What is the Difference Between Alluvial Fan and Delta

The main difference between alluvial fan and delta is that alluvial fans are formed from the deposition of water-transported materials, whereas delta is formed from the deposition of sediments carried by rivers at an estuary.

The terms alluvial fan and delta refer to the landforms which are formed from the deposition of material that comes with flowing water. Generally, alluvial fans form at elevated places such as mountains while delta forms at the regions near the ocean.

Key Areas Covered

1. What is an Alluvial Fan 
     – Definition, Features, Formation
2. What is a Delta
     – Definition, Features, Formation
3. What is the Difference Between Alluvial Fan and Delta
      – Comparison of Key Differences

Key Terms

Alluvial Fan, Delta, Sediments, Water Bodies

What is an Alluvial Fan

An alluvial fan is a landform that has a triangular shape and forms due to the deposition of materials that are transported by water. These materials are known as “alluvium”. These are usually unconsolidated sedimentary deposits. Typically, these landforms form in elevated regions such as mountains. This is because these regions have a rapid change in slope at a short distance. Due to this rapid gradient of landscape slope, the rivers that flow on these regions have a high velocity; therefore, the coarse materials can remain in the flow. When the river reaches a planar region, the stream loses the energy to move its sediment materials. Ultimately, the sediments spread out on the ground, forming the alluvial fan. An alluvial fan has three major regions: proximal fan, medial fan and distal fan.

Figure 1: A Small Alluvial Fan

An alluvial fan can exist in a wide range of sizes. For example, some alluvial fans are on the scale of meters at the base and on kilometre-scale at the edge of the fan. Sometimes, two or more alluvial fans tend to combine and form a continuous apron. Often, alluvial fans form in desert regions. This is because deserts often experience flash floods from nearby thunderstorms in local hills. However, these landforms can also form in humid regions—E.g. near the Koshi river in Nepal. Moreover, alluvial fans are often subject to floods; sometimes, they are dangerous than usual flooding.

What is a Delta

Delta or river delta are landforms created due to the deposition of sediments carried by water in the river. These landforms form at estuary (the point where a river connects to the ocean). This deposition occurs as the river water flow leaves its mouth and enters a slow-moving water region such as the ocean. Not only at estuaries, but these landforms also form at places where the river meet a lake, sea, reservoir, and rarely, another river.

Figure 2: Structure of a Delta

The shape and the size of this deposition depend on the watershed process and the receiving mechanism. The size, geometry and location of the receiving water body also affect the formation of the delta. From ancient times to today, these rivers deltas have been very important in human civilizations since agricultural lands are always located near the water bodies. Moreover, these landforms are important ecologically since there are a number of different animal and plant species living in these regions.

Difference Between Alluvial Fan and Delta

Definition

An alluvial fan is a landform that has a triangular shape and forms due to the deposition of materials that are transported from water. In contrast, a delta or river delta is a landform created due to the deposition of sediments carried by water in the river.

Location

Alluvial fans form at mountain regions while deltas form at ocean regions

Elevation

While alluvial fans have a higher elevation, deltas have a lower elevation.

Formation

Moreover, alluvial fans form from the deposition of coarse material that comes with water flow at a high velocity while deltas form from the deposition of sediments  that occurs when a flowing water body meets a non-flowing water body.

Size and Shape

Alluvial fans can vary in size, but their shape is always triangular. Deltas are usually smaller than alluvial fans and their shape varies with the nature of moving and non-moving water bodies. 

Conclusion

Alluvial fan and delta are landforms that form from the deposition of sediment materials. The main difference between alluvial fan and delta is that alluvial fans form from the deposition of water-transported materials whereas delta form from the deposition of sediment carried by rivers at an estuary.

Reference:

1. “Alluvial Fan.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., 18 Apr. 2020, Available here.

Image Courtesy:

1. “Mini Alluvial Fan Imprinted with Footprints” By Wing-Chi Poon – at Salt Creek Interpretive Trail, Death Valley National Park, California, USA (CC BY-SA 2.5) via Commons Wikimedia
2. “Line5066 – Flickr – NOAA Photo Library” By Alaska ShoreZone Program NOAA/NMFS/AKFSC; Courtesy of Mandy Lindeberg, NOAA/NMFS/AKFSC. – NOAA Photo Library: line5066, Public Domain) via Commons Wikimedia