What is alluvium? The definition of this term can be given in many ways. It all depends on who exactly is interested in this topic. For a student, for a student, for a housewife, for a simple layman, definitions may sound different.
Probably any person at least once in his life was on the river. And if this happened in the spring, during the flood, then he would certainly notice a huge amount of various material (stones, rock fragments, blocks, sand, silt, tree branches and shrubs, well, if not different anthropogenic debris), carried away by the river downstream . In principle, all this is alluvium.
So alluvium is all that a river carries with it? No, not really. Then, perhaps, alluvium is part of the channel that the river makes for itself in the mother rock? Not at all like that.
Scientific definition of the term
Well, now let's give him a scientific definition. Alluvium is sediment deposited by water flows, consisting of rounded and sorted detrital material, as well as organic matter. This word itself comes from the Latin alluvio, which means "sediment", "alluvium".
Alluvium of Plain and Mountain Rivers
There are two main types of alluvium, which depend primarily on tectonics and the relief of the area where the river flows. This is an alluvium of mountain and lowland rivers.
Alluvium mountain rivers
Rivers in the mountains are usually characterized by a high flow rate; their sediments consist mainly of boulders and pebbles. The remaining smaller and softer rocks do not have time to linger by the river and are carried away downstream.
The sediments of mountain rivers have the following characteristics:
- consist of coarse clastic material in which pebble predominates;
- diverse mineral composition of fragments;
- poor sorting of material;
- no clear layering.
Alluvium of lowland rivers.
Plain rivers have a lower flow velocity and, accordingly, they are not able to transport coarse fragments over long distances.
Therefore, sediments of lowland rivers have other characteristics:
- consist of small-clastic material, in which sands and sandy loam prevail;
- fairly homogeneous mineral composition;
- good sorting of material;
- the presence of coarse oblique layering, turning into a small oblique layering.
Zonality of river alluvium and its characteristics
Zoning is characteristic of almost any natural phenomenon or object. Although it is precisely for alluvial soils that it is less pronounced than for others, and alluvium is their main constituent component. This, however, does not exclude the influence of zonality on alluvium, primarily on its mineral composition and acidity.
True, the larger the river and its floodplain, the weaker the zoning of alluvial deposits.
On average, in northern moist areas, alluvial soils usually have an acid reaction, are characterized by the absence of carbonates and non-salinity. Moving south, in more arid regions, they first acquire a neutral, and then an alkaline reaction, characterized by saturated carbonates.
Delta, floodplain, old and channel alluvium
Alluvial deposits in lowland rivers are complex and diverse. Therefore, according to the nature of precipitation and the places of their accumulation, alluvial deposits are usually divided into channel, delta, floodplain and old.
Delta alluvium is formed in river deltas and is characterized by a sandy clay composition.
Channel alluvium is formed in river beds and consists mainly of sand and coarser debris, such as boulders, gravel and pebbles. He formed sandbanks, spits and islands on the river.
Floodplain alluvium is formed during the flood period and consists of a variety of loams, clays and fine-grained sands enriched with organic matter.
The old alluvium is deposited at the bottom of the old women and consists of silt with a lot of organic matter.
Alluvial deposits are widespread throughout the world. Since ancient times, it was precisely on their development that all the main ancient world civilizations began to emerge, such as Ancient Egypt in the Nile Valley or Ancient Mesopotamia in the valleys of the Tigris and Euphrates rivers.
In the modern world, the most productive agricultural lands are located in areas with floodplain alluvium. It also often contains placers of minerals and even precious minerals.