Many watercourses carry heavy sediment loads, as evidenced by the formation of large deltas by the world's major rivers. For example, the annual loads of the Parana in south america and the Ganges-Brahmaputra system in Bangladesh are each approximately 250 million tons of dry solids. Silt accumulations can create navigational and other hazards, and can even divert a river from its original channel. The sediment carried by rivers can gradually fill in reservoirs, smother spawning beds, clog or damage water supply intakes and treatment plants, and foreclose recreational uses. The introduction of sediment into watercourses can result from natural causes such as heavy run-off as well as from human conduct such as overgrazing or deforestation. Water systems may be blocked, or the flow of water seriously reduced, by the deposition of fine material carried by the water. This may curtail the use of water systems for transport, for fishing, as reservoirs, for irrigation, or for drainage. Catastrophic sediment movements which disrupt agricultural patterns and transport facilities are a major result of large-scale flooding.
The incidence of silting is high when the flow of water is slow and when there is a large amount of material suspended in the water. The salinity of the water and tidal flow also affect the rate of sedimentation. The material, or silt, may arise from excessive soil erosion or maybe ordinary land run-off, or sewage. Silting occurs in natural water systems, but in man-made water systems results through bad planning and construction. The flow of water may be reduced by weeds, whose growth has been stimulated by the excess of nutrients suspended in the water. The reduced flow allows increased sedimentation, which further stimulates weed growth. Silting may be part of the complete eutrophication of a water system. Reduced flow, causing sedimentation, may also result from mechanical blockage from grills or channels being blocked by debris.
As well as directly reducing the usefulness of a water course, silting may have a number of indirect adverse effects. If drainage and irrigation channels are blocked, land may be flooded, or the water-table may rise, leading to the waterlogging of productive land, to the creation of marshland or saline or alkaline soils. Siltation in reservoirs may block the intakes of the turbines used to generate hydroelectric power cause dams to be decommissioned, for example the Sanmenxia Reservoir in China had to be decommissioned four years after it was completed and the Laoying reservoir silted up before its dam was completed. Silting by contaminated sediments and their excavation may also cause problems.
The third longest river in the world, the Yangtze in China, carries down some 530 million tonnes of silt every year. This poses one of the biggest long-term environmental and technical problems. The 600 km long semi-stationary waters upstream of a dam that is being constructed on the river are expected to be clogged by silt and polluted by a steady build-up of huge quantities of raw sewage and factory pollutants, thus becoming the "biggest open sewer in the world". Large quantities of silt will also build up behind the main dam wall itself.