• 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 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.

    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.

    Shore+river+saltwater =ESTUARY

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

    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.

    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.

  • Term Part of Speech Definition Encyclopedic Entry
    accumulate Verb

    to gather or collect.

    agriculture Noun

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

    Encyclopedic Entry: agriculture
    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.

    civilization Noun

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

    Encyclopedic Entry: civilization
    coastal plain estuary Noun

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

    crust Noun

    rocky outermost layer of Earth or other planet.

    Encyclopedic Entry: crust
    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.

    diet Noun

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

    Encyclopedic Entry: diet
    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.

    ecosystem Noun

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

    Encyclopedic Entry: ecosystem
    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.

    estuary Noun

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

    Encyclopedic Entry: estuary
    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.

    flood plain Noun

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

    Encyclopedic Entry: flood plain
    food web Noun

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

    Encyclopedic Entry: food web
    freshwater estuary Noun

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

    game Noun

    wild animals hunted for food.

    glacier Noun

    mass of ice that moves slowly over land.

    Encyclopedic Entry: glacier
    habitat Noun

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

    Encyclopedic Entry: habitat
    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.

    irrigation Noun

    watering land, usually for agriculture, by artificial means.

    Encyclopedic Entry: irrigation
    keystone species Noun

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

    Encyclopedic Entry: keystone species
    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.

    mouth Noun

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

    Encyclopedic Entry: mouth
    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.)

    pollution Noun

    introduction of harmful materials into the environment.

    Encyclopedic Entry: pollution
    port Noun

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

    Encyclopedic Entry: port
    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.

    reef Noun

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

    Encyclopedic Entry: reef
    ria Noun

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

    runoff Noun

    overflow of fluid from a farm or industrial factory.

    Encyclopedic Entry: runoff
    salinity Noun


    Scandinavia Noun

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

    sediment Noun

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

    Encyclopedic Entry: sediment
    seiche Noun

    standing wave in an enclosed body of water.

    silt Noun

    small sediment particles.

    Encyclopedic Entry: silt
    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.

    storm surge Noun

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

    Encyclopedic Entry: storm surge
    strait Noun

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

    Encyclopedic Entry: strait
    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


    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.

    urban area Noun

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

    Encyclopedic Entry: urban area
    vanish Verb

    to disappear.

    wetland Noun

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

    Encyclopedic Entry: wetland