What is a delta?
A delta is a landform at the mouth of a river, where it slows down and drops sediment as it enters a sea or lake. Over time, the sediment builds up new land.
As a river flows, it carries sand, silt and clay. When it reaches calmer water at its mouth, it slows down and can no longer carry all this material, so the sediment settles. The river often splits into many smaller channels, called distributaries, spreading sediment over a wide, fan-shaped area.
The name comes from the Greek letter delta, Δ, because the Greek historian Herodotus noticed that the Nile's mouth was shaped like a triangle. Deltas have very fertile soil and often support large populations and farms. The Ganges-Brahmaputra Delta in Bangladesh and India is the largest in the world.
Famous deltas
Other well-known deltas include the Mississippi Delta in the United States, the Mekong Delta in Vietnam and the Okavango Delta in Botswana, which flows into a desert basin rather than the sea. Many deltas are threatened by rising sea levels and dams that trap the sediment they need.
Did You Know?
The Okavango Delta is a UNESCO World Heritage Site and one of Africa's richest areas for wildlife, including elephants, lions and hippos.