The process by which a stream or river deposits sediment, raising the elevation of its bed over time. Aggradation occurs when sediment supply exceeds the transport capacity of the flow.
Aggradation is the geomorphic process by which sediment accumulates on the bed and banks of a river channel, floodplain, or other depositional surface, progressively raising the surface elevation. It occurs when the sediment load delivered to a reach exceeds the stream's capacity to transport that material downstream, creating a net positive sediment budget. Common causes of aggradation include upstream land use changes that increase erosion and sediment supply, dam removal that releases impounded sediment, tectonic uplift in source areas, volcanic eruptions that deliver large volumes of debris, and sea level rise that reduces stream gradients near the coast. Aggradation can reduce channel capacity and increase flood risk, as the raised bed elevation forces floodwaters to spread across wider areas. In alluvial fans and deltas, aggradation is the dominant process building new land surfaces through sediment deposition. Managing aggradation is a significant challenge in reservoir operations, where sedimentation gradually reduces water storage capacity; globally, reservoirs lose an estimated 0.5-1% of storage capacity annually to sedimentation. The opposite process, where erosion lowers the channel bed, is termed degradation or incision.