A river delta is a triangular landform created by the deposition of the sediments that are carried by the waters of a river , where the river merges with a body of slow-moving water or with a body of stagnant water. The creation of a river delta occurs at the river mouth , where the river merges into an ocean , a sea , or an estuary , into a lake , a reservoir , or (more rarely) into another river that cannot carry away the sediment supplied by the feeding river. Etymologically, the term river delta derives from the triangular shape (Δ) of the uppercase Greek letter delta . In hydrology , the dimensions of a river delta are determined by the balance between the watershed processes that supply sediment and the watershed processes that redistribute, sequester, and export the supplied sediment into the receiving basin.
61-594: The Gediz Delta is the river delta at the confluence of the Gediz River with the Gulf of İzmir , in İzmir Province in western Turkey . It is a 14,900 ha area of land that occupies coastal parts of Foça , Menemen , and Çiğli districts. It is one of the largest areas of coastal wetlands in Turkey and has a biodiversity of plants and birds. It is a Ramsar site since 1998 and an Important Bird Area since 2000. It
122-432: A density current that deposits its sediments as turbidites . When the river water is less dense than the basin water, as is typical of river deltas on an ocean coastline, the delta is characterized by hypopycnal flow in which the river water is slow to mix with the denser basin water and spreads out as a surface fan. This allows fine sediments to be carried a considerable distance before settling out of suspension. Beds in
183-409: A clay platelet has a slight negative charge where the edge has a slight positive charge when two platelets come into close proximity with each other the face of one particle and the edge of the other are electrostatically attracted." Flocs then have a higher combined mass which leads to quicker deposition through a higher fall velocity, and deposition in a more shoreward direction than they would have as
244-557: A cloudy water column which travels under the tidal influence as the wave orbital motion is in equilibrium. The Null-point hypothesis has been quantitatively proven in Akaroa Harbour, New Zealand, The Wash , U.K., Bohai Bay and West Huang Sera, Mainland China, and in numerous other studies; Ippen and Eagleson (1955), Eagleson and Dean (1959, 1961) and Miller and Zeigler (1958, 1964). Large-grain sediments transported by either bedload or suspended load will come to rest when there
305-493: A common location for civilizations to flourish due to access to flat land for farming, freshwater for sanitation and irrigation , and sea access for trade. Deltas often host extensive industrial and commercial activities, and agricultural land is frequently in conflict. Some of the world's largest regional economies are located on deltas such as the Pearl River Delta , Yangtze River Delta , European Low Countries and
366-495: A delta but enter into the sea in the form of an estuary . Notable examples include the Gulf of Saint Lawrence and the Tagus estuary. In rare cases, the river delta is located inside a large valley and is called an inverted river delta . Sometimes a river divides into multiple branches in an inland area, only to rejoin and continue to the sea. Such an area is called an inland delta , and often occurs on former lake beds. The term
427-519: A depth of −13 m relative to mean sea level at the 9 km point down the transect of the central axis. The predominant storm wave energy has unlimited fetch for the outer harbour from a southerly direction, with a calmer environment within the inner harbour, though localised harbour breezes create surface currents and chop influencing the marine sedimentation processes. Deposits of loess from subsequent glacial periods have in filled volcanic fissures over millennia, resulting in volcanic basalt and loess as
488-533: A fan. The more often the flow changes course, the shape develops closer to an ideal fan because more rapid changes in channel position result in a 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
549-399: A few main distributaries. Once a wave-dominated or river-dominated distributary silts up, it is abandoned, and a new channel forms elsewhere. In a tidal delta, new distributaries are formed during times when there is a lot of water around – such as floods or storm surges . These distributaries slowly silt up at a more or less constant rate until they fizzle out. A tidal freshwater delta is
610-545: A flat arid area splits into channels that evaporate as it progresses into the desert. The Okavango Delta in Botswana is one example. See endorheic basin . The generic term mega delta can be used to describe very large Asian river deltas, such as the Yangtze , Pearl , Red , Mekong , Irrawaddy , Ganges-Brahmaputra , and Indus . The formation of a delta is complicated, multiple, and cross-cutting over time, but in
671-573: A hypocynal delta dip at a very shallow angle, around 1 degree. Fluvial-dominated deltas are further distinguished by the relative importance of the inertia of rapidly flowing water, the importance of turbulent bed friction beyond the river mouth, and buoyancy . Outflow dominated by inertia tends to form Gilbert-type deltas. Outflow dominated by turbulent friction is prone to channel bifurcation, while buoyancy-dominated outflow produces long distributaries with narrow subaqueous natural levees and few channel bifurcations. The modern Mississippi River delta
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#1732783393458732-439: A mature delta with a distributary network. Another way these distributary networks form is from the 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
793-423: A sedimentary deposit formed at the boundary between an upland stream and an estuary, in the region known as the "subestuary". 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
854-447: 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 on small scale by crossbedding . Human activities in both deltas and the river basins upstream of deltas can radically alter delta environments. Upstream land use change such as anti-erosion agricultural practices and hydrological engineering such as dam construction in
