• delta
    The triangle-shaped Nile Delta is a perfect example of an arcuate delta.

    Photograph courtesy Jacques Descloitres, MODIS Rapid Response Team, NASA/GSFC

    Delta Blues
    Delta blues is a style of music developed by African-American artists living and performing in the Mississippi Delta region of the southern United States. The Mississippi Delta actually has nothing do do with deltas at all! The Mississippi Delta is a flood plain between two rivers in northwestern Mississippi, the Mississippi and the Yazoo. The Mississippi Delta is more than 480 kilometers (300 miles) north of the delta of the Mississippi River in Lousiana.

    Slide guitar is one of the standard instruments used by delta blues musicians, while familiar topics include poverty and injustice. Robert Johnson, widely recognized as one of the greatest guitarists of all time, played the Delta blues. Listen to Robert Johnson here.

    Deltas are wetlands that form as rivers empty their water and sediment into another body of water, such as an ocean, lake, or another river. Deltas can also empty into land, although this is less common. 
     
    A river moves more slowly as it nears its mouth, or end. This causes sediment, solid material carried downstream by currents, to fall to the river bottom. 
     
    The slowing velocity of the river and the build-up of sediment allows the river to break from its single channel as it nears its mouth. Under the right conditions, a river forms a deltaic lobe. A mature deltaic lobe includes a distributary network—a series of smaller, shallower channels, called distributaries, that branch off from the mainstem of the river.
     
    In a deltaic lobe, heavier, coarser material settles first. Smaller, finer sediment is carried farther downstream. The finest material is deposited beyond the river's mouth. This material is called alluvium or silt. Silt is rich in nutrients that help microbes and plants—the producers in the food web—grow. 
     
    As silt builds up, new land is formed. This is the delta. A delta extends a river's mouth into the body of of water into which it is emptying. 
     
    A delta is sometimes divided into two parts: subaqueous and subaerial. The subaqueous part of a delta is underwater. This is the most steeply sloping part of the delta, and contains the finest silt. The newest part of the subaqueous delta, furthest from the mouth of the river, is called the prodelta.
     
    The subaerial part of a delta is above water. The subaerial region most influenced by waves and tides is called the lower delta. The region most influenced by the river's flow is called the upper delta.
     
    This nutrient-rich wetland of the upper and lower delta can be an extension of the river bank, or a series of narrow islands between the river's distributary network. 
     
    Like most wetlands, deltas are incredibly diverse and ecologically important ecosystems. Deltas absorb runoff from both floods (from rivers) and storms (from lakes or the ocean). Deltas also filter water as it slowly makes its way through the delta's distributary network. This can reduce the impact of pollution flowing from upstream. 
     
    Deltas are also important wetland habitats. Plants such as lilies and hibiscus grow in deltas, as well as herbs such as worts, which are used in traditional medicines. 
     
    Many, many animals are indigenous to the shallow, shifting waters of a delta. Fish, crustaceans such as oysters, birds, insects, and even apex predators such as tigers and bears can be part of a delta's ecosystem. 
     
    Not all rivers form deltas. For a delta to form, the flow of a river must be slow and steady enough for silt to be deposited and build up. The Ok Tedi, in Papua New Guinea is one of the fastest-flowing rivers in the world. It does not form a delta as it becomes a tributary of the Fly River. (The Fly, on the other hand, does form a rich delta as it empties into the Gulf of Papua, part of the Pacific Ocean.)
     
    A river will also not form a delta if exposed to powerful waves. The Columbia River in Canada and the United States, for instance, deposits enormous amounts of sediment into the Pacific Ocean, but strong waves and currents sweep the material away as soon as it is deposited. 
     
    Tides also limit where deltas can form. The Amazon, the largest river in the world, is without a delta. The tides of the Atlantic Ocean are too strong to allow silt to create a delta on the Amazon. 
     
    Types of Deltas
     
    There are two major ways of classifying deltas. One considers the influences that create the landform, while the other considers its shape.
     
    Influence
    There are four main types of deltas classified by the processes that control the build-up of silt: wave-dominated, tide-dominated, Gilbert deltas, and estuarine deltas.
     
    In a wave-dominated delta, the movement of waves controls a delta's size and shape. The Nile delta (shaped by waves from the Mediterranean Sea), Mississippi delta (shaped by waves from the Gulf of Mexico), and Senegal delta (shaped by waves from the Atlantic Ocean) are all wave-dominated deltas.
     
