An estuary is an area where a freshwater river or stream meets the ocean. In estuaries, the salty ocean mixes with a freshwater river, resulting in brackish water. Brackish water is somewhat salty, but not as salty as the ocean.

An estuary may also be called a bay, lagoon, sound, or slough.

Water continually circulates into and out of an estuary. Tides create the largest flow of saltwater, while river mouths create the largest flow of freshwater.

When dense, salty seawater flows into an estuary, it has an estuarine current. High tides can create estuarine currents. Saltwater is heavier than freshwater, so estuarine currents sink and move near the bottom of the estuary.

When less-dense freshwater from a river flows into the estuary, it has an anti-estuarine current. Anti-estuarine currents are strongest near the surface of the water. Heated by the sun, anti-estuarine currents are much warmer than estuarine currents.

In estuaries, water level and salinity rise and fall with the tides. These features also rise and fall with the seasons. During the rainy season, rivers may flood the estuary with freshwater. During the dry season, the outflow from rivers may slow to a trickle. The estuary shrinks, and becomes much more saline.

During a storm season, storm surges and other ocean waves may flood the estuary with saltwater. Most estuaries, however, are protected from the ocean's full force. Geographical features such as reefs, islands, mud, and sand act as barriers from ocean waves and wind.

Types of Estuaries

There are four different kinds of estuaries, each created a different way: 1) coastal plain estuaries; 2) tectonic estuaries; 3) bar-built estuaries; and 4) fjord estuaries.

Coastal plain estuaries (1) are created when sea levels rise and fill in an existing river valley. The Chesapeake Bay, on the East Coast of the United States, is a coastal plain estuary.

Chesapeake Bay was formed at the end of the last ice age. Massive glaciers retreated, leaving a carved-out landscape behind. The Atlantic Ocean rushed to fill in the wide coastal plain around the Susquehanna River, creating a large estuary known as a ria: a drowned river mouth.

Tectonic activity, the shifting together and rifting apart of the Earth's crust, creates tectonic estuaries (2). California's San Francisco Bay is a tectonic estuary.

The San Francisco Bay lies at the junction of the San Andreas fault and the Hayward fault. The complex tectonic activity in the area has created earthquakes for thousands of years. The San Andreas fault is on the coastal side of the bay, where it meets the Pacific Ocean at a strait known as the Golden Gate. The Hayward fault lies on the East Bay, near where the Sacramento and San Joaquin Rivers enter the estuary. The interaction of the San Andreas and Hayward faults contributes to downwarping, the process of an area of the Earth sinking.

Like the Chesapeake, the San Francisco Bay was only filled with water during the last ice age. As glaciers retreated, land around the bay experienced post-glacial rebound—without the massive weight of the glacier on top of it, the land gained elevation. The Pacific Ocean rushed in through the Golden Gate to flood the downwarped valley.

When a lagoon or bay is protected from the ocean by a sandbar or barrier island, it is called a bar-built estuary (3). The Outer Banks, a series of narrow barrier islands in North Carolina and Virginia, create sandy, bar-built estuaries.

The Outer Banks protect the region's coast from waves and wind brought by Atlantic Ocean hurricanes. The islands and sandbars also protect the delicate, brackish ecosystems created by the outflow of many rivers, such as the Roanoke and Pamlico. For these reasons, engineers monitor the shifting sandbars of the Outer Banks, and constantly work to maintain them.

Fjord estuaries (4) are a type of estuary created by glaciers. Fjord estuaries occur when glaciers carve out a deep, steep valley. Glaciers retreat and the ocean rushes into fill the narrow, deep depression. Puget Sound is a series of fjord estuaries in the U.S. state of Washington.

Like fjords found in Alaska and Scandinavia, the fjord estuaries of Puget Sound are very deep, very cold, and very narrow. Unlike many of those fjords, Puget Sound's fjord estuaries also have inflows from local rivers and streams. Many of these streams are seasonal, and fjord estuaries remain mostly salty.

Freshwater Estuaries

Some estuaries not located near oceans. These freshwater estuaries are created when a river flows into a freshwater lake.

Although freshwater estuaries are not brackish, the chemical composition of lake and river water is distinct. River water is warmer and less dense than lake water. The mixing of the two freshwater systems contributes to lake turnover—the mixing of the waters of a lake.

