries the contribution of fresh water with nutrients is considerable, such as in the La Plata River, where the Paraná and Uruguay rivers contribute 20,000 m ³ / sec.

Types of estuaries

Given the complexity of marine and fluvial factors that act, there are different types of estuaries.

By the breadth of its tides

We speak of micro-tidal estuaries when the difference in height between high tide and low tide is less than 2 m. While in the mesotidal areas the difference is 2 to 4 m and in the macrotidal between 4 and 6 m.

Likewise, there are hypertidal estuaries where the difference between high tide and low tide is greater than 6m.

Because of its topography (shape of the land)

Taking into account the relief of the coast in the estuary, estuaries of coastal plains, fjords and estuaries with barriers are presented. The former are formed by flooding of the valley in which the mouth occurs.

These estuaries of coastal plains are funnel-shaped towards the sea and a depth no greater than 30 m, except towards the mouth. If the coastal valley is flooded by the sea instead of the river, an estuary is formed.

Fjord. Source: Sam Beebe / CC BY (https://creativecommons.org/licenses/by/2.0)

Fjords occur in regions that were covered by ice in the Pleistocene. The weight of these ice masses excavated pre-existing valleys making them narrow, deep and rectangular in shape, with almost vertical walls.

Barrier estuaries, on the other hand, are similar to coastal plain estuaries, but sedimentation has formed a transverse barrier at the mouth.

Tectonic estuaries

There are also estuaries caused by tectonic movements, for example uplifts of the land. For example, the estuary that constitutes the San Francisco Bay in the United States.

Location

Estuaries are located at the mouths of large rivers on the coasts of all continents.

Flora

Since the estuary is an ecosystem defined by geomorphological and hydrological characteristics, present at different latitudes, the flora varies greatly from one to another. In all cases, marsh and halophytic plants are frequent.

Grasslands of Spartina sp. Source: Pacific Southwest Region US Fish and Wildlife Service / Public domain

On the other hand, estuaries are home to various ecosystems associated with their own species, such as mangroves, seagrass meadows, muddy tidal flats, and salt marshes. For example, Spartina grasslands, combined with different species of algae, are common in temperate marshes.

Mangroves and underwater grasslands

In those estuaries located in tropical and subtropical areas, whose waters do not exceed 20 ºC, the mangrove ecosystem develops on their coasts. Similarly you can find submerged meadows of aquatic herbs such as Thalassia and Zostera.

Fauna

As with flora, fauna varies depending on where the estuary occurs, be it tropical, temperate or cold latitude. A prominent animal in many tropical and subtropical estuaries is the river dolphin, of which there are four genera (superfamily Platanistoidea).

Birds

An abundant group in estuarine areas are birds, with several associated species of seabirds. Common among these are the gannet (Morus bassanus) and the gull (family Laridae).

Alcatraz (Morus bassanus). Source: Andrew C / CC BY (https://creativecommons.org/licenses/by/2.0)

In general, the most abundant aquatic species are those euryhaline, that is, those that support wide variations in salinity. This is natural if we consider the differences in this factor across the estuary.

On the other hand, there are many species of fish, turtles, crustaceans and mollusks, both river and marine.

Breeding area

For some marine species, the estuary constitutes an area for the development of their larvae that go up the river and return to the sea as adults. Such is the case of the Atlantic shad (Brevoortia tyrannus), a fish from the herring group.

Weather

Estuaries can originate in any climate, both in tropical, temperate or cold regions, depending on the latitude at which they are located. But, due to its coastal nature, its climate is tempered by the influence of oceanic masses.

In such a way that even in a cold area, its climate will be less extreme than the characteristic inland climate.

Examples of estuaries in the world

- Rio de La Plata estuary (Argentina and Uruguay)

This estuary arises at the joint mouth of the Paraná and Uruguay rivers in the Atlantic, between Argentina and Uruguay. It is one of the largest in the world, with 40 km wide in its internal area and 200 km in the external area.

On its coasts there are marshes populated with Spartina species and other plants that support salinity (halophytes).

Weather

It presents a temperate climate tempered by the incidence of the oceanic mass of the western Atlantic.

Fauna

The river dolphin called Franciscana or Plata dolphin (Pontoporia blainvillei) is typical of its waters and the dolphin (Tursiops gephyreus) is also located. On the other hand, there are various species of aquatic turtles such as the seven-keeled turtle (Dermochelys coriacea) and the loggerhead turtle (Caretta caretta).

Among the fish, 72 species are recognized, including the anchovy (Engraulis anchoita) and the white croaker (Micropogonias furnieri).

- Estuary of the Guadalquivir (Spain)

It is formed at the mouth of the Guadalquivir River in the Province of Andalusia, Spain, near the city of Seville. In ancient times the estuary opened to a lagoon or gulf (Tartessian Gulf or Lacus Ligustinus), which settled.

Guadalquivir Estuary (Spain). Source: Jándalo / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)

The so-called Guadalquivir marshes, part of the Doñana reserve, were established on these marshy sediments.

Halophytic grasses, such as wiregrass (Spartina spp.) Dominate this ecosystem. On its margins there are also trees such as holm oak (Quercus rotundifolia) and white poplar (Populus alba).

Weather

The area of ​​the Guadalquivir estuary is subject to an oceanic Mediterranean climate, with hot summers and mild winters.

Fauna

Fish such as the eel (Anguilla anguilla), the jarabugo (Anaecypris hispanica), the colmilleja (Cobitis paludica) and the common trout (Salmo trutta) abound. While in the surrounding estates you can find the Iberian lynx (Lynx pardinus), roe deer (Capreolus capreolus) and wild boar (Sus scrofa).

Similarly, birds such as the Iberian imperial eagle (Aquila adalberti), the white-headed duck (Oxyura leucocephala) and the flamingo (Phoenicopterus roseus) abound.

References

1. Boschi, E. (1987). The estuarial ecosystem of the Rio de La Plata (ARGENTINA and Uruguay). Annals of the Institute of Marine Sciences and Limnology.
2. Calow P (Ed.) (1998). The encyclopedia of ecology and environmental management.
3. Cole, S. (1998). The Emergence of Treatment Wetlands. Environmental Science & Technology.
4. RAMSAR Agreement (Seen on September 21, 2019). ramsar.org/es
5. Cowardin, LM, Carter, V., Golet, FC & LaRoe, ET (1979). Classification wetlands and deepwater habitats of the United States.
6. Malvárez AI and Bó RF (2004). Documents of the course-workshop "Ecological bases for the classification and inventory of wetlands in Argentina".
7. Ramsar Convention Secretariat (2016). Introduction to the Convention on Wetlands.
8. World Wild Life (Viewed on March 26, 2020). worldwildlife.org ›ecoregions