Siltation of a water body is the process of accumulation of mineral, plant and animal residues on the bottom of the water body, coming from the catchment area and formed in the water body itself. Silt composition and consistency are determined by the activity of aerobic bacteria inhabiting the upper layer of silt. The rate of siltation of the water body depends on the terrain, the rate of water runoff from the catchment area, the presence of organic matter in the water, etc.

Represents water pollution dominated by silt or clay particles. It refers to both an increased concentration of suspended sediments and an increased accumulation (temporary or permanent) of fine sediments on the bottom where they are undesirable. Siltation is most often caused by soil erosion or sediment spillage.

The source of increased sediment transport to an area may be erosion on land or activity in water.

In rural areas, the source of erosion is usually soil degradation due to intensive or inadequate farming practices, resulting in soil erosion, especially on fine-grained soils such as loess. This will result in increased amounts of silt and clay in water bodies where water drains from the area. In urban areas, the source of erosion is usually construction activity, which involves clearing the original vegetation covering the land and temporarily creating something resembling an urban desert from which fine particles are easily washed out during rainstorms.

In water, the main source of pollution is sediment spillage from dredging, transportation of dredged material by barge and deposition of dredged material in or near the water. Such deposition may be carried out to get rid of unwanted material, for example by dumping material dredged from harbors and shipping channels into the sea. Sediments may also be used to reinforce shorelines, to create artificial islands or to restore beaches.

Climate change also affects the rate of siltation.

Another important cause of siltation is sewage and other sewage lagoons that are discharged into water bodies from households or businesses without septic tanks or treatment facilities.