Delta Definitions and Concepts

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Definitions and concepts:

y Delta (def.): A subaqueous and subaerial accumulation of river-derived sediments at themouth of a river. The receiving basin (fig. 1) is the base level for the fluvial system and maybe an ocean, sea or lake. Once base level is reached deposition results from the loss ofmomentum and the ability of the river to carry sediment. The term was first applied (circa450 BC) to the by Herodotus who observed the deltoid shape Nile River delta. Deltas formwherever sediment at a river mouth is being supply faster than it can be removed by marineprocesses. Most deltas do not exhibit the classic deltoid form but have a wide range ofmorphologies and features that reflect the environment of the receiving basin.

y Numerous variables, such as the hydrographic regime of the receiving basin, influence deltaformation As a result delta morphology is extremely variable. (For a summary of thesevariables see Coleman, and Prior, 1980 )

y Im portance of deltas : Deltas have historically been fertile regions for agriculture, andharbor productive estuaries for fishing. More recently deltas have been exploited for oil ,gas, and fresh water. Over pumping of reservoirs and aquifers beneath deltas have added tosubsidence and exacerbated the erosion related to increasing sea level. Engineering in thedrainage basin to divert waters to more arid agricultural regions has reduced both waterand sediment discharge at river mouths further increasing rates of erosion.

y Freshwater ( Gilbert-type ) deltas formed by homopycnal inflow into a low energy receivingbasin typically have steep delta fronts and clearly defined topset, foreset and bottomsetbeds. Marine deltas are influence by hypopycnal flow, where a buoyant plume of freshwaterspread out over the denser salt water. Sediment is also redistributed by wave, tides andlittoral currents.


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Components of a fluvial syste m

There are four components of fluvial system. Theenvironment of each region will have some effecton the delta system.

y Drainage basin: The region drained by ariver and its tributaries. The tributariesform a collection system that transferssediment and water to higher orderstreams. Sediment and river dischargeare a function of the size, relief,tectonics, climate and geology of thedrainage basin.

y A lluvial valley: The principle region ofsediment transport through the drainagebasin. Damming and diversion ofdischarge within the alluvial valley isgreatly altering the sediment and waterdischarge to the delta.

y Delta: The region of sediment dispersaland sedimentation. Shape of the deltaplain is governed the energy of thereceiving basin and fluvial output.Avulsion (def.): Delta lobe switching.

y Receiving basin: Base level for the fluvialsystem. Slope, orientation, andhydrodynamics of the receiving basin allinfluence delta morphology.

Figure 1. Components of a river system. Modifiedfrom Coleman and Wright (1971) .

Ph ysiograp h ic zones of a marine delta plain

The principal physiograpic zones of a marine delta are outlined below and illustrated in figure 1.

y S ubaerial delta plain: region above low tide limit; built on top of subaqueous delta plain(analogous to topset beds of a Gilbert-type delta)

o U pper delta plain : above the influence of marine processes o Lower delta plain : between low tide and high tide limits

dominated by marine and fluvial processes


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y Distributaries : One of a number of bifurcating channels that carry sediment to the deltafront.

o distributary systems develops when channel deposition caused by decreasedmomentum forms distributary mouth bars that cause the channel to bifurcate

y Distributor mout h bars : Sandy deposits immediately seaward of each distributary moutho

In the seaward direction these deposits grade into the silty delta front and prodelta clays y I nterdistributary regions: may include bays, marsh, salt pans, dune fields, beach ridges,

mangrove forests, etc. depending on hydrographic regime and climate y Natural levees: Narrow elevated margins of c h annels. y Crevasse splays y Beac h ridges y M ud flats

F actors influencing t h e morph ology of a delta

Climate:

y In addition to influencing reverie processes, the climactic setting of the delta plain governsvegetation and salinity. In an arid region the delta plain may be characterized by salt flats.On the other hand in a tropical climate the same region would contain mangrove forests.

Riverine processes

y These govern how rapidly a delta can build out and withstand the onslaught of waves andother currents.

o Water discharge o Sediment yield

Energy of the receiving basin

o waves, and littoral currents: These currents redistribute sediment parallel to thecoast and with the aid of wind create beach ridges.

o Tides: Tidal currents carrying sediment perpendicular to the coast and also increasethe area over which waves and do their work.

Characteristics of the receiving basin

o Shelf width and slope: These characteristics influence how readily sediment islost to the floor of the ocean basin. A broad shallow delta, such as the Niger Riverdelta, could only develop on a broad stable shelf.


