What is a River Delta? Formation, Types, and East African Examples
Discover the formation of river deltas, their types, and examples from East Africa. Learn how sediment deposition creates distributaries and swampy landscapes.
What is a Delta?
A river delta is a large, flat, low-lying landform formed by the deposition of sediments such as silt, sand, gravel, and boulders at a river’s mouth, where it flows into a lake, ocean, or sea. The continuous buildup of sediments causes the river to split into multiple smaller channels called distributaries. Many deltas are covered with swamps and lagoons and often take on a triangular shape, resembling the Greek letter Δ (delta).
Deltas play a crucial role in ecosystems and human settlements, providing fertile land for agriculture, rich biodiversity, and important water sources. In East Africa, notable deltas include the Nile Delta in Egypt and Sudan, the Rufiji Delta in Tanzania, and the Tana Delta in Kenya. These deltas support fishing, farming, and wildlife conservation while also facing challenges like erosion and rising sea levels.
Conditions Favouring the Formation of Deltas: Key Factors and East African Examples
The formation of deltas is a complex process that requires a combination of natural conditions that allow for sediment deposition at the mouth of a river. Below are the key conditions that favor delta formation, along with examples from East Africa to illustrate these conditions.
1. Large Amount of Sediments Transported and Deposited by the River
- A river must carry a substantial amount of sediment, including silt, clay, and sand, which it deposits at its mouth as the flow slows down.
Example: The Tana River Delta in Kenya forms because the Tana River carries a significant sediment load from its catchment area, depositing it at the mouth where it meets the Indian Ocean.
2. Minimal Tidal or Ocean Currents to Remove Deposited Sediments
- For delta formation to occur, the tidal or ocean currents must be weak enough to allow sediments to accumulate rather than being washed away.
Example: The Rufiji River Delta in Tanzania benefits from relatively calm ocean currents, which allow the sediments to build up over time, creating an expansive delta.
3. Gentle or Low Gradient to Allow Sediment Accumulation at the River Mouth
- A low gradient (flat terrain) at the river mouth allows the river to slow down enough for sediments to settle and accumulate.
Example: The Zambezi River Delta in Mozambique exhibits a gentle slope at its mouth, creating the perfect conditions for sediment buildup and delta formation.
4. Presence of a Sheltered Coast
- A sheltered coast, protected from strong waves and storms, helps reduce the movement of sediments, facilitating the growth of a delta.
Example: The Nile Delta in Egypt has a sheltered coastline along the Mediterranean, which has been crucial in the accumulation of sediments over thousands of years.
5. Presence of a Shallow Adjoining Sea
- A shallow sea adjoining the river mouth allows for sediment deposition, as the river's sediment load can settle before it is carried away into deeper waters.
Example: The Tana River Delta in Kenya is adjacent to the shallow Indian Ocean, creating a conducive environment for sediment buildup and delta development.
6. Absence of Barriers Like Dykes at the Mouth of the River and Large Lakes in the Course of the River
- The absence of natural barriers, such as dykes, dams, or large lakes, ensures that sediment transport is unimpeded, allowing for the natural development of deltas.
Example: The Shire River Delta in Malawi benefits from an open flow path to Lake Malawi, without significant barriers preventing sediment deposition at the mouth.
Formation of deltas
Deposition at the river mouth results from both the reduction in speed as the river enters the sea and also from coagulation of fine material mixing with salt water. Deposition divides the river channel into several distributaries. Spits, bars and lagoons are formed. The levees of the river extend into the sea via the distributaries.
The lagoon begins to be filled with sediments and they become swampy. The delta begins tassume a more solid appearance. With continued deposition over decades, the old part of the delta becomes colonised by plants and its height is slowly raised as a result of this. Swamps gradually disappear and this part of the delta becomes dry land.
Types Of Deltas.
Arcuate delta:-
This is characterised by having
· Coarse sediments such as gravel and sand.
· Many distributaries.
· Triangular shape.
· .Rounded seaward edge.
Examples include the Rufigi delta in Tanzania, Tana, Yala and Sondu deltas in Kenya, Semliki delta at the southern tip of Lake Albert, the Nile delta in Egypt and Niger delta in Nigeria.
Estuarine delta
This is formed by deposition of sediments in a submerged river mouth. The deposits form sand and islands around which wind several distributaries e.g. River Rufigi in Tanzania, River Zambezi in Mozambique and River Congo in the DRC. Deposited material forms Islands
Bird's foot delta
This is composed of:-
· Fine materials mainly silt
· Few very long distributaries bordered by leaves.
· Birds foot shape
Rivers with mouths in Lake Victoria generally have bird's foot type of deltas e.g. Nyando, Nyaidho, Ombeyi and Miriu in Kono plains, Nzoia at Busia and others. Rivers Turkwell and Kerio joining Lake Turkana, and Rivers Gilgil and Malewa joining Lake Naivasha have bird's foot type of deltas.
Inland delta.
This is a delta formed at any point on the river course where a river joins a lake e.g, where the Victoria Nile joins Lake Albert and where River Omo joins Lake Turkana.
Assessing the importance of deltas in Africa
- They attract dense settlements due to the fertile soils. Cairo for example has one of the larges concentrations of people in Africa. Other cities in the Nile Delta include Alexandria, ZagarTanta and El Mansura.
- Deltas contain fertile alluvial sila and this has given rise to crop cultivation e.g. in the Nile det grapes, flowers and vegetables are grown.