Freshwater estuaries are not affected by tides, but large bodies of water do experience predictable standing waves called seiches. Seiches, sometimes nicknamed sloshes, rhythmically move back and forth across a lake.

The Great Lakes, in the United States and Canada, experience seiches and have many freshwater estuaries. Old Woman Creek National Estuarine Research Center, in Huron, Ohio, was established to study the habitat created by a natural freshwater estuary. At the research center, Old Woman Creek empties into Lake Erie.

Estuary Ecosystems

Many plant and animal species thrive in estuaries. The calm waters provide a safe area for small fish, shellfish, migrating birds and shore animals. The waters are rich in nutrients such as plankton and bacteria. Decomposing plant matter, called detritus, provides food for many species.

The estuarine crocodile, for example, is an apex predator of tropical Australian and Southeast Asian estuaries. The estuarine crocodile is the largest reptile in the world. A specimen caught in the Philippines in 2011 measured 6.4 meters (21 feet).

Like most apex predators, estuarine crocodiles eat almost anything. This means the estuary must support a wide variety of food webs. Estuarine crocodiles do not usually consume producers—sea grasses, seaweeds, mushrooms, and plankton in the estuary. However, they do prey on consumers in the second trophic level, which rely on these plants and other photosynthetic organisms for food: insects, mollusks, birds, and fruit bats. Estuarine crocodiles also prey on consumers at the third trophic level, such as boars and snakes (and, rarely, people).

Estuarine crocodiles are ideally adapted to the brackish water of river estuaries. They can survive equally well in freshwater and saltwater ecosystems. During the rainy season, estuarine crocodiles live in freshwater rivers and streams. They feed on fish such as barramundi, and terrestrial species such as kangaroos and monkeys. During the dry season, estuarine crocodiles swim to river mouths and even out to sea. Fish remain the main component of their diet. Some estuarine crocodiles have even been known to attack and consume sharks.

Estuarine crocodiles have also adapted to seasonally vanishing estuaries. The reptiles can go months without eating. Estuarine crocodiles can simply not eat when the estuary shrinks and food becomes scarce.

Estuaries and People

Estuaries are excellent sites for community living. They provide freshwater for drinking and hygiene. Access to both rivers and oceans helps the development of trade and communication.

In fact, the earliest civilizations in the world developed around estuaries. Ur, in what is now Iraq, developed around 3800 BCE near the estuary of the Euphrates River where it met the Persian Gulf.

Ur was a sophisticated urban area, with a population of more than 60,000 at its height. Its estuary was the most important port on the Persian Gulf. All ships carrying trade goods from places such as India and the Arabian Peninsula had to pass through Ur. The estuary's wetlands and flood plains provided a rich source of wild game and allowed for the development of irrigation and agriculture.

Today, Ur is an archaeological site well inland from the Persian Gulf coast. The landscape has changed, and the estuary of the Euphrates is more than 300 kilometers (186 miles) away.

Many modern cities have grown around estuaries, including Jakarta, Indonesia, New York City, New York; and Tokyo, Japan. These urban areas have undergone rapid change, and put their estuaries at environmental risk through land reclamation, pollution, and overfishing.

Land Reclamation
Communities have filled in the edges of estuaries for housing and industry since the times of Ur. This process is called land reclamation.

Jakarta's 10 million residents have one of the highest population densities in the world. To create more space for homes and businesses, Indonesian officials have dredged the Ciliwung River and Java Bay. The sand and silt dredged from the river bottom and seafloor fortify the city's beaches and create new land.

Land reclamation comes at a price, however. Jakarta's fisheries are disrupted by the dredging. This reduces the potential profits for restaurants and markets, as well as fishers.

Destroying the estuary also creates the conditions for flooding. Estuaries provide a natural barrier to ocean waves, which can erode the shoreline and destroy coastal homes and businesses. Jakarta is particularly at risk for tsunami damage, as the area experiences frequent earthquakes.

Pollution
Pollution accumulates in estuaries. The Hudson-Raritan Estuary, where the Hudson and Raritan rivers meet the Atlantic Ocean, is one of the most-trafficked and most-polluted estuaries in the world.

