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Definition: delta from Philip's Encyclopedia

Fan-shaped body of alluvium deposited at the mouth of a river. A delta is formed when a river deposits sediment as its speed decreases while it enters the sea. Most deltas are extremely fertile areas, but are subject to frequent flooding.

Summary Article: delta
From The Hutchinson Unabridged Encyclopedia with Atlas and Weather Guide

River sediments deposited when a river flows into a standing body of water with no strong currents, such as a lake, lagoon, sea, or ocean. A delta is the result of fluvial and marine processes. Deposition is enhanced when water is saline because salty water causes small clay particles to adhere together. Other factors influencing deposition include the type of sediment, local geology, sea-level changes, plant growth, and human impact. Some examples of large deltas are those of the Mississippi, Ganges and Brahmaputra, Rhône, Po, Danube, and Nile rivers. The shape of the Nile delta is like the Greek letter delta or Δ, and gave rise to the name.

The arcuate (arc-shaped) delta of the Nile, found in areas where longshore drift keeps the seaward edge of the delta trimmed and relatively smooth, is only one form of delta. Others include birdfoot (projecting) deltas, where the river brings down enormous amounts of fine silt, as at the Mississippi delta; and tidal (linear) deltas, like that of the Mekong, China, where most of the material is swept to one side by sea currents. Cuspate deltas are pointed, and are shaped by regular opposing, gentle water movement, as seen at the Ebro and Tiber deltas in Italy.

The material deposited as a delta can be divided into three types.

Bottomset beds The lower parts of the delta are built outwards along the sea floor by turbidity currents, which are loaded with material. These bottomset beds are composed of very fine material.

Foreset beds Inclined, sloping layers of coarse material are deposited over the bottomset beds. Each bed is deposited above and in front of the previous one, the material moving by rolling and saltation. The delta thus builds up in a seaward direction.

Topset beds These are made up of fine material and are a continuation of the river's flood plain. They are extended and built up by the work of numerous distributaries formed when the main river splits into several smaller channels.


The Mississippi Delta

Landform features in drainage basins



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