River 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.

[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 the river, the word delta is first attested in the English-speaking world in the late 18th century, in the work of Edward Gibbon.

[11][12] 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.

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.

This results in additional deposition on the upstream end of the mouth bar, which splits the river into two distributary channels.

This results in the smoothing of the planform (or map-view) shape of the delta as the channels move across its surface and deposit sediment.

[16] Topography along passive margins tend to be more gradual and widespread over a greater area enabling sediment to pile up and accumulate over time to form large river deltas.

[17] Active margin river deltas tend to be much smaller and less abundant but may transport similar amounts of sediment.

This sediment deposition can generate a variety of landforms, such as deltas, sand bars, spits, and tie channels.

[citation needed] Deltas are typically classified according to the main control on deposition, which is a combination of river, wave, and tidal processes,[18][19] depending on the strength of each.

[20] The other two factors that play a major role are landscape position and the grain size distribution of the source sediment entering the delta from the river.

River water entering the lake rapidly deposits its coarser sediments on the submerged face of the delta, forming steeping dipping foreset beds.

The finer sediments are deposited on the lake bottom beyond this steep slope as more gently dipping bottomset beds.

[29] Elsewhere, similar structures occur, for example, at the mouths of several creeks that flow into Okanagan Lake in British Columbia and form prominent peninsulas at Naramata, Summerland, and Peachland.

[32] A tidal freshwater delta[33] is a sedimentary deposit formed at the boundary between an upland stream and an estuary, in the region known as the "subestuary".

[34] Drowned coastal river valleys that were inundated by rising sea levels during the late Pleistocene and subsequent Holocene tend to have dendritic estuaries with many feeder tributaries.

Each tributary mimics this salinity gradient from its brackish junction with the mainstem estuary up to the fresh stream feeding the head of tidal propagation.

[35][36] The combination of processes that create a tidal freshwater delta result in a distinct morphology and unique environmental characteristics.

Many tidal freshwater deltas that exist today are directly caused by the onset of or changes in historical land use, especially deforestation, intensive agriculture, and urbanization.

[37] These ideas are well illustrated by the many tidal freshwater deltas prograding into Chesapeake Bay along the east coastline of the United States.

Research has demonstrated that the accumulating sediments in this estuary derive from post-European settlement deforestation, agriculture, and urban development.

[48] In some cases, a river flowing into a flat arid area splits into channels that evaporate as it progresses into the desert.

In addition to interference with delta aggradation, pumping of groundwater,[59] oil, and gas,[60] and constructing infrastructure all accelerate subsidence, increasing relative sea level rise.

[62] There are small-scale efforts to correct these issues, improve delta environments and increase environmental sustainability through sedimentation enhancing strategies.

[63][64] Historical data documents show that during the Roman Empire and Little Ice Age (times when there was considerable anthropogenic pressure), there was significant sediment accumulation in deltas.

[citation needed] Urban areas and human habitation tend to be located in lowlands near water access for transportation and sanitation.