Alluvium

Alluvium is a loose soil, or sediment, not fused into a rock, formed by erosion, reshaped by water in some form, and redeposited in a riverside setting or by a river. This is suitably represented by the name Alluvium meaning “to wash against”. In simple terms, the soil or sediment deposited or redeposited by continuous erosion from water (for example of a running river) and solidifying but not completely fused into a rock is generally called Alluvium. It is typically made up of many various materials which include fine particles such as silt, clay and large sand and gravel particles. When such alluvium is over time deposited and solidified or lithified, into a rock, it is called an alluvial deposit.

Characteristics of the Alluvium

Alluvium occurs in deposits of a variety of landforms. The recent alluvium or the new alluvium is deposited on all level floodplains which are near to a stream or a river. Whereas the older alluvium is found on alluvial terrace landforms which are higher than any of the river-associated floodplains and are not subject to frequent erosion or washing away due to flooding. An important point to note is that sediment forms and deposits in a perennial river system are not typically referred to as alluvial. Thus, if any of the sediment formations can be credited to another very well-described geological event, then it is not referred to as alluvial. 

As is clear from the alluvium meaning - “wash against” in Latin, alluvium is the subaerial deposition of sediments due to river currents in the flood plains. The typical composition of alluvium or alluvial soil includes clays, silts, sands, gravels and occasional cobbles. Many times the composition is a mixture of all these particles along with some organic matter. This type of deposition of a landform is poorly sorted and the variations in particle shape are of different degrees. But in very general forms they all are rounded to some extent. 

Alluvium is associated with all the channels where water is present or was present in the past. The term “alluvial channels” based on alluvium is a very general term for all the channels that derive their deposition or form in some part because of flowing water irrespective of their time duration i.e. either perennial or short-lived. A variety of landforms are associated with alluvium which includes alluvial fans, braided channels, deltas, meander cutoffs, levees, point bars and terraces.

The soil of alluvium landforms is generally rich in potash and poor in phosphorus. The colour of the alluvial soils varies from light grey to ash grey. The shades depend on different factors namely, the texture of the materials and the time taken for attaining certain maturity after deposition. Due to various factors such as their location around the floodplains and the deposited particles with organic matter and mineral content, alluvial soils are one of the highly used soils for cultivation.

How Old or New is Alluvium?

Alluvium is the most recent form of a deposition. Geologically it comes in the Quaternary, which is the most recent of the three periods of the geologic Cenozoic era of the most recent Phanerozoic Eon. It is mostly referred to as the “cover” simply because it covers or hides the underlying lithified rock under it. 

Most of the sediments that are present in a given basin, but are not lithified into rocks are typically lumped together as alluvium. Depending on the time passed from their deposition, the alluvium is categorized into two types: Khadar and Bangar. They are explained below:

• Khadar: The new alluvium is known as Khadar. It is formed by the deposition brought by the annual floods in a given flood plain. It contains the newly deposited fine silts which aid in enriching the soil. 

• Bhangar: As the new alluvium is known as Khadar, the old alluvium is known as Bhangar. It is a system of older deposition which are found away from the flood plains and were probably deposited by flowing water in the past.

Both the Khadar and Bhangar contain calcareous concretions (called Kankars). In India, both types of Alluvium are found in the upper and middle Ganga plain. Some of the alluvium is also found in the Brahmaputra valley. In the composition of the alluvium, the sand content decreases while one goes from west to east. 

Alluvium is also distributed in different parts of the world. For example, the alluvium of the Pliocene age occurs in parts of Idaho. Alluvium from the late Miocene age is found in the valley of San Joaquin River, California. An example of alluvium deposition is shown in the given picture:

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The Alluvium Conclusion 

From the given article it is quite clear that Alluvium is a type of geological deposition (yet to be lithified into a rock) of sands, silts and other particles including organic matter by flowing water. It is rich in minerals such as potash and is widely used for cultivation.