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 mainstream 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 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.
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 wort, 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. This river 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.
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) and Senegal delta (shaped by waves from the Atlantic Ocean) are both 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-Brahmaputra 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.
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.
Stronger waves form a cuspate delta, which is more pointed than the arcuate delta, and is tooth-shaped. 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.
Another untraditional looking delta is the inverted 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 lobe 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, at least, the past 5,000 years.
Deltas and People
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 an 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. 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.
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.
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. According to Drawing Louisiana’s New Map 62 square kilometers (24 square miles) of wetland was lost each year between 1990 and 2000—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.
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 is actually a flood plain between two rivers in northwestern Mississippi, the Mississippi and the Yazoo, it is sometimes to as the Yazoo-Mississippi Delta.
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.
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.
to soak up.
a buildup of something.
to oversee, manage, or be in charge of.
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.
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.
shaped like an arc or bow, or part of a circle.
natural process involving the abandonment of one river channel and the formation of a new one.
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.
deepest part of a shallow body of water, often a passageway for ships.
to identify or arrange by specific type or characteristic.
rough or composed of large, jagged particles.
people and land separated by distance or culture from the government that controls them.
trade, or the exchange of goods and services.
sharing of information and ideas.
maintaining a steady, reliable quality.
familiar or comfortable all over the world, or to people from all over the world.
Encyclopedic Entry: crop crustacean Noun
type of animal (an arthropod) with a hard shell and segmented body that usually lives in the water.
steady, predictable flow of fluid within a larger body of that fluid.
Encyclopedic Entry: current cuspate Adjective
pointed or tapering to a sharp end.
structure built across a river or other waterway to control the flow of water.
to argue or disagree in a formal setting.
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.
area of land that receives no more than 25 centimeters (10 inches) of precipitation a year.
Encyclopedic Entry: desert devastate Verb
construction or preparation of land for housing, industry, or agriculture.
stream that branches off from the main stem of a river or other flowing fluid.
varied or having many different types.
in the direction of a flow, toward its end.
having to do with money.
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).
conditions that surround and influence an organism or community.
to wear away.
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.
good or service traded to another area.
the art, science, and business of cultivating the land for growing crops.
able to produce crops or sustain agriculture.
nutrient-rich chemical substance (natural or manmade) applied to soil to encourage plant growth.
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.
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
water that is not salty.
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.
system or order of a nation, state, or other political unit.
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.
person who moves to a new country or region.
to bring in a good or service from another area for trade.
characteristic to or of a specific place.
Encyclopedic Entry: indigenous 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.
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.
body of water surrounded by land.
Encyclopedic Entry: lake 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.
largest river or channel in a watershed or drainage basin.
central place for the sale of goods.
very large or heavy.
tiny organism, usually a bacterium.
to observe and record behavior or data.
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.
newest, most aquatic-facing portion of a delta, featuring the finest sediment.
completely or extremely.
something that is left over.
river management Noun
the art and science of controlling the flow, path, and power of rivers.
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.
small sediment particles.
Encyclopedic Entry: silt soil Noun
top layer of the Earth's surface where plants can grow.
to stop flowing.
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.
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.
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.
having to do with city life.
measurement of the rate and direction of change in the position of an object.
moving swell on the surface of water.
area of land covered by shallow water or saturated by water.
Encyclopedic Entry: wetland