Typically, when the river switches channels in this manner, some of its flow remains in the abandoned channel. If the river breaches its natural levees (such as during a flood), it spills out into a new course with a shorter route to the ocean, thereby obtaining a steeper, more stable gradient. As the deltaic lobe advances, the gradient of the river channel becomes lower because the river channel is longer but has the same change in elevation (see slope).Īs the gradient of the river channel decreases, the amount of shear stress on the bed decreases, which results in the deposition of sediment within the channel and a rise in the channel bed relative to the floodplain. Over time, this single channel builds a deltaic lobe (such as the bird's-foot of the Mississippi or Ural river deltas), pushing its mouth into the standing water. As a result, sediment drops out of the flow and is deposited as alluvium, which builds up to form the river delta. This flow expansion results in a decrease in the flow velocity, which diminishes the ability of the flow to transport sediment. When the flow enters the standing water, it is no longer confined to its channel and expands in width. River deltas form when a river carrying sediment reaches a body of water, such as a lake, ocean, or a reservoir. Formation Ī delta forms where a river meets a lake Īs a generic term for the landform at the mouth of river, the word delta is first attested in the English-speaking world in the late 18th century, in the work of Edward Gibbon. The Roman author Arrian's Indica states that "the delta of the land of the Indians is made by the Indus river no less than is the case with that of Egypt". ( Koinē Greek: καλεῖ δὲ τὴν νῆσον δέλτα, romanized: kalei de tēn nēson délta, lit.'he calls the island a delta'). According to the Roman geographer Strabo, the Cynic philosopher Onesicritus of Astypalaea, who accompanied Alexander the Great's conquests in India, reported that Patalene (the delta of the Indus River) was "a delta". The Greek historian Polybius likened the land between the Rhône and Isère rivers to the Nile Delta, referring to both as islands, but did not apply the word delta. Despite making comparisons to other river-systems' deltas, Herodotus did not describe them as "deltas". Herodotus's description of Egypt in his Histories mentions the Delta fourteen times, as "the Delta, as it is called by the Ionians", including describing the outflow of silt into the sea and the convexly curved seaward side of the triangle. The triangular shape of the Nile Delta was known to audiences of classical Athenian drama the tragedy Prometheus Bound by Aeschylus refers to it as the "triangular Nilotic land", though not as a "delta". They are also ecologically important, with different species' assemblages depending on their landscape position.Ī river delta is so named because the shape of the Nile Delta approximates the triangular uppercase Greek letter delta. They can provide coastline defense and can impact drinking water supply. River deltas are important in human civilization, as they are major agricultural production centers and population centers. The size, geometry, and location of the receiving basin also plays an important role in delta evolution. The size and shape of a delta is controlled by the balance between watershed processes that supply sediment, and receiving basin processes that redistribute, sequester, and export that sediment. It is so named because its triangle shape, resembles the Greek letter Delta. This occurs where a river enters an ocean, sea, estuary, lake, reservoir, or (more rarely) another river that cannot carry away the supplied sediment. Sacramento–San Joaquin (California) Delta at flood stage, early March 2009.Ī river delta is a landform shaped like a triangle, created by deposition of sediment that is carried by a river as the flow leaves its mouth and enters slower-moving or stagnant water.
0 Comments
Leave a Reply. |
Details
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |