Deltas

Deltas are deposits of sediments (particles of sand, gravel, and silt) at the mouths of rivers that flow into the ocean. The mouth of a river is the end where the body of water flows into the sea. Deltas are shaped by interactions of the river's fresh water with the ocean water, tides, and waves. Throughout history, deltas have been important places for human settlement. They are also vital habitats for many animals and plants. In the fifth century b.c.e, Greek naturalist Herodotus coined the name delta to describe the triangular shape of the sediments deposited at the end of the Nile River. The capital Greek letter delta (Δ) resembles a triangle. Most deltas are triangular in shape because rivers deposit larger amounts of sediment where they meet the sea, then fan out into the mouth of the sea to deposit the remaining sediment.

Formation of deltas

As rivers flow through their beds (a channel occupied by a river), the water breaks up rocks and pebbles, which the river then carries with it as it flows. These pieces of rock, sediment, include sand, pebbles, and silt. When the fast-flowing waters of the river reach the ocean, they push against the ocean water. This decreases the speed of the river water, which spreads out along the coastline. As the speed of the river water slows, the sediments that it carries settle to the bottom of the ocean. Over time, the sediments accumulate and form a delta.

In most deltas, the river water is less dense (packed together) than the sea water because it contains less salt. As it flows out into the ocean, it floats on top of the ocean water. This is calledhypopycnal flow. (The prefix hypo means "under" or "less" and the root word pycn means "density.") In places where hypopycnal flow occurs, the salt water that lies under the fresh water is called a salt wedge. The bigger sediments, like gravel and pebbles, are deposited at the tip of the salt wedge (nearest to shore) and the smaller sediments, like sand, are deposited farther out along the salt wedge. The smallest silt grains (fine sediment particles) are transported far offshore with the river water.

The structure of a delta

Paths of flowing water called channels run through the sediments of deltas. These channels are called distributaries and they may be large and relatively permanent or small, transient features. The sides of the channels are made up of piles of sediments called levées. The areas between the distributaries are called interdistributary areas.

Types of deltas

The structure of the distributaries and interdistributary areas depends on where the river empties into the ocean and the forces that affect the river water as it flows into the ocean. There are three major types of deltas: river-dominated, tide-dominated, and wave-dominated. Many deltas are formed by a combination of these forces.

Life in the Ganges Delta

The Ganges and the Brahmaputra Rivers combine as they flow into the Bay of Bengal in the Indian Ocean at Bangladesh. They carry enormous amounts of sediment and the delta that results is the largest in the world. It is estimated that the total amount of sediments entering the delta is one billion tons per year. The area of the delta is about 3,500 square miles (9,000 square kilometers), about the same size as Yellowstone National Park. The source of all this sediment is erosion from the Himalayas and the Tibetian Plateau. The delta is fed by an impressive set of distributaries, but the tides are the major controlling force. Tides may fluctuate by about 7 feet (2 meters) each day.

The area of the delta is called the Sunderbans and it is biologically diverse both in animal and plant life. Many varieties of fish and shellfish are found in the region as well as wild boar, crocodile, sea turtles, deer, and monkeys. A variety of birds migrate through the many islands of the Sunderbans, including storks, heron, egrets, and cormorants. The largest population of Royal Bengal tigers lives in the Sunderbans, where they number about 400. These animals have adapted to the environment, which is often completely flooded, by becoming powerful swimmers. The Sunderbans is home to one of the largest mangrove forests in the world. Mangrove trees are able to live in salt water and their roots are home to many juvenile species of fish and invertebrates.

Many people live in the Sunderbans. Nomadic fishermen train otters to help them catch fish. Woodcutters live in houses built as high as 10 feet (3 meters) above the ground to stay out of the paths of tigers and crocodiles. Honey collectors search for honey during the months of April and May.

Both monsoons (seasonal heavy rains) and cyclones (hurricanes), affect this delta. The people who live in the Sunderbans are impoverished, and even though flood and cyclone warnings are issued, each year thousands of people lose their lives as waters rise. In 1970, a terrible tragedy occurred in the Ganges-Brahmaputra Delta when nearly 500,000 people were killed by the flooding that followed a cyclone. Again in 1991, another flood killed 100,000 people who lived on low-lying islands of the delta.

River-dominated deltas extend outward from the coast as the river water jets out into the ocean. The sediments deposited by the river tend to form levées that hold channels of water. The aerial view of a river-dominated delta looks like the foot of a bird with several branching channels. River-dominated deltas often have sand bars (long deposits of sand) that are perpendicular to the river. The Mississippi River Delta in Louisiana is an example of a river-dominated delta.

