Estuary facts for kids
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.
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.
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.
They are found in cold climates.
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 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.
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.
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.
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.
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Estuary Facts for Kids. Kiddle Encyclopedia.