A landform created by the deposition of sediment where a river enters a standing body of water such as an ocean, sea, or lake. Deltas are ecologically rich, economically important, and highly vulnerable to climate change.
A delta is a depositional landform that develops at the mouth of a river where the flow enters a standing body of water and deposits its sediment load. The reduction in flow velocity as the river spreads into the receiving basin causes sediment to settle, progressively extending the land surface seaward or lakeward. Deltas are classified by the dominant processes shaping them: river-dominated (Mississippi), wave-dominated (Nile), and tide-dominated (Ganges-Brahmaputra). Major river deltas are among the most densely populated and agriculturally productive regions on Earth; the Nile, Ganges-Brahmaputra, Mekong, and Mississippi deltas collectively support hundreds of millions of people. However, deltas face severe threats from relative sea level rise, land subsidence due to groundwater and hydrocarbon extraction, upstream dam construction that traps sediment, and coastal erosion. The Mississippi River delta has lost approximately 5,000 km² of wetland area since the 1930s due to levee construction, subsidence, and reduced sediment supply. Delta ecosystems provide critical services including storm surge protection, fisheries habitat, nutrient processing, and carbon sequestration. Sediment management and controlled river diversions are key strategies being explored for delta restoration and sustainability.