Running Water Depositional Landforms: Alluvial Fans, Deltas, Floodplains, Natural Levees, Meanders, Oxbow Lakes, Braided Channels
Geography › Geomorphic Processes and Landform Evolution · Topic 3.20
Rivers deposit their load where their speed and energy fall, as in the middle and lower courses, at the foot of mountains and at the mouth. Depositional landforms include alluvial fans and cones where a stream leaves the hills, floodplains and natural levees along the banks, meanders and oxbow lakes on the plain, braided channels where a river is choked with sediment, and deltas at the mouth. Together these make the fertile alluvial plains that support dense populations.
5 min read · UPSC Prelims · Geography · Geomorphic Processes and Landform Evolution
At a Glance
- Alluvial fan Fan of sediment at a mountain foot
- Floodplain Flat land flooded by the river
- Natural levee Raised bank from flood deposits
- Oxbow lake Cut-off meander loop
- Delta Deposit at the river mouth
- Exam link UPSC Prelims, Geography: Geomorphic Processes and Landform Evolution
Where Does It Fit?
This topic completes the pair on running water by describing the landforms of deposition. It follows the topic on erosional forms and ties to the later macro on the drainage systems of India. Previous topic: Running Water Erosional Landforms: V-Shaped Valleys, Gorges, Canyons, Potholes, Incised Meanders, River Terraces. Next topic: gypsum Process Solution by carbonic acid Sinkhole Depression where rock dissolves or collapses Doline and uvala…">Karst (Groundwater) Topography: Sinkholes, Dolines, Uvalas.
What Are Alluvial Fans, Floodplains and Levees?
| Landform | How it forms | Features |
|---|---|---|
| Alluvial fan (cone) | A stream leaving a steep valley onto a plain slows and spreads its load | Fan-shaped deposit; coarse at the apex and fine at the edge; the Himalayan bhabar belt is a line of fans |
| Floodplain | The river overflows its banks and drops silt over the valley floor | Flat, fertile land; renewed by every flood |
| Natural levee | The heaviest sediments are dropped right at the bank during floods | Low ridges along the bank; the river can flow above the plain |
| Terraces | Remnants of old floodplains | Described in Running Water Erosional Landforms: V-Shaped Valleys, Gorges, Canyons, Potholes, Incised Meanders, River Terraces |
Levees can shut off tributaries, which then flow parallel to the main river for long distances, called yazoo streams. In India, the Kosi in Bihar, which has built a huge fan and shifted its course by over 100 km in two centuries, is called the sorrow of Bihar.
How Do Meanders, Oxbow Lakes and Braided Channels Form?
- Meanders: on a gentle gradient, the river swings in loops. It erodes the outer, faster bank (cut bank) and deposits sediment on the inner, slower bank (point bar), so the loop grows and migrates.
- Oxbow lakes: if the neck of a loop is cut through in a flood, the river takes the short route and the abandoned loop becomes a curved lake. Examples are seen on the Ganga plain, and kanwar tal in Bihar is a large such lake.
- Braided channels: where the river has more load than it can carry, it deposits bars and islands in its bed and splits into many shifting channels, as with the Brahmaputra, the Kosi and the Indus in places. Majuli, in the Brahmaputra, is among the largest river islands in the world.
- Natural levees and floodplains: described above, and part of the same system.
What Are the Types of Deltas?
A delta is a deposit formed when a river enters a sea or lake and slows. The shape depends on the balance between river, waves and tides.
| Type | Shape | Examples |
|---|---|---|
| Arcuate | Fan-shaped with a curved edge | Ganga-Brahmaputra, Nile, Indus, Rhine |
| Bird-foot | Long fingers; where the river dominates | Mississippi |
| Cuspate | Pointed, tooth shape; waves dominate | Tiber |
| Estuarine | Rivers flow in a funnel-shaped mouth, with no real delta | Narmada, Tapi, Hooghly |
The Ganga-Brahmaputra delta, including the Sundarbans, is the largest delta in the world, and the Sundarbans mangroves are shared by India and Bangladesh. The eastern-flowing rivers of the Peninsula, the Mahanadi, Godavari, Krishna and Kaveri, make deltas, while the westward-flowing Narmada and Tapi, which run in rift valleys, end in estuaries, as in Plate Boundaries: Divergent, Convergent, Transform.