915-795: Is 26 km from İzmir city center. The delta is home to more than 250 bird species and hosts approximately 80,000 birds in winter. It provides shelter for 10% of the world's flamingo population. Remains of the former Ionian town of Leucae is located on the delta. Çamaltı Salt Pan, which is located on the delta, provides one third of Turkey's salt production. River delta River deltas are important in human civilization , as they are major agricultural production centers and population centers. They can provide coastline defence and can impact drinking water supply. They are also ecologically important, with different species' assemblages depending on their landscape position. On geologic timescales , they are also important carbon sinks . A river delta
976-551: Is a good example of a fluvial-dominated delta whose outflow is buoyancy-dominated. Channel abandonment has been frequent, with seven distinct channels active over the last 5000 years. Other fluvial-dominated deltas include the Mackenzie delta and the Alta delta. A Gilbert delta (named after Grove Karl Gilbert ) is a type of fluvial-dominated delta formed from coarse sediments, as opposed to gently sloping muddy deltas such as that of
1037-432: 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. Deposition (geology) Deposition is the geological process in which sediments , soil and rocks are added to a landform or landmass . Wind, ice, water, and gravity transport previously weathered surface material, which, at the loss of enough kinetic energy in
1098-506: Is because sediment grain size analysis throughout a profile allows inference into the erosion or accretion rates possible if shore dynamics are modified. Planners and managers should also be aware that the coastal environment is dynamic and contextual science should be evaluated before the implementation of any shore profile modification. Thus theoretical studies, laboratory experiments, numerical and hydraulic modelling seek to answer questions pertaining to littoral drift and sediment deposition,
1159-591: Is called by the Ionians ", including describing the outflow of silt into the sea and the convexly curved seaward side of the triangle. Despite making comparisons to other river systems deltas, Herodotus did not describe them as "deltas". 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. According to
1220-404: Is due to the influence of hydraulic energy, resulting in a seaward-fining of sediment particle size, or where fluid forcing equals gravity for each grain size. The concept can also be explained as "sediment of a particular size may move across the profile to a position where it is in equilibrium with the wave and flows acting on that sediment grain". This sorting mechanism combines the influence of
1281-468: Is insufficient bed shear stress and fluid turbulence to keep the sediment moving; with the suspended load this can be some distance as the particles need to fall through the water column. This is determined by the grain's downward acting weight force being matched by a combined buoyancy and fluid drag force and can be expressed by: Downward acting weight force = Upward-acting buoyancy force + Upward-acting fluid drag force where: In order to calculate
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#17327833934581342-407: Is known as the hypothesis of asymmetrical thresholds under waves; this describes the interaction between the oscillatory flow of waves and tides flowing over the wave ripple bedforms in an asymmetric pattern. "The relatively strong onshore stroke of the waveforms an eddy or vortex on the lee side of the ripple, provided the onshore flow persists, this eddy remains trapped in the lee of the ripple. When
1403-448: Is made up partly of the microscopic calcium carbonate skeletons of marine plankton , the deposition of which induced chemical processes ( diagenesis ) to deposit further calcium carbonate. Similarly, the formation of coal begins with the deposition of organic material, mainly from plants, in anaerobic conditions. The null-point hypothesis explains how sediment is deposited throughout a shore profile according to its grain size. This
1464-548: 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". Herodotus 's description of Egypt in his Histories mentions the Delta fourteen times, as "the Delta, as it
1525-662: The Greater Tokyo Area . The Ganges–Brahmaputra Delta , which spans most of Bangladesh and West Bengal and empties into the Bay of Bengal , is the world's largest delta. The Selenga River delta in the Russian republic of Buryatia is the largest delta emptying into a body of fresh water, in its case Lake Baikal . Researchers have found a number of examples of deltas that formed in Martian lakes . Finding deltas
1586-532: 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. 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. The industrial revolution has only amplified the impact of humans on delta growth and retreat. Ancient deltas benefit
1647-700: 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'). The Roman author Arrian 's Indica states that "the delta of
1708-503: The Mississippi. For example, a mountain river depositing sediment into a freshwater lake would form this kind of delta. It is commonly a result of homopycnal flow. Such deltas are characterized by a tripartite structure of topset, foreset, and bottomset beds. 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
1769-472: 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 lesser 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
1830-1256: The basins feeding deltas have reduced river sediment delivery to many deltas in recent decades. 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. Declines in river sediment delivery are projected to continue in the coming decades. The extensive anthropogenic activities in deltas also interfere with geomorphological and ecological delta processes. 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 , oil , and gas , and constructing infrastructure all accelerate subsidence , increasing relative sea level rise. Anthropogenic activities can also destabilise river channels through sand mining , and cause saltwater intrusion . 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,
