淤泥

In the Unified Soil Classification System (USCS) and the 美國國家公路與運輸協會 Soil Classification system, the sand–silt distinction is made at the 0.075 mm particle size (i.e., material passing the #200 筲箕). Silts and clays are distinguished mechanically by their 塑性變形.

Silt is easily transported in 水 or other liquid and is fine enough to be carried long distances by air in the form of 灰塵. Thick deposits of silty material resulting from deposition by 風蝕 are often called 黃土. Silt and clay contribute to 濁度 in water. Silt is transported by 溪s or by water currents in the 洋. When silt appears as a pollutant in water the phenomenon is known as siltation.

Silt, deposited by annual floods along the 尼罗河, created the rich, fertile soil that sustained the 古埃及ian civilization. Silt deposited by the 密西西比河 throughout the 20th century has decreased due to a system of 堤, contributing to the disappearance of protective 湿地 and 離岸沙洲 in the 密西西比河三角洲 region surrounding 新奥尔良.[9]

In southeast Bangladesh, in the 諾阿卡利縣, cross dams were built in the 1960s whereby silt gradually started forming new land called "chars". The district of Noakhali has gained more than 73平方（28平方英里） of land in the past 50 years.

With Dutch funding, the Bangladeshi government began to help develop older chars in the late 1970s, and the effort has since become a multi-agency operation building roads, 涵洞s, embankments, cyclone shelters, toilets and ponds, as well as distributing land to settlers. By fall 2010, the program will have allotted some 100平方（20,000英畝） to 21,000 families.[10]

A main source of silt in urban rivers is disturbance of soil by 建筑施工 activity.[11] A main source in rural rivers is 侵蚀作用 from plowing of farm fields, 皆伐 or 刀耕火耨 treatment of 森林s.