alluvium

Alluvium refers to loose, unconsolidated sediments deposited by flowing water, typically by rivers and streams. It includes clay, silt, sand, and gravel laid down in downstream settings such as floodplains, river channels, alluvial fans, and deltas. Alluvial deposits are usually poorly sorted and stratified, and they may contain organic material derived from surrounding landscapes.

Deposition of alluvium occurs when the velocity of the transporting water decreases, such as during floods, in bends and wide expansions of rivers, or where streams spill onto floodplains. The grain sizes reflect the sediment supply and transport conditions and can range from clay to gravels. Over long timescales, alluvium can become compacted and may undergo lithification to form sedimentary rocks; most alluvium, however, remains unconsolidated and forms fertile soils and aquifers.

Alluvial deposits are globally widespread in river valleys and are responsible for major landforms such as floodplains, alluvial fans at mountain fronts, and deltas at coasts. Their properties—particle size distribution, sorting, and layering—depend on climate, discharge regime, and sediment supply. Alluvial soils are often highly productive for agriculture due to nutrient content and good drainage, and alluvium frequently constitutes important groundwater reservoirs through aquifers and perched water tables. The study of alluvium intersects geomorphology, hydrology, and resource management, and it is distinct from colluvial deposits produced by gravity rather than running water.