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o Tectonics: subsiding or stable platform. A subsiding basin (e.g. Mississippiembayment) tends to stabilize the location of the depocenter and decrease thefrequency and distance of delta lobe swiching., known as avulsion. The elongatebirdfoot delta of the Mississippi is in part related to local basin subsidence.However, it has also been artificially elongated by USAC projects designed to

prevent avulsion into the Appalachacola basin. Now much of the sediment that wouldnormally feed the delta plain is being carried beyond the continental shelf.

o Receiving basin geometry and region of sediment input (axial for from thesides)

Classifications of deltas

A number of classifications have evolve as geologist begin to understand all the variables influencingtheir development.

y Scott (1969) y Fisher and others (1969) y Galloway (1975) y Coleman, J.M., and Prior, D.B., 1980 y See Wright (1985)

See Hayes and Kana (1976) for review of Fisher and Galloway.

Originally used by Galloway (1975)and further developed by Wright (1985) the ternary plot infigure 3 stresses the importance of waves, tides, and sediment input. The classification of Colemanand Prior, 1980 (to be discussed in class), which defines six difference delta models, the mostcomprehension and incorporates other variables such as climate, topography and tectonics of thereceiving basin, littoral currents the other factors mentioned above.

Table 1: River deltas of the world. Compiled from Coleman 1968, Coleman and Prior 1980, Davies1996 and other sources.

R iver Country A d

(10 3km)

Deltaarea(km2)

Q mean(cms)

S ed

Q

x106 metri

ctons/ yr

T idal

range

(m)

WavePower x107 er

gs

climate ofdelta area

Receiving basin(Coast )


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Ocean

Copper U.S.Alaska 584mi2

Nelson Canada 1,072 --- 2300 --- 5.20 subarctic Hudson Bay

(Am-TEC) Amur Russia 1,855 estuar y 12400

20 2.30 subarctic Sea ofOkhotsk (MSC)

Ob-Urtysh Russia 2,975 2,849 15800 20 0.70 subarctic ObskakaGuba/Kara Sea(Am-TEC)

Yenisey(Jenisej) Russia

2,580 2,460 19000 11 0.40 tundra Kara Sea (Am-TEC)

Lena Russia 2,421 43,563 1,402 12 0.21 tundra Laptev Sea(Am-TEC)

Volga Russia 1,614 27,224 7,736 282 0 drysubtropic Caspian Sea

Dnepr Russia 801 640 1370 9 0.09 -- drysubtropical

Amu Darya

(Oxus)River

Russia 1500 5 drysubtropical Aral Sea

Pearl (Hsi)

Yangtze * China 1,959 estuar y 34000 478 4.20 humidsubtropical East China Sea(MSC)

Huang Ho China 865 36,272 2,571 2,080 1.13 .218 drysubtropical

Yellow Sea(MSC)

Mekong * * S.Vietnam 516 93,781 14,168 187 2.6 humidtropical

South ChinaSea (MSC)

Red N.Vietnam 144 11,908 3,913 1.9 humidsubtropical

ChaoPhraya Thailand

92 11,329 831 2.4 .736

Klang Malaysia .9 .1,817 1,100 4.2 .218 humidtropical

Indus *

Rann ofKutch

W.Pakistan 888

29.524 4,274 480 2.6 14.15


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Ganges-Brahmaputra*

Bangladesh 1,597

105,641

34,500 1,670

3.60 .586 humidtropical Bay ofBengal/IndianOcean

Irrawaddy Burma 342 21,00 12,558 285 2.7 .193 humidtropical

Bay ofBengal/IndianOcean (MSC)

Mahakam Borneo 5000 8 MagassarStraights

Murray-Darling Australia

1,057 --- 35 2.80 drysubtropical

GreatAustralianBight/IndianOceab

Burdekin Australia 267 2,112 476 2.2 6.414 dry tropical Coral Sea(MSC)

Ord Australia 78 3,896 166 5.8 1.062 desert CambridgeGulf/TimorSea

Danube Romania 713 2740 6250 80 0.09 .034 humidcontinental Black Sea

Rhine Netherlands 160 --- 2200 --- 5.50 marine westcoast

North Sea(Am-TEC)

Rhone France 96 1,683 1700 --- 0.20 Mediterranean,subtropical

MediterraneanSea

Po Italy 847 13,398 1,484 61 .73 Drysubtropical Adriatic Sea

Ebro Spain 90 624 552 0 1.55 Drysubtropical MediterraneanSea

1 Davis, R.A, Jr. 1997


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Figure 3. Deltas of the world. Sediment discharge values from Davis, 1980.

90% of major rivers (Ad>105km 2) debauch along:

y Amero-trailing edge coasts (46.6%) y Marginal coasts (34.5%) y Afro-trailing edge coasts (8.6%)

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Delta Definitions and Concepts