Grain size criteria

In the Udden-Wentworth scale (due to Krumbein), silt particles range between ¹⁄₂₅₆ and ¹⁄₁₆ mm (3.9 to 62.5 μm), larger than clay but smaller than a sand. ISO 14688 grades silts between 0.002 mm and 0.06 mm, with clay particles being smaller. In actuality, silt is chemically distinct from clay, and unlike clay, grains of silt are approximately the same size in all dimensions; furthermore, their size ranges overlap. Clays are formed from thin plate-shaped particles held together by electrostatic forces, so present a cohesion. According to the USDA Soil Texture Classification system, the sand-silt distinction is made at the 0.05 mm particle size.^[7] The USDA system has been adopted by the Food and Agriculture Organization (FAO). In the Unified Soil Classification System (USCS) and the AASHTO Soil Classification system, the sand-silt distinction is made at the 0.075 mm particle size (i.e. material passing the #200 sieve). Silts and clays are distinguished by their plasticity.

Environmental impacts

Silt can occur as a deposit or as material transported by a stream or by a current in the ocean. Silt is easily transported in water and is fine enough to be carried long distances by air as 'dust'. Thick deposits of silty material resulting from aeolian deposition are often called loess (a German term) or limon (French). Silt and clay contribute to turbidity in water.

The main source of river siltation and storm sewer sedimentation in urban areas is disturbance of soil by construction activity. The main cause of river siltation in rural areas is erosion from extensive plowing of farm fields, clearcut logging or slash and burn treatment of tropical forests. When the total ground surface is stripped of vegetation, the upper soils are vulnerable to both wind and water erosion. In a number of regions of the earth, entire sectors of a country have been rendered unproductive; for example, on the Madagascar high central plateau, comprising approximately ten percent of that country's land area, virtually the entire landscape is sterile of vegetation, with gully erosive furrows typically in excess of 50 meters deep and one kilometer wide. Shifting cultivation is a farming system which sometimes incorporates the slash and burn method in some regions of the world. The resulting sediment load can cause extensive fish kills, hampering economic development.

Silt, deposited by annual floods along the Nile River, created the rich and fertile soil that sustained the ancient Egyptian civilization. This silt was depended on for this purpose. A decrease in silt deposited by the Mississippi River throughout the 20th century due to a system of levees has contributed to the disappearance of protective wetlands and barrier islands in the delta region surrounding New Orleans.[1]