Alluvial fan
Topic: Landforms and their Evolution: Rivers, Groundwater, Glaciers, Waves and Wind · NCERT: Class 9, Ch 2 "Shaping of the Earth's Surface"; Class 11, Ch 6 "Landforms and their Evolution"
Meaning
An alluvial fan is a cone-shaped heap of coarse sediment such as boulders, gravel and sand. It forms where a fast mountain stream suddenly comes out onto a gentle plain. The stream slows down and can no longer carry its heavy load, so it drops it. The stream then splits into many shifting branches called distributaries, which spread the load out into a fan. In humid areas fans are low with gentle slopes. In dry areas they are high and steep.
Example
The Bhabar belt along the foot of the Shiwaliks, from the Indus to the Teesta, is a chain of alluvial fans joined together. Streams often sink into its porous pebbles and disappear.
Don't confuse with
- Delta: a delta forms where a river meets the sea. It is made of fine, well-sorted layers. A fan forms at the foot of a mountain and is made of coarse, poorly sorted material.
Related concepts
- Distributary
- Delta
- Estuary
- Floodplain
- Natural levee
- Point bar
- Meander
- Slip-off slope and undercut bank
- Oxbow lake
- Braided channel