Why Do These Landforms Matter?
The floodplains and deltas of the Ganga-Brahmaputra, Indus, Nile, Yangtze and Mekong are among the most densely populated parts of the world, because the soils are fertile and water is plentiful. They are also at risk. Floods spread over floodplains, rivers shift course, and deltas sink and are cut by the sea. Embankments protect land but may raise the riverbed with trapped silt, which makes floods worse in the long run. The Ganga-Brahmaputra delta, the largest in the world, includes the Sundarbans with its mangroves, and is threatened by rising sea level. The rivers that built these plains are described in the macro on the drainage of India, and the role of deposition in soil in Black soil From basalt (Deccan Traps) Red soil From granite and gneiss…">Relationship Between Rocks and Soils. As an analytical point, deposition is the creative side of rivers, and floods are part of the same process.
Why Is This Topic Important?
- Terms Deltas and levees are asked
- India Kosi, Brahmaputra, Sundarbans
- Hazard Floods and shifting rivers
What Came Next?
The next topic moves to groundwater and describes the sinkholes, dolines and uvalas of karst topography. Next topic: Karst (Groundwater) Topography: Sinkholes, Dolines, Uvalas.
Key People and Terms
- Alluvial fan Fan-shaped sediment deposit
- Levee Raised natural bank
- Oxbow lake Abandoned meander
- Braided river River with many shifting channels
- Delta Deposit at the river mouth
- Distributary Branch of a river in a delta
Exam Corner
Points to Remember
- Oxbow lakes come from cut-off meanders.
- Levees are raised banks of coarse silt.
- The Ganga-Brahmaputra is the largest delta.
- The Kosi is the sorrow of Bihar.
- Majuli is a river island on the Brahmaputra.
Do Not Confuse With
- Delta and alluvial fan At a river mouth versus at a mountain foot
- Levee and terrace Bank deposit versus old floodplain step
- Meandering and braided rivers Single looping channel versus many channels
Memory Hook
Fan at the foot, levee at the bank; oxbow cuts, delta thanks.
Mains Angle
Questions ask for fluvial landforms and floods. These points add depth.
- Forms Fans to deltas
- Importance Fertile plains
- Risks Floods and shifting courses
A Question You May Face
An original practice question, not a past paper question.
“Explain the formation of the depositional landforms of rivers and discuss their importance and hazards.”
How to Answer
- Forms Fans, levees, oxbows, deltas.
- Process Loss of energy.
- Value Fertile plains.
- Hazard Floods.
GS Relevance
Frequently Asked Questions
How is an oxbow lake formed?
An oxbow lake forms when a river meander becomes so curved that the neck between two bends is cut through during a flood. The river takes the shorter path, the loop is sealed off by deposits, and it remains as a crescent-shaped lake.
What are the types of deltas?
The main types of deltas are arcuate, fan-shaped with a curved edge, as in the Ganga and Nile; bird-foot, as in the Mississippi; and cuspate, as in the Tiber. Rivers such as the Narmada and Tapi form estuaries and not deltas.
Which is the largest delta in the world?
The Ganga-Brahmaputra delta, in India and Bangladesh, is the largest delta in the world. It includes the Sundarbans, the largest mangrove forest in the world, and it is an arcuate or fan-shaped delta formed by the sediments of the two great rivers.
What is a natural levee?
A natural levee is a low ridge along the bank of a river, built when floodwater spills over the bank and drops its heaviest sediments there first. Levees raise the banks above the floodplain, and over time can cause the river to flow higher than the surrounding land.
What is a braided river?
A braided river is a river that splits into many shifting channels separated by bars and islands of sand and gravel. It occurs where the river carries more sediment than it can transport, as with the Brahmaputra, the Kosi and the Indus in some stretches.
PYQ Practice — Statement Analysis
1 The Narmada and Tapi form large deltas.
They form estuaries.
2 An oxbow lake is formed by deposition at the mouth of a river.
It is a cut-off meander.
3 A natural levee is formed by deposition during floods.
Coarse sediment drops near the bank.
4 An alluvial fan forms at the mouth of a river in the sea.
It forms at the foot of a mountain; a delta forms at the mouth.
Past Mains Questions on This Topic
This topic comes under Geomorphic Processes and Landform Evolution. UPSC Mains questions asked on the matching syllabus topic, year-wise with marks:
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