alluvial

Alluvial refers to sediments, soils, or landforms deposited by running water, typically rivers and streams. The term covers loose, unconsolidated materials that have been transported and deposited by fluvial action as flow velocity decreases, allowing heavier particles to settle first and finer material to be deposited later. Alluvial deposits often show clear stratification and consist of clay, silt, sand, and gravel.

Common alluvial landforms include floodplains, alluvial fans, and deltas. Floodplains form beside rivers and are repeatedly reworked during floods, producing fertile soils but also risk of inundation. Alluvial fans develop where mountain streams emerge onto flatter terrain, spreading sediment in a fan-shaped apron. Deltas form at river mouths where sediment supply meets standing water, building outward deposits.

Alluvial soils are typically highly productive for agriculture due to nutrient richness and good depth, though they can be prone to erosion or salinization in arid regions; they also serve as important groundwater aquifers in many areas.

In archaeology and geology, alluvial sequences preserve layered records of past floods and environmental change, and stratigraphic study can reveal the timing of deposition. Alluvial processes are contrasted with related terms such as colluvial (gravity-driven), aeolian (wind-driven), and glacial (ice-driven) deposits.

Etymology: from Latin alluvium and alluvies, meaning washed against or washed in, reflecting deposition by water. See also: alluvium, floodplain, delta.