Estuary

An estuary is a special place where fresh water from rivers mixes with salt water from the sea, creating a body of brackish water. These areas are partly enclosed by land and act as a bridge between river environments and the ocean. Because of the mix of fresh and salt water, estuaries are rich in nutrients, making them very important for many plants and animals.

Most estuaries were formed during a time called the Holocene, when sea levels rose about 10,000–12,000 years ago, flooding valleys that had been carved by rivers or glaciers. They can have many names, such as bays, harbors, lagoons, inlets, or sounds, but not all of these are true estuaries.

Unfortunately, many estuaries face problems. Human activities like soil erosion, deforestation, overgrazing, and overfishing can harm these areas. Pollution from nutrients, heavy metals, and other harmful substances can also damage estuaries, making it important to protect these valuable places.

Definition

An estuary is a special place where the sea meets rivers. It is a coastal area that is partly enclosed and has water that is a mix of salt from the sea and fresh water from rivers. This mixing happens because the sea’s tides bring in salty water, while rivers add fresh water.

Estuaries are important homes for many plants and animals. They connect the land and the sea, creating a changing environment where both sea life and river life can thrive. The amount of fresh water, the tides, and other natural forces shape how these areas work and affect the plants and animals that live there.

Estuarine ecosystems

Estuaries are special places where rivers meet the sea, creating a mix of fresh and salty water. This mixing makes the environment always changing, which helps many plants and animals live there.

These areas support many different kinds of life, from those that live in fresh water to those that live in the sea. They help keep nature balanced by providing homes for young fish and other sea creatures, and by cleaning the water. Estuaries also give birds and other special animals places to live, and they help protect land from strong storms and rising sea levels.

Classification based on geomorphology

Drowned river valleys

Main article: Ria

Drowned river valleys, also called coastal plain estuaries, form when sea levels rise and seawater moves into river valleys. This type is common in places with temperate climates. Examples include the Severn Estuary in the United Kingdom and the Ems Dollard along the Dutch-German border. In the U.S., you can find them in the Hudson River, Chesapeake Bay, and Delaware Bay along the Mid-Atlantic coast, as well as Galveston Bay and Tampa Bay along the Gulf Coast.

Lagoon-type or bar-built

See also: Mouth bar

Bar-built estuaries happen where sand and sediment build up, creating barriers like islands or spits that separate the estuary from the ocean. These are common in tropical and subtropical areas. They are often shallow and have only small openings to the sea. Examples are found along the Atlantic and Gulf coasts of the U.S. where there is lots of sediment and small tidal ranges.

Fjord-type

Fjords are deep valleys carved by glaciers. When these valleys fill with water, they become fjord-type estuaries. They usually have steep sides and rocky bottoms. The deepest part is often near the mouth, where rocks or glaciers once left barriers. These estuaries can be very deep inside but shallower at the entrance. You can find them in places like Alaska, the Puget Sound region of Washington state, British Columbia, eastern Canada, Greenland, Iceland, New Zealand, Chile, and Norway.

Tectonically produced

Some estuaries are formed when the land moves due to forces under the earth’s surface, like earthquakes or volcanoes. When the land sinks or gets blocked by moving pieces, water can fill the area and create an estuary. An example is San Francisco Bay, shaped by movements along the San Andreas Fault.

Classification based on water circulation

See also: Estuarine water circulation

Salt wedge

"Salt wedge" redirects here; not to be confused with Salt wedging (geology).

See also: Saltwater intrusion

In a salt wedge estuary, the river pushes out much more water than the sea, and tides do not play a big role. Freshwater stays on top of the saltwater, getting thinner as it moves toward the sea. The heavier saltwater moves toward land along the bottom, forming a wedge shape. This creates waves that mix the two waters a little. The Mississippi River and the Mandovi estuary in Goa during the rainy season are examples.

