How are Alluvial Fans and Deltas formed?

Alex Williams
7 Min Read

The alluvial fan is a deposit of gravel, sand, and also small pieces of sediment found in a triangle shape. This sand is called alluvium. However, this alluvium sand is deposited and is known as stream fans. It creates the familiar triangle-shaped feature. Further, let us see how are alluvial fans and deltas formed and how are alluvial fans and deltas alike. Moreover, the difference between a delta and an alluvial plain will be discussed below.

1. How are Alluvial Fans and Deltas Formed?

Alluvial fans are made with flowing water that combines with mountains, hills, or steep walls of canyons. The rushing water takes alluvium to a flat plain, the stream leaves its channel to spread out the surface. Take a look at the following points to see how are alluvial fans and deltas formed:

  • Alluvial fans are created as flowing water interacts with mountains, hills, or the steep walls of canyons, and a delta is formed from the deposition of sediments which is carried by rivers or seas.
  • The streams that carry alluvium can flow due to rainwater, a fast-moving creek or a powerful river, or even runoffs from any industry or agriculture.
  • The running water carries alluvium to a flat plain. The stream leaves the channel to spread out. The alluvium is deposited as the stream fans out by creating a triangle-shaped feature. Delta is mostly formed when rivers meet seas or any larger bodies of water. This is your answer to how are Alluvial fans and Deltas formed.

2. What is a Difference between a Delta and an Alluvial Plain?

  • While answering how are alluvial fans and deltas formed, you have learned that the alluvial fans are formed from the removal of water-transported material. This is the main difference between a delta and an alluvial plain as the delta is formed from the deposition of sediments which is carried by rivers at a waterway.
  • The terms alluvial fan and delta both signify the landforms. They are formed from the deposition of material that comes with flowing water. So, alluvial fans are formed at high places such as mountains while delta is formed in the regions near the ocean.
  • Alluvial fans are found in the mountain regions and Deltas are found in the Ocean regions.
  • Alluvial fans have a higher elevation whereas deltas have lower elevations. 

3. How are Alluvial Fans and Deltas Alike?

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Delta and Alluvial landforms both are depositional landforms. Alluvial fans are formed at the foothills whereas the delta is found at the mouth of streams. (See What is an Abandoned Cutoff Meander Loop?)

4. How are Deltas Formed?

Deltas are wetlands that are empty rivers and their water and sediments flow into another body of water such as an ocean, lake, or river. Deltas can also empty into lands. A river moves very slowly when it reaches a point. This can cause sediments; solid materials get carried down by currents to fall to the river bottom.  (See Is Sundarban the Largest Delta in the World?)

5. What are the Types of Alluvial Fans?

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  • Debris Dominated – The fan involves the flows of a dense gluey mixture of water, mud, sand, and gravel. They are mixed with standing stones and commonly woody debris.
  • Floodwater – Floodwater will spill the fan across the surface. The floodwater contains fine sediment than debris flow, and it cannot carry large boulders. Sometimes water flows down the fans in a thin continuous sheet. 

6. What is Soil Erosion?

Soil erosion is the process in which soil materials such as rock, sand, and sediments are taken away by natural forces like water or wind. The best example would be the soil erosion created at Grand Canyon. Due to this the upper layer of the soil gets washed away which has nutrients and all the minerals. Also, check out how do animals cause erosion?

7. What is an Apex?

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The narrow point of the alluvial fan which is closest to the mountains is called the apex. The broader part is called an apron. The alluvium deposited near the apex will be inferior to the material that makes up the apron. Alluvial fans are more likely to form in deserts because there is plenty of loose alluvium and no vegetation at all which could stop the water channels from shifting. (See What are Geologic Features?)

8. What are the Types of Deltas?

  • Arcuate: The arcuate is the fan-shaped deltas. The broad portion of the fan is facing the water. The activity of the river is balanced with the wind.
  • Birds foot delta: It is a delta that appears to be like a bird’s foot claw. This shape is created when the waves are weak and the river flows are strong. These are mostly near the ocean coasts because the waves are stronger than the river.
  • Cuspate deltas: These are formed when sediments are deposited into a straight line with strong waves. The waves push the sediments to spread creating a tooth-like shape.

Both alluvial fans and deltas are found in triangular-shaped sedimentary deposits, including gravel, sand, and even smaller fragments. The deposit of coarse material brought by water movement is known as an alluvial fan. When a moving water body collides with a still water body, a delta is created. Hope this article informed you about how are alluvial fans and deltas formed and also explained how are alluvial fans and deltas alike. (See What are Parts of a River System?)

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