    Tide-dominated deltas usually form in areas with a large tidal range, or area between high tide and low tide. The massive Ganges-Bramahputra delta, in India and Bangladesh, is a tide-dominated delta, shaped by the rise and fall of tides in the Bay of Bengal.
     
    Gilbert deltas are formed as rivers deposit large, coarse sediments. Gilbert deltas are usually confined to rivers emptying into freshwater lakes. They are usually steeper than the normal flat plain of a wave-dominated or tide-dominated delta. This type of delta was first identified by the geologist Grove Karl Gilbert, who described mountain streams feeding ancient Lake Bonneville. (Utah's Great Salt Lake is the only remnant of Lake Bonneville.)
     
    Estuarine deltas form as a river does not empty directly into the ocean, but instead forms an estuary. An estuary is a partly enclosed wetland that features a brackish water (part-saltwater, part-freshwater) habitat. The Yellow River forms an estuary, for instance, as it reaches the Bohai Sea off the coast of northern China.
     
    Shape
    The term delta comes from the upper-case Greek letter delta (Δ), which is shaped like a triangle. Deltas with this triangular or fan shape are called arcuate (arc-like) deltas. The Nile River forms an arcuate delta as it empties into the Mediterranean Sea.
     
    Strong waves make a cuspate delta a pointed, tooth-shaped version of the arcuate. The Tiber River forms a cuspate delta as it empties into the Tyrrhenian Sea near Rome, Italy.
     
    Not all deltas are triangle-shaped. A bird-foot delta has few, widely spaced distributaries, making it look like a bird's foot. The Mississippi River forms a bird-foot delta as it empties into the Gulf of Mexico.
     
    Inverted deltas look like the opposite of a classic arcuate delta. The distributary network of an inverted delta is inland, while a single stream reaches the ocean or other body of water. The delta of the Sacramento-San Joaquin River in northern California is an inverted delta. The rivers and creeks of the Sacramento and San Joaquin distributary networks meet in Suisun Bay, before flowing to the Pacific Ocean through a single gap in the Coast Range, the Carquinez Strait.
     
    Inland deltas, which empty into a plain, are extremely rare. The Okavango delta in Botswana is probably the most well-known—and so unusual it is recognized as one of the "Seven Natural Wonders of Africa." Water from the Okavango River never reaches another body of water. The delta spreads water and silt across a flat plain in the Kalahari Desert before being evaporated.
     
    An abandoned delta forms as a river develops a new channel, leaving the other to dry up or stagnate. This process is called avulsion. Avulsion occurs when the slope of a channel decreases and the sediment build-up increases. These forces allow the channel to overflow its banks or levees and find a steeper, more direct route to the ocean or other body of water. The process of avulsion in deltaic lobes is called delta switching. Over time, delta switching can create entirely new deltaic lobes. Delta switching has resulted in seven or eight distinct deltaic lobes of the Mississippi River over the past 5,000 years.
     
    Deltas and People
     
    Deltas are incredibly important to the human geography of a region. They are important places for trade and commerce, for instance.
     
    The booming city of Vancouver, British Columbia, Canada, sits on the delta of the Fraser River as it empties into the Strait of Georgia, part of the Pacific Ocean. The Fraser delta helps make Vancouver one of the busiest, most cosmopolitan ports in the world, where goods from the interior of Canada are exported, and goods from around the world are imported.
     
    The Pearl River Delta, sometimes called the Delta of Guangdong, is another heavily urbanized river delta. The Pearl River delta is one of the fastest-growing centers of China's economy. The Pearl River delta includes both of China's two special administrative regions, the former British colony of Hong Kong and the former Portuguese colony of Macau. Hong Kong and Macau are welcoming to western business, and provide a entryway to the Chinese market. The Pearl River delta region is growing so quickly, it frequently experiences labor shortages as immigrants from the Chinese interior settle in the area, seeking a better life and higher wages.
     
    Deltas have a rich accumulation of silt, so they are usually fertile agricultural areas. The world's largest delta is the Ganges–Brahmaputra delta in India and Bangladesh, which empties into the Bay of Bengal. Bangladesh sits almost entirely on this delta, and about two-thirds of the population live and work there. Fish, other seafood, and crops such as rice and tea are the leading agricultural products of the delta.
     
    Similarly, the inverted delta of the Sacramento and San Joaquin Rivers in northern California is one of the most agriculturally rich areas in the U.S. The soil supports crops from asparagus to zucchini, wine grapes to rice.
     
    Disappearing Deltas
     
    Extensive river management threatens deltas. River management involves monitoring and administering a river's flow (often through the use of dams). River management increases the amount of land available for agricultural or industrial development, and controls access to water for drinking, industry, and irrigation
     
    Engineers and government officials must consistently debate the interests of agriculture, industry, the environment, and citizen safety and health when putting delta wetlands at risk. 
     
