River delta
Landform formed by sediment deposition at a river mouth.
A river delta is a landform, typically triangular, created by the deposition of sediments carried by a river where it merges with a body of slow-moving or stagnant water, such as an ocean, sea, estuary, lake, reservoir, or another river. River deltas are important in human civilization as major agricultural production centers and population centers, provide coastline defense, impact drinking water supply, and are ecologically important, with different species' assemblages depending on landscape position. On geologic timescales, they are also important carbon sinks.
- Etymology
- Derives from the triangular shape (Δ) of the uppercase Greek letter delta, first applied to the Nile Delta.
- First english usage
- Late 18th century, in the work of Edward Gibbon.
- Formation mechanism
- Sediment deposition when river flow expands and slows upon entering standing water.
- Primary control types
- River, wave, and tidal processes.
- Common location
- Most large river deltas discharge to intra-cratonic basins on trailing edges of passive margins.
Lore & Background
The term 'river delta' originates from the triangular shape of the Nile Delta, which resembles the uppercase Greek letter delta (Δ). Classical Athenian drama, specifically Aeschylus's tragedy Prometheus Bound, refers to the Nile Delta as the 'triangular Nilotic land,' though not as a 'delta.' Herodotus's Histories mentions the Delta fourteen times, describing it as 'the Delta, as it is called by the Ionians,' and notes the outflow of silt into the sea. The Greek historian Polybius likened the land between the Rhône and Isère rivers to the Nile Delta, but did not apply the word 'delta.' According to Strabo, the Cynic philosopher Onesicritus of Astypalaea reported that Patalene (the delta of the Indus River) was 'a delta.' 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.' As a generic term for the landform, the word 'delta' is first attested in English in the late 18th century, in the work of Edward Gibbon.
Reader's Guide
River deltas form when a river carrying sediment reaches a body of water, such as a lake, ocean, or reservoir. The flow expands, velocity decreases, and sediment drops out as alluvium, building up the delta. Over time, a single channel builds a deltaic lobe, such as the bird's-foot of the Mississippi or Ural river deltas. As the lobe advances, the river channel gradient decreases, leading to sediment deposition within the channel and destabilization. During floods, the river may breach its natural levees and switch to a new, shorter course, creating a distributary network. Alternatively, deposition of mouth bars can split the river into two distributary channels. These processes redistribute sediment, smoothing the delta's planform shape toward a fan. The more often the flow changes course, the closer the shape approaches an ideal fan. Most large river deltas, such as the Mississippi, Nile, Amazon, Ganges, Indus, Yangtze, and Yellow River, discharge along passive continental margins due to gradual topography and greater basin area, allowing sediment to accumulate. Active margins tend to have steeper topography and smaller deltas. Deltas are classified by the main control on deposition: river, wave, or tidal processes, with additional factors including landscape position and sediment grain size.
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