Pollution from ships routinely spills into the Hudson-Raritan Estuary, just south of New York City. Debris in the estuary, including fuel, garbage, sewage, and ballast, remained unregulated for decades.

Runoff from agriculture and industry in New York and New Jersey also contributed a toxic estuarine environment. Industrial waste and pesticides travel downstream and settle in the water and sediment of the estuary.

Today, strict regulations and community activities are working to protect and restore the Hudson-Raritan Estuary. The restoration of oyster beds is an important part of many projects.

Oysters are a keystone species in the estuary, filter feeders that naturally help regulate toxins in the water. Millions of oyster beds greeted Henry Hudson when he entered the river in 1609. By the middle of the 20th century, however, the few remaining oysters were too toxic for human consumption. Today, several environmental groups are establishing oyster beds to repopulate the region's native species and reduce pollution in the estuary.

Overfishing
Many estuaries have been overfished. Pacific bluefin tuna are not endangered, but their range has been drastically reduced. Japan provides one of the largest markets for bluefin tuna, and the fish used to swim in the estuary of Tokyo Bay.

Bluefin tuna are large, predatory fish. They require an expansive habitat and many kilograms of food every day. As Tokyo's population grew and technology made it easier to catch more fish with less time and money, Tokyo Bay's bluefin tuna population shrank.

Today, there is not a bluefin tuna population in Tokyo Bay. However, Japanese scientists have established a successful tuna fish farming technique. Farm-raised tuna does not have a direct environmental impact on the Tokyo Bay estuary.

Indonesian, American, and Japanese governments and environmental groups struggle to promote sustainable development in estuaries. Sustainable development aims to preserve the environment while satisfying people's economic standard of living.

estuary
Shore+river+saltwater =ESTUARY

Edo
Tokyo, the most populous city in the world, was originally known as Edo, which means "estuary." Tokyo Bay is an estuary formed where the Sumida and Arakawa rivers flow into the Pacific Ocean.

Largest Estuary in the World
Because the definition of "estuary" is fluid, determining which one is the world's largest is an ongoing debate. However, many scientists say that that St. Lawrence River, which connects the Great Lakes to the Atlantic Ocean, is the world's largest estuary. The St. Lawrence River is about 1,197 kilometers (744 miles) long.

Between Land
Some Native Americans called estuaries the "Between-Land" because they are not quite land and not quite water.

accumulate
Verb

to gather or collect.

Noun

the art and science of cultivating the land for growing crops (farming) or raising livestock (ranching).

anti-estuarine current
Noun

warm, surface current flowing from a river or stream into an estuary.

apex predator
Noun

species at the top of the food chain, with no predators of its own. Also called an alpha predator or top predator.

archaeological site
Noun

place where evidence of the past is being studied by scientists.

ballast
Noun

heavy material, usually water, used to provide stability for large ships or other oceangoing vessels.

bar-built estuary
Noun

place where a river mouth is at least partly protected from the ocean by sandbars or barrier islands.

barrier island
Noun

long, narrow strip of sandy land built up by waves and tides that protects the mainland shore from erosion.

brackish water
Noun

salty water, usually a mixture of seawater and freshwater.

circulate
Verb

to move around, often in a pattern.

Noun

complex way of life that developed as humans began to develop urban settlements.

coastal plain estuary
Noun

place where the ocean rushed in to flood a low-lying river valley.

Noun

rocky outermost layer of Earth or other planet.

debris
Noun

remains of something broken or destroyed; waste, or garbage.

dense
Adjective

having parts or molecules that are packed closely together.

detritus
Noun

non-living organic material, often decomposing.

Noun

foods eaten by a specific group of people or other organisms.

downwarping
Noun

process of an area of land sinking.

dredge
Verb

to remove sand, silt, or other material from the bottom of a body of water.

earthquake
Noun

the sudden shaking of Earth's crust caused by the release of energy along fault lines or from volcanic activity.

Noun

community and interactions of living and nonliving things in an area.

engineer
Noun

person who plans the building of things, such as structures (construction engineer) or substances (chemical engineer).

erode
Verb

to wear away.

estuarine current
Noun

cold, dense bottom current flowing from the ocean into an estuary.

