Deltas

Description

• A delta is a feature of river deposition found at the end of a river at its mouth where it reaches the sea or a lake. Deltas are fan-shaped, low-lying areas of flat land jutting out to sea. They can be very large and can span hundreds of km across. They are comprised of a range of coarse to fine alluvial material and are characterised by a complicated criss-cross network of distributaries.

Indus River Delta-(example)

Series of complex distributaries in a criss-cross pattern.

Coarse and fine alluvial material

•Location: Pakistan•Size: 40000 sq. km

Formation

• A delta is alluvial landform where the mouth of a river flows into an ocean. As the river reaches its mouth it loses energy and competence so therefore slows down. This reduction in speed results in the river depositing its load. As the sea is unable to remove this build up of sediment fast enough, a delta is formed. When rivers like the Mississippi reach the sea, the meeting of fresh and salt water produces an electric charge which causes clay particles to collect together and to settle on the seabed. This is a process called flocculation.

• As these particles of sediment flocculate and therefore increase in size, they are deposited under the influence of gravity. Over time these factors result in the build up of sediment both outwards and in height. Later, a system of multiple, dividing channels form within the delta and these are called distributaries. These divide at points of channel blockage and come together again to form a maze of active and inactive channels.

• http://www.crescent.edu.sg/crezsphere/applets/Geography/deposit.html

Different types of deltas

• Bird’s foot- where delta formation is river-dominated and less subject to tidal or wave action, a delta may take on a bird’s foot shape.

The Mississippi delta is an example of a bird’s foot delta.

The shape of the delta is like a claw.

• Arcuate delta- has a rounded, convex outer margin. A good example is the Nile delta which is dominated a little more by tidal and wave action.

The land around the river mouth arches out into the sea.

The river splits many times on the way to the sea, creating a fan effect.

• Cuspate delta- where the material brought down by a river is spread out evenly on either side of its channel. It is like a cup and is shaped by gentle, regular, but opposing, sea currents or longshore drift. An example of a cuspate delta is the Tiber.

Pointed like a tooth or a cup.

Human impacts of deltas

Positive impacts• Deltas provide fertile land and are associated with good

fishing grounds offshore, and with oil and gas deposits beneath the surface.

Negative impacts• Deltas are by their nature very dynamic landforms due to

the fact that they are composed of unstable, unconsolidated sediments and are subject to channel migration as well as to subsidence by the sea.

• Deltas flatness makes the land surrounding it high flood-risk areas.

Ganges delta-Human uses

• A large part of the nation of Bangladesh lies in the Ganges Delta, and many of the country's people depend on the delta for survival. In fact it is believed that up to of 300 million people are supported by the Ganges Delta.

Agriculture and fishing• Approximately two-thirds of the Bangladesh people work

in agriculture, and grow crops on the fertile floodplains of the delta. The major crops that are grown in the Ganges Delta are rice, tea and a vegetable called jute.

• Fish farms have also been set up for the poor by scientists to improve the local fishing methods. Shrimp and salmon are the major types of fish farmed here and most of them are exported and sold.

• However, people have to be careful on the river delta as severe flooding also occurs. In 1998, the Ganges flooded the delta, killing about 1,000 people and leaving more than 30 million people homeless. The rice crops were also destroyed which left may of the surviving people starving.