Deltas

• A delta is a gently sloping depositional feature that is found at a river’s mouth where it empties into a sea or ocean that extends to the surface and is shaped like the Greek letter delta (Δ).
• It is important to note that although some deltas are indeed shaped like the Greek letter delta some deltas as pointed below have other shapes as well for example the Estuary delta.
• They are low lying swampy plains that gradually become colonised by various types of plants.
• The growth of a delta interferes with the flow of a river resulting in the river splitting up into several distributaries not unlike the ones resulting from braiding.
• A distributary is a channel that splits and rejoins with other channels of the same river.
• An example is the Niger Delta in West Africa and the Nile Delta in Egypt.

Formation of deltas

• most of the load carried by rivers is deposited into the oceans, seas and lakes into which the rivers empty.
• Sometimes the load is carried far away into the mouth of the river before it sinks to the bottom.
• Deltas are formed when the load instead sinks at the bottom of the mouth of the river.
• When this happens layers of sediment collect and pile up to form a gently sloping platform.
• With time the platform extends to the surface to form a delta.

Conditions necessary for the formation of a delta.

1. The river must have a large load.
2. The velocity of the river must be low enough to allow its load to be deposited in the river’s mouth.
3. The river’s load must be deposited faster than it can be removed by the action of tides and currents.
4. The chances for the formation of a delta are greatly enhanced when clay particles are part of a river’s load resulting in them coagulating and thickening as they mix and react with seawater and settle at the bed in a process called flocculation.

• The Congo River has a large velocity at the point at which it meets the sea and thus has no delta as most of its load is carried off into the sea.
• The River Niger has low velocity at its mouth resulting in the formation of an extensive delta.

Stages in the formation of a delta.

Stage 1

• Deposition in the river’s mouth results in the river forming several distributaries.
• The delta begins to form when the initial sediment collects at the bottom near the river’s mouth.
• As depositions continues layer upon layer a slow platform results.
• Deposition on the banks of the distributaries forms levees.
• The area between the distributaries may result in the formation of lagoons.

Stage 2

• The lagoons begin filling with sediment which causes further division into distributaries and to the formation of smaller distributaries.
• The delta starts to take a more solid appearance although it may still be swampy and usually covered with water loving vegetation.

Stage 3

• Further in-filling of the lagoons plus the growth of vegetation results in the older parts of the delta coming to stand above water level forming dry land.
• Continued development of a delta can lead to it merging with the flood plain and forming deltaic plains

Types of deltas

There are four types of deltas viz:

Arcuate, Bird’s foot,Estuarine and Cuspate Deltas.

Arcuate

Arcuate Delta. Image credit Wikispaces.com

• Is a triangular shaped delta with an arc-shaped shoreline.
• The Niger delta is an example of such a delta.
• They have coarse and fine sediment in the form of a cone.
• It is crossed by many distributaries.
• It is useful for man to settle on as it provides arable and fertile lands as well as pastures for animals.
• However the waterlogged conditions tend to favour pests and diseases.

Birds foot

Satellite view of the Mississippi River. Image credit adammandalmen.com

• Is formed where there are weak tidal currents which allow the distributaries to extend further seawards resulting in a form that looks much like the shape of a bird’s foot.
• An example is the Mississippi River delta.
• It consists of very fine material like silt and several distributaries bordered by levees jutting out from the shore.

Estuarine

• Is made up of a single channel with braids formed from deposits made onto the river’s channel as it reaches the ocean.
• An example is the Zambezi River delta in Mozambique.
• The delta is formed from material deposited in the submerged mouth of a river.
• It forms the shape of an Estuary.

Cuspate delta

• Sometimes a cuspate delta may be formed.
• These develope where there is limited distributary development in coasts that have moderate current.
• An example is the Medjerda River delta in Tunisia.

To learn more about landforms resulting from river processes go to this page.

To access more topics go to the Geography Notes page.