Partially mixed

When tides become stronger, the river sends out less water than the sea. This causes the whole water column to mix, so the saltiness changes more from one end to the other instead of up and down. This makes the water only partly separated by layers. The Chesapeake Bay and Narragansett Bay are examples.

Well-mixed

If tides are very strong, they can mix the water so well that there are no layers of different saltiness. The freshwater and seawater mix completely. The lower parts of Delaware Bay and the Raritan River in New Jersey are examples of this type.

Inverse

Inverse estuaries happen in very dry places where evaporation takes away more water than rivers add. This creates a zone with the most salt, and water from both the river and the sea moves toward this zone. The water pushes down and spreads along the bottom in both directions. Spencer Gulf in South Australia, the Saloum River, and the Casamance River in Senegal are examples.

Intermittent

The type of estuary can change a lot depending on how much freshwater flows into it. It can shift from being mostly sea-like to any of the other types.

Physiochemical variation

Estuary water changes a lot, especially in how salty it is, how much dirt it carries, and how much oxygen it holds. Near the rivers, the water is almost not salty at all, but near the sea, it can be as salty as 3.4%. This changing salt level makes it hard for plants and animals to live there.

Dirt from rivers often settles in areas called mudflats, where it’s tough for plants like algae to grow. This dirt can also block the breathing and eating of some creatures, but special types of animals have learned to live with it. Sometimes, too much dirt from human activities can use up all the oxygen in the water, making it hard for fish and other life to survive.

Implications of eutrophication on estuaries

Further information: Eutrophication § Coastal waters

Eutrophication happens when too many nutrients, like nitrogen, enter estuaries. This can cause harmful algae to grow a lot. When these algae die, they break down and use up oxygen in the water. This creates areas with very little oxygen, called hypoxic zones, which can harm plants and animals that need oxygen to live.

Plants in estuaries, like salt marsh grasses and mangrove trees, can also be hurt. With too many nutrients, they grow more above ground but have weaker roots below. This makes the soil less stable and can cause erosion. Animals that depend on these plants for food or shelter can also suffer. Some fish may do better with more nutrients, but others may struggle, especially if oxygen levels drop too low. This can affect fishing and the jobs of many people who depend on healthy estuaries.

Implications for marine life

Estuaries are always changing, with temperature, salt levels, and water movement shifting every day with the tides. These changes make estuaries rich places for plants and animals, but also challenging for many to live in all year. Fish communities in estuaries change with the seasons. In winter, tougher fish that live in the sea stay nearby, while in summer many different fish move in to take advantage of the plentiful food.

Estuaries are important homes for many creatures. Some fish, like Pacific Herring, lay their eggs there. Young flatfish and rockfish grow up in estuaries, and fish like salmon and lampreys pass through on their journeys. Birds such as the black-tailed godwit also depend on estuaries.

Living in estuaries can be tough because salt levels and mud can change a lot. Some fish and small animals have special ways to handle these changes. Many also hide in the mud to stay safe. However, the mud often has very little oxygen, making it hard for some creatures to survive there.

Tiny plants called phytoplankton float in the water and are a key food source. They are carried in and out by the tides. The main types are diatoms and dinoflagellates, which are found in the mud. Another important food source in estuaries comes from bits of dead plants and animals settling to the bottom.

Human impact

Further information: Ecosystem service § Estuarine and coastal ecosystem services, and Marine ecosystem § Threats

Many big cities around the world are near estuaries. However, human activities can harm these important areas. Pollution from many sources, like waste and chemicals, can flow into estuaries and hurt the plants and animals that live there. Some harmful materials can build up in the bodies of fish and other sea life, making them unsafe to eat.

Too many nutrients from farms and waste can cause parts of the water to have too little oxygen, creating areas where most sea life cannot survive. Overfishing has also damaged some estuary populations, like the oysters in Chesapeake Bay, which used to clean the water but are now much fewer in number. Some estuaries have also shrunk because of dams and water diversions.

Examples

Africa

Asia

Europe

North America

Oceania

South America