Noun

mouth of a river where the river's current meets the sea's tide.

fault
Noun

a crack in the Earth's crust where there has been movement.

filter feeder
Noun

aquatic animal that strains nutrients from water.

fish farming
Noun

art and science of raising and harvesting fish and other seafood, such as shrimp or crabs.

fjord estuary
Noun

place where a river or freshwater stream flows into a deep, steep gorge carved by a glacier and filled with seawater.

Noun

flat area alongside a stream or river that is subject to flooding.

Noun

all related food chains in an ecosystem. Also called a food cycle.

freshwater estuary
Noun

place where a river mouth flows into a large freshwater lake.

game
Noun

wild animals hunted for food.

Noun

mass of ice that moves slowly over land.

Noun

environment where an organism lives throughout the year or for shorter periods of time.

hurricane
Noun

tropical storm with wind speeds of at least 119 kilometers (74 miles) per hour. Hurricanes are the same thing as typhoons, but usually located in the Atlantic Ocean region.

hygiene
Noun

science and methods of keeping clean and healthy.

ice age
Noun

long period of cold climate where glaciers cover large parts of the Earth. The last ice age peaked about 20,000 years ago. Also called glacial age.

industry
Noun

activity that produces goods and services.

Noun

watering land, usually for agriculture, by artificial means.

Noun

organism that has a major influence on the way its ecosystem works.

lake turnover
Noun

process of the dense lower layer of a lake rising to become the upper, less-dense layer.

land reclamation
Noun

process of creating new land for housing or industry by draining parts of rivers, lakes, or the ocean.

Noun

place where a river empties its water. Usually rivers enter another body of water at their mouths.

overfish
Verb

to harvest aquatic life to the point where species become rare in the area.

pesticide
Noun

natural or manufactured substance used to kill organisms that threaten agriculture or are undesirable. Pesticides can be fungicides (which kill harmful fungi), insecticides (which kill harmful insects), herbicides (which kill harmful plants), or rodenticides (which kill harmful rodents.)

Noun

introduction of harmful materials into the environment.

Noun

place on a body of water where ships can tie up or dock and load and unload cargo.

post-glacial rebound
Noun

process in which land that was crushed by a glacier regains its shape.

prey
Noun

animal that is hunted and eaten by other animals.

producer
Noun

organism on the food chain that can produce its own energy and nutrients. Also called an autotroph.

Noun

a ridge of rocks, coral, or sand rising from the ocean floor all the way to or near the ocean's surface.

ria
Noun

low, wetland area near the mouth of a river. Rias are often called "drowned river valleys."

Noun

overflow of fluid from a farm or industrial factory.

salinity
Noun

saltiness.

Scandinavia
Noun

region and name for some countries in Northern Europe: Iceland, Norway, Sweden, Finland, and Denmark.

Noun

solid material transported and deposited by water, ice, and wind.

seiche
Noun

standing wave in an enclosed body of water.

Noun

small sediment particles.

slough
Noun

marshy wetland largely defined by thick mud.

sound
Noun

body of water, larger than a bay, partially surrounded by land.

specimen
Noun

individual organism that is a typical example of its classification.

standard of living
Noun

amount of goods and services a person in a specific community or geographic area is able to afford.

standing wave
Noun

type of wave that does not move or lose strength.

Noun

abnormal rise in sea level accompanying a hurricane or other intense storm. Also called a storm tide.

Noun

narrow passage of water that connects two larger bodies of water.

sustainable development
Noun

human construction, growth, and consumption that can be maintained with minimal damage to the natural environment.

tectonic activity
Noun

movement of tectonic plates resulting in geologic activity such as volcanic eruptions and earthquakes.

tectonic estuary
Noun

place where tectonic activity causes downwarping, and the ocean rushes in to fill the downwarped land.

terrestrial
Adjective

having to do with the Earth or dry land.

toxic
Adjective

poisonous.

trophic level
Noun

one of three positions on the food chain: autotrophs (first), herbivores (second), and carnivores and omnivores (third).

tsunami
Noun

ocean waves triggered by an earthquake, volcano, or other movement of the ocean floor.

Noun

developed, densely populated area where most inhabitants have nonagricultural jobs.

vanish
Verb

to disappear.

Noun

area of land covered by shallow water or saturated by water.