1891-422: 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 gradient of the river channel decreases, the amount of shear stress on the bed decreases, which results in the deposition of sediment within
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1952-562: The central axis of the harbour, or if classified into grain class sizes, "the plotted transect for the central axis goes from silty sands in the intertidal zone to sandy silts in the inner nearshore, to silts in the outer reaches of the bays to mud at depths of 6 m or more". See figure 2 for detail. Other studies have shown this process of the winnowing of sediment grain size from the effect of hydrodynamic forcing; Wang, Collins and Zhu (1988) qualitatively correlated increasing intensity of fluid forcing with increasing grain size. "This correlation
2013-450: The channel and a rise in the channel bed relative to the floodplain . This destabilizes the river 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. Typically, when the river switches channels in this manner, some of its flow remains in the abandoned channel. Repeated channel-switching events build up
2074-427: The down-slope gravitational force of the profile and forces due to flow asymmetry; the position where there is zero net transport is known as the null point and was first proposed by Cornaglia in 1889. Figure 1 illustrates this relationship between sediment grain size and the depth of the marine environment. The first principle underlying the null point theory is due to the gravitational force; finer sediments remain in
2135-406: The drag coefficient, the grain's Reynolds number needs to be discovered, which is based on the type of fluid through which the sediment particle is flowing, laminar flow, turbulent flow or a hybrid of both. When the fluid becomes more viscous due to smaller grain sizes or larger settling velocities, the prediction is less straightforward and it is applicable to incorporate Stokes Law (also known as
2196-619: The economy due to their well-sorted sand and gravel . Sand and gravel are often quarried from these old deltas and used in concrete for highways , buildings, sidewalks, and landscaping. More than 1 billion tons of sand and gravel are produced in the United States alone. Not all sand and gravel quarries are former deltas, but for ones that are, much of the sorting is already done by the power of water. Urban areas and human habitation tend to be located in lowlands near water access for transportation and sanitation . This makes deltas
2257-559: The fines are suspended and reworked aerially offshore leaving behind lag deposits of the main bivalve and gastropod shells separated out from the finer substrate beneath, waves and currents then heap these deposits to form chenier ridges throughout the tidal zone, which tend to be forced up the foreshore profile but also along the foreshore. Cheniers can be found at any level on the foreshore and predominantly characterise an erosion-dominated regime. The null point theory has been controversial in its acceptance into mainstream coastal science as
2318-414: 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 is deposited as alluvium , which builds up to form the river delta. Over time, this single channel builds a deltaic lobe (such as
2379-414: The flow reverses, the eddy is thrown upwards off the bottom and a small cloud of suspended sediment generated by the eddy is ejected into the water column above the ripple, the sediment cloud is then moved seaward by the offshore stroke of the wave." Where there is symmetry in ripple shape the vortex is neutralised, the eddy and its associated sediment cloud develops on both sides of the ripple. This creates
2440-410: The fluid, is deposited, building up layers of sediment. This occurs when the forces responsible for sediment transportation are no longer sufficient to overcome the forces of gravity and friction , creating a resistance to motion; this is known as the null-point hypothesis. Deposition can also refer to the buildup of sediment from organically derived matter or chemical processes . For example, chalk
2501-506: The fresh stream feeding the head of tidal propagation. As a result, the tributaries are considered to be "subestuaries". The origin and evolution of a tidal freshwater delta involves processes that are typical of all deltas as well as processes that are unique to the tidal freshwater setting. 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
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2562-531: The frictional force, or drag force) of settling. The cohesion of sediment occurs with the small grain sizes associated with silts and clays, or particles smaller than 4ϕ on the phi scale. If these fine particles remain dispersed in the water column, Stokes law applies to the settling velocity of the individual grains, although due to seawater being a strong electrolyte bonding agent, flocculation occurs where individual particles create an electrical bond adhering each other together to form flocs. "The face of
2623-466: The geomorphology and ecosystem. Deltas are typically classified according to the main control on deposition, which is a combination of river, wave , and tidal processes, depending on the strength of each. 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. Fluvial-dominated deltas are found in areas of low tidal range and low wave energy. Where
2684-451: The individual fine grains of clay or silt. Akaroa Harbour is located on Banks Peninsula , Canterbury, New Zealand , 43°48′S 172°56′E / 43.800°S 172.933°E / -43.800; 172.933 . The formation of this harbour has occurred due to active erosional processes on an extinct shield volcano, whereby the sea has flooded the caldera, creating an inlet 16 km in length, with an average width of 2 km and
2745-406: The lake bottom beyond this steep slope as more gently dipping bottomset beds. Behind the delta front, braided channels deposit the gently dipping beds of the topset on the delta plain. While some authors describe both lacustrine and marine locations of Gilbert deltas, others note that their formation is more characteristic of the freshwater lakes, where it is easier for the river water to mix with
