River delta

Sacramento (California) Delta at flood stage, early-March 2009

A river delta is a landform that forms from deposition of sediment carried by a river as the flow leaves its mouth and enters slower-moving or stagnant water.[1][2] This occurs where a river enters an ocean, sea, estuary, lake, reservoir, or (more rarely) another river that cannot carry away the supplied sediment. 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.[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. They can provide coastline defense and can impact drinking water supply.[5] They are also ecologically important, with different species' assemblages depending on their landscape position.

Formation

Delta forms where river meets lake[6]

River deltas form when a river carrying sediment reaches either (1) a body of water, such as a lake, ocean, or reservoir, (2) another river that cannot remove the sediment quickly enough to stop delta formation, or (3) an inland region where the water spreads out and deposits sediments. The tidal currents also cannot be too strong, as sediment would wash out into the water body faster than the river deposits it. The river must carry enough sediment to layer into deltas over time. The river's velocity decreases rapidly, causing it to deposit the majority, if not all, of its load. This alluvium builds up to form the river delta.[7] When the flow enters the standing water, it is no longer confined to its channel and expands in width. This flow expansion results in a decrease in the flow velocity, which diminishes the ability of the flow to transport sediment. As a result, sediment drops out of the flow and deposits. 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 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).

As the slope of the river channel decreases, it becomes unstable for two reasons. First, gravity makes the water flow in the most direct course down slope. If the river breaches its natural levees (i.e., during a flood), it spills out into a new course with a shorter route to the ocean, thereby obtaining a more stable steeper slope.[8] Second, as its slope gets lower, the amount of shear stress on the bed decreases, which results in deposition of sediment within the channel and a rise in the channel bed relative to the floodplain. This makes it easier for the river to breach its levees and cut a new channel that enters the body of standing water at a steeper slope. Often when the channel does this, some of its flow remains in the abandoned channel. When these channel-switching events occur, a mature delta develops a distributary network.

Another way these distributary networks form is from deposition of mouth bars (mid-channel sand and/or gravel bars at the mouth of a river). When this mid-channel bar is deposited at the mouth of a river, the flow is routed around it. This results in additional deposition on the upstream end of the mouth-bar, which splits the river into two distributary channels. A good example of the result of this process is the Wax Lake Delta.

In both of these cases, depositional processes force redistribution of deposition from areas of high deposition to areas of low deposition. 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. Because the sediment is laid down in this fashion, the shape of these deltas approximates a fan. The more often the flow changes course, the shape develops as closer to an ideal fan, because more rapid changes in channel position results in more uniform deposition of sediment on the delta front. The Mississippi and Ural River deltas, with their bird's-feet, are examples of rivers that do not avulse often enough to form a symmetrical fan shape. Alluvial fan deltas, as seen by their name, avulse frequently and more closely approximate an ideal fan shape.

Most large river deltas discharge to intra-cratonic basins on the trailing edges of passive margins due to the majority of large rivers such as the Mississippi, Nile, Amazon, Ganges, Indus, and Yangtzee discharging along passive continental margins.[9] This phenomenon is due to three big factors: topography, basin area, and basin elevation.[9] Topography along passive margins tend to be more gradual and widespread over a greater area enabling sediment to pile up and accumulate overtime to form large river deltas. Topography along active margins tend to be steeper and less widespread, which results in sediments not having the ability to pile up and accumulate due to the sediment traveling into a steep subduction trench rather than a shallow continental shelf.

There are many other smaller factors that could explain why the majority of river deltas form along passive margins rather than active margins. Along active margins, orogenic sequences cause tectonic activity to form over-steepened slopes, brecciated rocks, and volcanic activity resulting in delta formation to exist closer to the sediment source.[9][10] When sediment does not travel far from the source, sediments that build up are coarser grained and more loosely consolidated, therefore making delta formation more difficult. Tectonic activity on active margins causes the formation of river deltas to form closer to the sediment source which may affect channel avulsion, delta lobe switching, and auto cyclicity.[10] Active margin river deltas tend to be much smaller and less abundant but may transport similar amounts of sediment.[9] However, the sediment is never piled up in thick sequences due to the sediment traveling and depositing in deep subduction trenches.[9]

Other Languages
Afrikaans: Rivierdelta
العربية: دلتا
aragonés: Delta fluvial
অসমীয়া: ব-দ্বীপ
asturianu: Delta fluvial
azərbaycanca: Delta (çay)
বাংলা: বদ্বীপ
Bân-lâm-gú: Saⁿ-kak-chiu
беларуская: Дэльта (рака)
беларуская (тарашкевіца)‎: Дэльта
भोजपुरी: नदी डेल्टा
Boarisch: Flussdelta
bosanski: Riječna delta
brezhoneg: Delta (stêr)
català: Delta fluvial
čeština: Říční delta
dansk: Floddelta
Deutsch: Flussdelta
dolnoserbski: Rěcny delta
eesti: Delta
Ελληνικά: Δέλτα ποταμού
español: Delta fluvial
Esperanto: Riverdelto
estremeñu: Delta
euskara: Ibai delta
한국어: 삼각주
հայերեն: Դելտա (գետ)
हिन्दी: डेल्टा
hornjoserbsce: Rěčny delta
hrvatski: Riječna delta
Ido: Delto
Bahasa Indonesia: Delta sungai
italiano: Delta fluviale
עברית: דלתה
ქართული: დელტა
қазақша: Өзен атырауы
Kiswahili: Delta
Kreyòl ayisyen: Dèlta
Кыргызча: Дельта
latviešu: Upes delta
lietuvių: Upės delta
македонски: Речна делта
Malagasy: Hefaka
മലയാളം: നദീമുഖം
Bahasa Melayu: Delta
монгол: Садраа
မြန်မာဘာသာ: မြစ်ဝကျွန်းပေါ်
Nederlands: Rivierdelta
日本語: 三角州
norsk: Elvedelta
norsk nynorsk: Elvedelta
oʻzbekcha/ўзбекча: Delta
polski: Delta rzeki
português: Delta
română: Deltă
русский: Дельта реки
Simple English: River delta
slovenčina: Delta (rieka)
slovenščina: Rečna delta
српски / srpski: Rečna delta
srpskohrvatski / српскохрватски: Riječna delta
suomi: Suisto
svenska: Floddelta
українська: Річкова дельта
اردو: دہانہ
Tiếng Việt: Châu thổ
ייִדיש: דעלטע
粵語: 三角洲
中文: 三角洲