The term “delta” comes from the ancient Greek. In the fifth century B.C., Herodotus used the term to describe the Nile Delta in Egypt, as it had a similar triangular shape to the Greek letter delta (?). Deltas are land forms created at or near the mouths of rivers. They are caused by sediment, typically silt, that is eroded into a river and carried to its mouth, where the sediment is deposited.
Alluvial sediment is a term for material, typically silt (but also sand, gravel or other material), that is deposited on a land form by water action. As a stream approaches its mouth it becomes wider, and the current moves slower. This slowing of the current allows for the deposit of alluvial sediment and the creation of landforms such as deltas and alluvial fans. Alluvial sediment is particularly abundant in the runoff from a flood.
There are two types of action created by stream systems--erosion and deposition. Delta landforms are created by both actions. Alluvial sediments are eroded upstream and carried down to the stream's mouth, where they are deposited. Water velocity is slowed near a river 's mouth when it enters a level plain, especially in a large river. The slow velocity causes sediment to settle and create sediment beds. When the sediment is excessive, such as during flooding episodes, the material will clog the flow of water and eventually create a delta.
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Alluvial fans are a form of a delta where the alluvial sediment is deposited onto level land or a plain. It is distinct in that deltas are formed in a body of water, and an alluvial fan is created on land. However, the principle of sedimentation and land form creation is similar. Deltas and alluvial fans can be thought of as two variants of the same type of land form.
Delta land forms are divided into upper and lower plains. An upper delta plain consists of lagoons, bogs, floodplains and braided stream channels. Lacustrine wetlands and marshes are also commonly formed in upper deltas. Typically, the soil in an upper delta is very rich, but the area is prone to flooding. The lower delta plain is located within the tidal zone and forms a brackish (salt-water) environment. Salt marshes are a lower delta plain land form.
Major deltas are formed at the mouths of the world's largest rivers, such as the Yellow River in China, the Nile in Egypt, the Amazon in South America and the Mississippi. Culturally, the most famous delta in the world is the Nile Delta in Egypt, in the cradle of ancient Egyptian civilization. According to Science Clarified, the Mississippi Delta drains 40 percent of the continental United States and deposits an estimated 159 million tons of sediment annually. However, the Huang He (Yellow River) Delta deposits 1.6 billion tons of sediment annually.