Where a river meets the sea, its current slows and drops its load of sand and silt. A delta is the low-lying landform built by that deposited sediment. Distributary channels split and rejoin, creating a characteristic fan or bird-foot shape.

The Nile, Mississippi, and Ganges-Brahmaputra deltas support dense populations because the soils are fertile and water is abundant. Yet deltas are also vulnerable to flooding, subsidence, and rising sea level. Sediment supply from upstream dams has shrunk many modern deltas.

Ecologically, deltas host rich wetlands and serve as nurseries for fish and birds. Engineers build levees and diversion structures to manage them, often with mixed results. The ongoing contest between river deposition and coastal erosion keeps deltas among the most dynamic landforms on Earth.

• Sediment landform at a river mouth
• Created by slowing current and deposition
• Often fertile and densely populated
• Vulnerable to sea-level rise and reduced sediment