- They act as tourist attractions hence earning the respective countries foreign exchange e.g. the Nile Delta in Egypt and Niger Delta in Nigeria.
- Many deltas contain petroleum deposits e.g. the Niger, Rufigi and Semliki deltas forming an importance source of energy.
- Delta lagoons contain silt good for fish food hence giving rise to fishing activities.
- Many deltas especially those in the tropics contain mangrove used as a source of poles used in the building industry.
- The swampy vegetation-covering delta is used as a raw material in the craft industry.
Negative
- Deltas tend to harbour disease-carrying vectors, such as mosquitoes causing malaria.
- Deltas are flat low-lying plains and hence subjected to flooding causing damage to cropland property and settlements.
- Deposition at the river mouth impedes navigation.
- The saline water from the sea reduces the productivity of deltas for agricultural purposes.
Alluvial fans
These are fan shaped deposits of fairly course materials laid down by rivers with a large load. They are similar to deltas except that they are formed on a land. They form where a river emerges from a steep narrow valley into a wide gentle plain.
A sharp increase in the channel's width and decrease in gradient causes a marked reduction in stream energy resulting in sudden deposition to form alluvial fans. They range in size from a few metres to several kilometres. Rivers forming alluvial fans flow through them in a system of braided channels e.g. the Lume fan formed by the Lume River as it reaches the Semliki plain on western part of Mount Rwenzori, Kilombero River fans in Tanzania which include Rondo, Rufiri, Lumemo and Luri, and the Ewaso Narok in Kenya. Alluvial fans may block a river valley to form a lake like. Lake Nyabihoko in western Uganda.
Similarities Between Deltas and Alluvial Fans: Key Features and East African Examples
Deltas and alluvial fans are both depositional landforms formed by rivers as they lose energy and deposit sediments. Despite their differences in location and composition, they share several characteristics. Below is a detailed comparison, with examples from East Africa.
1. Both Are Funnel-Shaped
- Deltas and alluvial fans have a triangular or fan-like shape, widening as sediments spread outward from the point of deposition.
Example: The Tana River Delta in Kenya and the alluvial fans at the base of Mount Elgon in Uganda both exhibit this characteristic funnel shape.
2. Both Are Made Up of River-Deposited Sediments
- Both landforms consist of sediments carried and deposited by rivers as they lose velocity.
- Finer materials like silt and clay are deposited further away, while coarser materials such as sand and gravel settle closer to the source.
Example: The Rufiji Delta in Tanzania contains fine sediments deposited by the Rufiji River, whereas alluvial fans in the Great Rift Valley of Kenya consist of coarse materials dropped by fast-moving rivers.
3. Both Divide into Distributaries Due to Deposition
- In both cases, sediment buildup on the riverbed forces the river to split into multiple smaller channels called distributaries.
Example: The Shire River Delta in Malawi has well-developed distributaries, just as alluvial fans in the Aberdare Ranges of Kenya display multiple branching streams.
4. Both Form Where the Gradient Has Been Significantly Reduced
- Both features develop where a river's slope or gradient decreases, leading to a reduction in flow speed and subsequent deposition of sediments.
Example: The Tana Delta forms where the river meets the Indian Ocean, while alluvial fans in northern Tanzania occur where rivers descend from the highlands into flatter plains.
5. Both Are Formed Where River Speed Has Decreased
- A key condition for the formation of deltas and alluvial fans is a loss of river velocity, which causes the river to drop its sediment load.
Example: The Semliki Delta in Uganda, where the Semliki River enters Lake Albert, slows down significantly, allowing sediment deposition. Similarly, alluvial fans in the Rwenzori Mountains form as rivers slow down on flatter terrain.
Deltas and alluvial fans share fundamental characteristics related to their formation by sediment deposition, funnel shape, distributary networks, and the role of river speed and gradient reduction. While deltas like the Tana and Rufiji form at river mouths, alluvial fans such as those in the Rift Valley develop on land.
These landforms play essential roles in shaping East Africa’s landscapes, influencing agriculture, water distribution, and human settlement patterns.
Differences between deltas and alluvial fans
Deltas form at the mouth of the river where it enters into the sea while alluvial fans develop on land far away from the sea.
Alluvial fans tend to have a steeper gradient than deltas, sloping up towards the apex.
Deltas have well marked distributaries while alluvial fans have less district distributaries.
Deltas are formed in the old or senile state of a river. Alluvial fans are formed in the youthful or torrent stage.
Deltas are associated with swamps, lagoons, spits and bars while the alluvial fans are not.
Deltas are mainly formed of fine sediments while alluvial fans are made of deposits of mainly coarse materials.
Delta formation is favoured by the absence of tidal currents, absence of obstructions, difference in sanity and presence of sheltered bays. These conditions are not applicable to alluvial fans.
Braided channel
This is an extremely wide shallow channel especially in the old stage of a river in which the five divides and subdivides into a series of minor inter-connecting channels separated by sand banks and islands of alluvium. Braiding is a result of deposition of sand and alluvium on the river be by a heavily loaded stream. Successive deposition leads to the gradual increase in their size form more or less islands.
These can be seen along rivers Tana, Galana and Nzoia in Kenya and Kilombero and Rufigi in Tanzania. The sand banks and islands of alluviumare often unstable and submerged during flood time.
Knick points, terraces and incised meanders.
Where rejuvenation has taken place landforms such as knick points, incised meanders, terraces and valley in valley are formed. These landforms are dealt with later in the topic under rejuvenation.