Tide-dominated deltas are deltas where the sediments deposited by a river are redistributed by tides. These deltas have channels cut by the river water as well as channels cut by tidal currents. The result is a shoreline that looks like the fringe on the end of a carpet. Tide-dominated deltas may also have coastal features like sand bars and shoals (a sandbank seen at low tide) oriented parallel to the tidal flow. The Ganges-Brahmaputra Delta in the Bay of Bengal is an example of a tide-dominated delta.

Wave-dominated deltas occur in places where the sediments deposited by the river water are redistributed by waves. Because waves tend to move sediments along the shoreline, the shape of wave-dominated deltas is usually a smooth coastline. If the waves that affect the delta break parallel to the shoreline, the sediments of the delta will tend to be symmetrical on either side of the river. If the waves break at an angle to the shore, the sediments of the delta will tend to accumulate on one side of the delta. The Nile River Delta in Egypt is an example of a delta that is wave-dominated.

Humans and deltas

Deltas play an important role in human life both historically and in present times. The land on deltas is typically very good for agriculture. On the parts of the delta close to the river, the soil is fertilized each year by nutrient-containing floodwaters when the river floods. In addition, by building aqueducts (canals or pipelines used to transport water) from the wetter lands near the river to the dryer lands farther from the water source, it is easy to expand the amount of land available for farming. This practice is called land reclamation. In particular, the fertile Nile delta has supported much of the agriculture in Egypt for thousands of years.

Trade is another reason why humans have lived on deltas. Because of the many distributaries and the access to a major river and the ocean, deltas are regions where goods can be easily exported both inland and overseas. Many of the world's largest ports are in deltas. Also, communication is relatively easy in deltas. Because the land is usually flat, it is easy to build roads. The many waterways make boat travel easy as well.

Life on deltas

The interdistributary areas of deltas can support a variety of different habitats, depending on whether they are closer to the freshwater of the river or the salt water of the ocean. If the interdistributary areas are close to the river and affected by annual floods, they are called floodplains. Other interdistributary areas near the river water may be freshwater marshes, freshwater swamps, or lakes. The interdistributary areas that are closer to the ocean are likely influenced by the tides. They may be tidal flats (flat, barren, muddy areas periodically covered by tidal waters), mangrove (a tree that grows in saltwater) swamps, salt marshes, or marine embayments (an indentation in the shoreline of the sea that forms a bay).

Because deltas have such a broad range of environments, they host a diversity of species. Many different species of plants flourish on deltas, from saltwater trees called mangroves, to sea grasses, swamp sedges, and shrubs. Deltas also serve as nursery grounds for many species of fish and invertebrates (animals without a spine) and many land animals such as snakes and birds. The marine (sea) areas are important habitats for burrowing worms and mollusks, crustaceans (sea animals with hard outer shells) that hunt for food along the seafloor, as well as a variety of different species of fish.

Deltas also serve important environmental roles. They remove harmful chemicals that are deposited in them by river pollution. These chemicals are absorbed by sediments and trapped as new sediments settle on top. Over time, bacteria break down many of these harmful substances and release chemicals that are not dangerous to the health of humans or other animals. Deltas are also known as nutrient recharge zones. When animals and plants die, they are buried in sediments and bacteria digest them. This converts the chemicals in their bodies into the raw materials needed for plants to grow.

Juli Berwald, Ph.D.

For More Information

Books

Challand, Helen J. Disappearing Wetlands. Chicago: Childrens Press, 1992.

Garrison, Tom. Oceanography. 3rd ed. Belmont, CA: Wadsworth, 1999.

Haslett, Simon K. Coastal Systems. New York: Routledge, 2000.

Pipkin, Bernard W. Geology and the Environment. St. Paul, MN: West Publishing Company, 1994.

Websites

"Conceptual Models of Australian Estuaries and Coastal Waterways." OzEstuaries Geosciences Australia.http://www.ozestuaries.org/conceptual_mods/conceptual.html (accessed on August 16, 2004).

"Ganges River Delta." NASA.http://www.earthkam.ucsd.edu/public/images/pdf/Ganges_River_Delta.pdf (accessed on August 16, 2004).

Pidwirny, Michael. "Fluvial Landforms." PhysicalGeography.net.http://www.physicalgeography.net/fundamentals/10z.html (accessed on August 16, 2004).