Why do deltas form where rivers meet the sea?

As a river reaches an ocean, lake, or large reservoir, its velocity decreases sharply and it loses the energy needed to carry sediment. Sand, silt, and clay settle out at the river mouth, creating distributary channels and a growing delta plain. Deltas develop best where sediment supply is high and waves or tides do not remove the material too quickly. Famous examples include the Nile Delta, the Mississippi Delta, and the Ganges-Brahmaputra Delta. These regions are fertile but often vulnerable to flooding, subsidence, and sea-level rise.