    River management in Egypt has radically altered the way land is farmed around the Nile delta, for instance. Construction of the Aswan Dam in the 1960s reduced annual flooding of the delta. This flooding had distributed silt and nutrients along the banks of the Nile. Today, Egypt is much more reliant on fertilizers and irrigation. The Nile delta is also shrinking as a result of the Aswan Dam and other river management techniques. Without silt and other sediments to fortify it in a prodelta, the waves of the Mediterranean Sea are eroding the delta faster than the Nile can replace it.
     
    In the United States, dams on the Colorado River nearly prevent it from reaching its delta on the Sea of Cortez, Mexico. The ecosystem (what was once the world's largest desert estuary) has been reduced to a fraction of its former area, and many indigenous species are vulnerable, threatened, or endangered.
     
    Finally, decades of river management prevent the Mississippi River from naturally flowing through its delta wetlands. Like the Nile delta, the Mississippi delta is also eroding. Between 1990 and 2000, the Mississippi River Delta lost 62 square kilometers (24 square miles) of wetlands per year—that's about one football field of mud washed into the Gulf of Mexico every 38 minutes. This situation contributed to the devastation caused by Hurricane Katrina in 2005.
  • Term Part of Speech Definition Encyclopedic Entry
    abandoned delta Noun

    landform created as a river develops a new channel, leaving the other to dry up or stagnate.

    absorb Verb

    to soak up.

    accumulation Noun

    a buildup of something.

    administer Verb

    to oversee, manage, or be in charge of.

    agriculture Noun

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

    Encyclopedic Entry: agriculture
    alluvium Noun

    gravel, sand, and smaller materials deposited by flowing water.

    alter Verb

    to change.

    ancient Adjective

    very old.

    annual Adjective

    yearly.

    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.

    arcuate Adjective

    shaped like an arc or bow, or part of a circle.

    avulsion Noun

    natural process involving the abandonment of one river channel and the formation of a new one.

    bank Noun

    a slope of land adjoining a body of water, or a large elevated area of the sea floor.

    bird-foot delta Noun

    area where a river flows into a larger body of water through long, isolated channels that branch outward like a bird's foot. Also called a birdfoot delta or bird's-foot delta.

    brackish water Noun

    salty water, usually a mixture of seawater and freshwater.

    channel Noun

    deepest part of a shallow body of water, often a passageway for ships.

    classify Verb

    to identify or arrange by specific type or characteristic.

    coarse Adjective

    rough or composed of large, jagged particles.

    colony Noun

    people and land separated by distance or culture from the government that controls them.

    commerce Noun

    trade, or the exchange of goods and services.

    communication Noun

    sharing of information and ideas.

    consistent Adjective

    maintaining a steady, reliable quality.

    cosmopolitan Adjective

    familiar or comfortable all over the world, or to people from all over the world.

    crop Noun

    agricultural produce.

    Encyclopedic Entry: crop
    crustacean Noun

    type of animal (an arthropod) with a hard shell and segmented body that usually lives in the water.

    current Noun

    steady, predictable flow of fluid within a larger body of that fluid.

    Encyclopedic Entry: current
    cuspate Adjective

    pointed or tapering to a sharp end.

    dam Noun

    structure built across a river or other waterway to control the flow of water.

    debate Verb

    to argue or disagree in a formal setting.

    delta Noun

    the flat, low-lying plain that sometimes forms at the mouth of a river from deposits of sediments.

    Encyclopedic Entry: delta
    deltaic lobe Noun

    landform created as a river deposits sediment into the body of water as it empties.

    delta switching Noun

    process of a delta distributary abandoning one channel and carving out another.

    desert Noun

    area of land that receives no more than 25 centimeters (10 inches) of precipitation a year.

    Encyclopedic Entry: desert
    devastate Verb

    to destroy.

    development Noun

    construction or preparation of land for housing, industry, or agriculture.

    distributary Noun

    stream that branches off from the main stem of a river or other flowing fluid.

    diverse Adjective

    varied or having many different types.

    downstream Noun

    in the direction of a flow, toward its end.

    economic Adjective

    having to do with money.

