River delta
A river delta is a landform shaped like a triangle, created by the deposition of sediment that is carried by a river and enters slower-moving or stagnant water.[1][2] This occurs at a river mouth, when it enters an ocean, sea, estuary, lake, reservoir, or (more rarely) another river that cannot carry away the supplied sediment. It is so named because its triangle shape resembles the uppercase Greek letter delta, Δ. The size and shape of a delta are controlled by the balance between watershed processes that supply sediment, and receiving basin processes that redistribute, sequester, and export that sediment.[3][4] The size, geometry, and location of the receiving basin also plays an important role in delta evolution.
River deltas are important in human civilization, as they are major agricultural production centers and population centers.[5] They can provide coastline defense and can impact drinking water supply.[6] They are also ecologically important, with different species' assemblages depending on their landscape position. On geologic timescales, they are also important carbon sinks.[7]
Etymology[edit]
A river delta is so named because the shape of the Nile Delta approximates the triangular uppercase Greek letter delta. 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".[8] 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.[8] Despite making comparisons to other river-systems' deltas, Herodotus did not describe them as "deltas".[8] 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.[8] According to the Greek 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" (Koinē Greek: καλεῖ δὲ τὴν νῆσον δέλτα, romanized: kalei de tēn nēson délta, lit. 'he calls the island a delta').[8] 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".[8]
As 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.[9]
The formation of a delta is complicated, multiple, and cross-cutting over time, but in a simple delta three main types of bedding may be distinguished: the bottomset beds, foreset/frontset beds, and topset beds. This three part structure may be seen in small scale by crossbedding.[25][50]
Existential threats to deltas[edit]
Human activities in both deltas and the river basins upstream of deltas can radically alter delta environments.[53] Upstream land use change such as anti-erosion agricultural practices and hydrological engineering such as dam construction in the basins feeding deltas have reduced river sediment delivery to many deltas in recent decades.[54] This change means that there is less sediment available to maintain delta landforms, and compensate for erosion and sea level rise, causing some deltas to start losing land.[54] Declines in river sediment delivery are projected to continue in the coming decades.[55]
The extensive anthropogenic activities in deltas also interfere with geomorphological and ecological delta processes.[56] People living on deltas often construct flood defences which prevent sedimentation from floods on deltas, and therefore means that sediment deposition can not compensate for subsidence and erosion. In addition to interference with delta aggradation, pumping of groundwater,[57] oil, and gas,[58] and constructing infrastructure all accelerate subsidence, increasing relative sea level rise. Anthropogenic activities can also destabilise river channels through sand mining,[59] and cause saltwater intrusion.[60] There are small-scale efforts to correct these issues, improve delta environments and increase environmental sustainability through sedimentation enhancing strategies.
While nearly all deltas have been impacted to some degree by humans, the Nile Delta and Colorado River Delta are some of the most extreme examples of the devastation caused to deltas by damming and diversion of water.[61][62]
Historical data documents show that during the Roman Empire and Little Ice Age (times where there was considerable anthropogenic pressure), there was significant sediment accumulation in deltas. The industrial revolution has only amplified the impact of humans on delta growth and retreat.[63]
Probable delta in Eberswalde crater, as seen by Mars Global Surveyor. Image in Margaritifer Sinus quadrangle.
Researchers have found a number of examples of deltas that formed in Martian lakes. Finding deltas is a major sign that Mars once had large amounts of water. Deltas have been found over a wide geographical range. Below are pictures of a few.[67]