What is alluvium? This term can be defined in many ways. It all depends on who exactly is interested in this topic. For a schoolchild, for a student, for a housewife, for a simple layman, definitions may sound different.
Probably, any person at least once in his life has been on the river. And if this happened in the spring, during the flood, then he would definitely notice a huge amount of various material (stones, fragments of rocks, boulders, sand, silt, branches of trees and shrubs, well, if not various anthropogenic debris) carried by the river downstream. In principle, all this is alluvium.
So alluvium is all the river carries with it? No, not really. Then, perhaps, alluvium is part of the channel that the river makes for itself in the parent rock? Not at all.
Scientific definition of the term
Well, now let's give it a scientific definition. Alluvium is sediment deposited by water flows, consisting of rounded and sorted detrital material, as well as organic matter. The word itself comes from the Latin alluvio,which means "applied", "alluvium".
Alluvium of lowland and mountain rivers
There are two main types of alluvium, which depend primarily on the tectonics and topography of the area where the river flows. This is alluvium from mountain and lowland rivers.
Alluvium of 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 in the river and are carried downstream.
Sediments of mountain rivers have the following features:
- consist of coarse clastic material dominated by pebbles;
- diverse mineral composition of the fragments;
- weak material sorting;
- no clear layering.
Alluvium of lowland rivers.
The lowland rivers have a lower flow rate and, accordingly, they are not able to carry coarse debris over long distances.
Therefore, sediments of lowland rivers have other features:
- consist of fine-clastic material dominated by sand and sandy loam;
- quite homogeneous mineral composition;
- good material sorting;
- presence of coarse cross-bedding, turning into fine cross-bedding.
Zonality of river alluvium and its characteristics
Zonality is characteristic of almost any natural phenomenon or object. Although it is for alluvialsoils, it is less pronounced than for other soils, and alluvium is their main component. However, this does not exclude the influence of zoning on alluvium, primarily on its mineral composition and acidity.
True, the larger the river and its floodplain, the less pronounced the zoning of alluvial deposits.
On average, in the northern humid regions, alluvial soils are usually acidic, characterized by the absence of carbonates and non-salinity. As they move south, in more arid regions, they first acquire a neutral, and then an alkaline reaction, are characterized by saturation with carbonates.
Delta, floodplain, oxbow 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 oxbow.
Deltaic alluvium is formed in river deltas and is characterized by a sandy-clayey composition.
River alluvium is formed in river beds and consists mainly of sand and coarser debris such as boulders, gravel and pebbles. They formed sandbars, 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.
Old alluvium is deposited at the bottom of oxbow lakes and consists of silt with a lot of organic matter.
Alluvial deposits are widespread throughout the world. long time agoit was 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.
