Siltation or sedimentation is a phenomenon that happens in deltas but as well in artificial lakes blocked by dams. Soil is transported when water flow in rivers and is packed in the bottom of the lakes. With time, silt layers pile up and fill up the artificial lakes them losing their power production capacity.
Dam siltation causes drought and irrigation problems. It causes also hydroelectric power production losses as well as lack of potable water. In big scale this means loss of food security and therefore migration. The biggest risks are dam failures which can case losses of human lives as well as losses in infrastructure.