    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).

    environment Noun

    conditions that surround and influence an organism or community.

    erode Verb

    to wear away.

    estuary Noun

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

    Encyclopedic Entry: estuary
    evaporate Verb

    to change from a liquid to a gas or vapor.

    export Noun

    good or service traded to another area.

    farming Noun

    the art, science, and business of cultivating the land for growing crops.

    fertile Adjective

    able to produce crops or sustain agriculture.

    fertilizer Noun

    nutrient-rich chemical substance (natural or manmade) applied to soil to encourage plant growth.

    filter Verb

    to remove particles from a substance by passing the substance through a screen or other material that catches larger particles and lets the rest of the substance pass through.

    fine Adjective

    very thin.

    flood Noun

    overflow of a body of water onto land.

    Encyclopedic Entry: flood
    food web Noun

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

    Encyclopedic Entry: food web
    fortify Verb

    to strengthen.

    frequent Adjective

    often.

    freshwater Noun

    water that is not salty.

    gap Noun

    steep-sided opening through a mountain ridge.

    Encyclopedic Entry: gap
    geologist Noun

    person who studies the physical formations of the Earth.

    Gilbert delta Noun

    landform created by the depositon of coarse sediments, as opposed to the fine sediments of regular deltas.

    government Noun

    system or order of a nation, state, or other political unit.

    habitat Noun

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

    Encyclopedic Entry: habitat
    herb Noun

    type of seasonal plant often used as a medicine or seasoning.

    high tide Noun

    water level that has risen as a result of the moon's gravitational pull on the Earth.

    human geography Noun

    the study of the way human communities and systems interact with their environment.

    Hurricane Katrina Noun

    2005 storm that was one of the deadliest in U.S. history.

    immigrant Noun

    person who moves to a new country or region.

    import Verb

    to bring in a good or service from another area for trade.

    indigenous Adjective

    native to or characteristic of a specific place.

    industry Noun

    activity that produces goods and services.

    inverted delta Noun

    landform noted for its opposite-seeming appearance: the wide end of the delta is inland, while the narrow end empties into a body of water. 

    irrigation Noun

    watering land, usually for agriculture, by artificial means.

    Encyclopedic Entry: irrigation
    island Noun

    body of land surrounded by water.

    Encyclopedic Entry: island
    labor Noun

    work or employment.

    lake Noun

    body of water surrounded by land.

    landform Noun

    specific natural feature on the Earth's surface.

    Encyclopedic Entry: landform
    levee Noun

    bank of a river, raised either naturally or constructed by people.

    Encyclopedic Entry: levee
    lower delta Noun

    portion of a delta defined by a region's tidal range.

    low tide Noun

    water level that has dropped as a result of the moon's gravitational pull on the Earth.

    mainstem Noun

    largest river or channel in a watershed or drainage basin.

    market Noun

    central place for the sale of goods.

    massive Adjective

    very large or heavy.

    microbe Noun

    tiny organism, usually a bacterium.

    monitor Verb

    to observe and record behavior or data.

    mouth Noun

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

    Encyclopedic Entry: mouth
    nutrient Noun

    substance an organism needs for energy, growth, and life.

    Encyclopedic Entry: nutrient
    plain Noun

    flat, smooth area at a low elevation.

    Encyclopedic Entry: plain
    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
    prevent Verb

    to keep something from happening.

    prodelta Noun

    newest, most aquatic-facing portion of a delta, featuring the finest sediment.

    radically Adverb

    completely or extremely.

    remnant Noun

    something that is left over.

    river management Noun

    the art and science of controlling the flow, path, and power of rivers.

    runoff Noun

    overflow of fluid from a farm or industrial factory.

    Encyclopedic Entry: runoff
    sediment Noun

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

    Encyclopedic Entry: sediment
    shipping Noun

    transportation of goods, usually by large boat.

    silt Noun

    small sediment particles.

    Encyclopedic Entry: silt
    soil Noun

    top layer of the Earth's surface where plants can grow.

    stagnate Verb

    to stop flowing.

    storm Noun

    severe weather indicating a disturbed state of the atmosphere resulting from uplifted air.

    tidal range Noun

    the difference in height between an area's high tide and low tide.

    tide Noun

    rise and fall of the ocean's waters, caused by the gravitational pull of the moon and sun.

    Encyclopedic Entry: tide
    trade Noun

    buying, selling, or exchanging of goods and services.

    tributary Noun

    stream that feeds, or flows, into a larger stream.

    Encyclopedic Entry: tributary
    upper delta Noun

    portion of a delta roughly defined by deposits from a river.

    urban Adjective

    having to do with city life.

    velocity Noun

    measurement of the rate and direction of change in the position of an object.

    wave Noun

    moving swell on the surface of water.

    wetland Noun

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

    Encyclopedic Entry: wetland
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