Alluvium

Alluvium (from the Latin alluvius, from alluere, "to wash against") is loose, unconsolidated (not cemented together into a solid rock) soil or sediment that has been eroded, reshaped by water in some form, and redeposited in a non-marine setting. Alluvium is typically made up of a variety of materials, including gravel, sand, silt, and clay. The process of deposition of alluvium is called alluviation.

Formation

Alluvium is deposited by flowing water from rivers and streams as the water slows down in velocity, allowing particles to settle out of suspension. This can occur at the point where the stream or river flows into a larger body of water, such as a lake or the ocean, or when the water flow decreases on a floodplain or delta. Alluvium can also be deposited by ephemeral streams during heavy rain events. The size of the particles deposited typically decreases the farther they travel from their source, with gravel and larger particles settling out first, followed by sand, and then silt and clay.

Characteristics

The characteristics of alluvial deposits can vary widely depending on the source material, the speed of the water flow, and the area's geography. Alluvial soils are often highly fertile, as they can be rich in nutrients carried from upstream. However, they can also contain contaminants such as heavy metals, depending on the geology of the area the river has flowed through.

Types of Alluvium

There are several types of alluvium, including:

• Floodplain alluvium: Deposited by rivers and streams over their floodplains during overflow events.
• Deltaic alluvium: Found at the mouths of rivers where they enter a standing body of water, forming deltas.
• Alluvial fan: Formed when a high-velocity stream suddenly slows down as it exits a narrow valley onto a flat plain, depositing large amounts of sediment in a fan-shaped area.
• Terrace deposits: Older alluvium deposits that remain at a higher elevation than the current river level, indicating previous river courses or changes in sea level.

Economic Importance

Alluvial deposits are of significant economic importance. They are often sites for the extraction of placer deposits of gold, diamonds, and other precious stones and metals. Alluvial soils in river floodplains and deltas are highly fertile and are important areas for agriculture, supporting a large portion of the world's food production.

Environmental Concerns

The extraction of minerals from alluvial deposits can lead to environmental degradation, including habitat destruction and water pollution. Additionally, the construction of dams and other modifications to river flow can disrupt the natural process of alluviation, affecting ecosystems and reducing the fertility of downstream alluvial soils.

See Also

• Sediment
• Erosion
• Floodplain
• Delta (landform)
• Alluvial fan