ZIMSEC O Level Geography Notes: 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.
- The river must have a large load.
- The velocity of the river must be low enough to allow its load to be deposited in the river’s mouth.
- The river’s load must be deposited faster than it can be removed by the action of tides and currents.
- 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.
- 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.
- 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.
- 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.