Nile River delta, as seen from Earth orbit. Photo courtesy of NASA.

A delta is the mouth of a river where it flows into an ocean, sea, or lake, building outwards (as a deltaic deposit) from sediment carried by the river and deposited as the water current is dissipated. Deltaic deposits of larger, heavily-laden rivers are characterised by the river channel dividing into multiple streams (distributaries), these divide and come together again to form a maze of active and inactive channels.

Delta formation

At the final course of a river, when it enters the sea, it mixes with the surrounding water, and its velocity of flow is checked, causing it to deposit its load of gravel, sand, silt and clay. The first materials deposited are the gravel and sand, as they are by far the heaviest and coarsest. Next to be dropped is the silt. Because it is fine, the clay is transported in suspension quite far out in the sea. When salt water causes the clay to flocculate, it becomes heavier and sinks. As layers upon layers of alluvial materials are deposited, a platform of alluvium is built up and it eventually rises above the water, which can now be called the delta. The water then overflows the banks into different channels called distributaries, which build up their own levees. The vegetation that later grows on the alluvium stabilizes the delta.

Where delta formation is river-dominated and less subject to tidal or wave action, a delta may take on a multi-lobed shape which resembles a bird's foot. The Mississippi delta is an example of this type.

The most famous delta is that of the Nile River, and it is this delta from which the term is derived, because the Nile delta has a very characteristic triangular shape, like the (upper-case) Greek letter delta (Δ). Other rivers with notable deltas include the Ganges/Brahmaputra combination (this delta spans most of Bangladesh), the Niger, the Amazon, the Mississippi, the Sacramento-San Joaquin, the Rhine, the Rhône, the Danube, the Ebro, the Mekong, the Irrawaddy, the Krishna-Godavari, and the Cauvery.

In rare cases the river delta is located inside of a large valley and is called an inverted river delta.

List of deltas

• Ganges-Brahmaputra Delta
• Mekong Delta
• Mississippi Delta
• Nile Delta
• Oil Rivers (Niger)
• Sacramento River Delta