2806-594: The lakewater faster (as opposed to the case of a river falling into the sea or a salt lake, where less dense fresh water brought by the river stays on top longer). Gilbert himself first described this type of delta on Lake Bonneville in 1885. 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 . In wave-dominated deltas, wave-driven sediment transport controls
2867-583: 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 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 . River deltas form when a river carrying sediment reaches a body of water, such as a lake, ocean, or a reservoir . When
2928-478: The main sediment types available for deposition in Akaroa Harbour Hart et al. (2009) discovered through bathymetric survey, sieve and pipette analysis of subtidal sediments, that sediment textures were related to three main factors: depth, distance from shoreline, and distance along the central axis of the harbour. This resulted in the fining of sediment textures with increasing depth and towards
2989-524: The onset of or changes in historical land use, especially deforestation , intensive agriculture , and urbanization . 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. Other rivers, particularly those on coasts with significant tidal range , do not form
3050-487: 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
3111-579: The river delta, causing the delta to retreat. For deltas that form further upriver in an estuary, there are complex yet quantifiable linkages between winds, tides, river discharge, and delta water levels. Erosion is also an important control in tide-dominated deltas, such as the Ganges Delta , which may be mainly submarine, with prominent sandbars and ridges. This tends to produce a "dendritic" structure. Tidal deltas behave differently from river-dominated and wave-dominated deltas, which tend to have
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#17327833934583172-414: The river water is nearly equal in density to the basin water, the delta is characterized by homopycnal flow , in which the river water rapidly mixes with basin water and abruptly dumps most of its sediment load. Where the river water has a higher density than basin water, typically from a heavy load of sediment, the delta is characterized by hyperpycnal flow in which the river water hugs the basin bottom as
3233-399: The sediment is never piled up in thick sequences due to the sediment traveling and depositing in deep subduction trenches. At the mouth of a river, the change in flow conditions can cause the river to drop any sediment it is carrying. This sediment deposition can generate a variety of landforms, such as deltas, sand bars, spits, and tie channels. Landforms at the river mouth drastically alter
3294-514: The sediment source. 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. Active margin river deltas tend to be much smaller and less abundant but may transport similar amounts of sediment. However,
3355-424: The shape of the delta, and much of the sediment emanating from the river mouth is deflected along the coastline. The relationship between waves and river deltas is quite variable and largely influenced by the deepwater wave regimes of the receiving basin. With a high wave energy near shore and a steeper slope offshore, waves will make river deltas smoother. Waves can also be responsible for carrying sediments away from
3416-438: The theory operates in dynamic equilibrium or unstable equilibrium, and many fields and laboratory observations have failed to replicate the state of a null point at each grain size throughout the profile. The interaction of variables and processes over time within the environmental context causes issues; "a large number of variables, the complexity of the processes, and the difficulty in observation, all place serious obstacles in
3477-656: The trailing edges of passive margins due to the majority of large rivers such as the Mississippi , Nile , Amazon , Ganges , Indus , Yangtze , and Yellow River discharging along passive continental margins. This phenomenon is due mainly to three factors: topography , basin area, and basin elevation. 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. Topography along active margins tends to be steeper and less widespread, which results in sediments not having
3538-400: The water column for longer durations allowing transportation outside the surf zone to deposit under calmer conditions. The gravitational effect or settling velocity determines the location of deposition for finer sediments, whereas a grain's internal angle of friction determines the deposition of larger grains on a shore profile. The secondary principle to the creation of seaward sediment fining
3599-405: The way of systematisation, therefore in certain narrow fields the basic physical theory may be sound and reliable but the gaps are large" Geomorphologists, engineers, governments and planners should be aware of the processes and outcomes involved with the null point hypothesis when performing tasks such as beach nourishment , issuing building consents or building coastal defence structures. This
3660-544: Was demonstrated at the low energy clayey tidal flats of Bohai Bay (China), the moderate environment of the Jiangsu coast (China) where the bottom material is silty, and the sandy flats of the high energy coast of The Wash (U.K.)." This research shows conclusive evidence for the null point theory existing on tidal flats with differing hydrodynamic energy levels and also on flats that are both erosional and accretional. Kirby R. (2002) takes this concept further explaining that
3721-732: Was first coined by Alexander von Humboldt for the middle reaches of the Orinoco River , which he visited in 1800. Other prominent examples include the Inner Niger Delta , Peace–Athabasca Delta , the Sacramento–San Joaquin River Delta , and the Sistan delta of Iran. The Danube has one in the valley on the Slovak–Hungarian border between Bratislava and Iža . In some cases, a river flowing into
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