intro to coasts / tectonic classification / hydrographic regime / waves /tides / beaches / barrier islands / inlets / deltas

Definitions and concepts:

Delta (def.):   A subaqueous and subaerial accumulation of river-derived sediments at the mouth of a river. The receiving basin (fig. 1) is the base level for the fluvial system and may be an ocean, sea or lake. Once base level is reached deposition results from the loss of momentum and the ability of the river to carry sediment. The term was first applied (circa 450 BC) to the by the Greek writer Herodotus who first notedthat the shape of the Nile River delta resembled that of the capital Greek letter delta. Deltas form wherever sediment at a river mouth is being supply faster than it can be removed by marine processes. Most deltas do not exhibit the classic deltoid form but have a wide range of morphologies and features that reflect the environment of the receiving basin.

• Numerous variables, such as the hydrographic regime of the receiving basin, influence delta formation As a result delta morphology is extremely variable. (For a summary of these variables see Coleman, and Prior, 1980 )
  
  
• Importance of deltas: Deltas have historically been fertile regions for agriculture, and harbor 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 to subsidence and exacerbated the erosion related to increasing sea level. Engineering in the drainage basin to divert waters to more arid agricultural regions has reduced both water and sediment discharge at river mouths further increasing rates of erosion.
  
  
• Freshwater (Gilbert-type) deltas formed by homopycnal inflow into a low energy receiving basin typically have steep delta fronts and clearly defined topset, foreset and bottomset beds. Marine deltas are influence by hypopycnal flow, where a buoyant plume of freshwater spread out over the denser salt water. Sediment is also redistributed by wave, tides and littoral currents.

Physiographic zones of a marine delta plain

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

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

• Upper delta plain: above the influence of marine processes
• Lower delta plain: between low tide and high tide limits
  • dominated by marine and fluvial processes
  • width of the lower delta plane is governed by tidal range

Subaqueous deltaic plain

• Prodelta: seaward-most portion of the subaqueous delta (analogous to bottomset beds in a Gilbert-type delta)
• typically composed of clay and silt
• Delta front: landward portion (analogous to the region of foreset deposition)
• typically composed of silt and sand

Active delta plain: occupied by active distributaries

• The area of active building dominated by fluvial and marine processes

Abandoned delta plain: not longer receives sediment from distributaries

• Dominated by marine processes and subsidence
• Erosion and redistribution of sediment occurs in areas of high wave and/or tidal energy. Through erosion and subsidence the region may return to open marine conditions.

Figure 1. Physiographic zones of a delta. Modified from Coleman and Wright (1971). Note sediments from the abandoned delta front have been reworked into barrier islands. Environmental Note: In the Mississippi River Delta subsidence is responsible for the formation of the back barrier bay. Eventually subsidence, which in the Gulf is also exacerbated by the pumping of oil, will outpace the ability of the barriers to retreat and the island will be destroyed.

Features of the delta plain:

The occurrence and characteristics of each of these features depends on climate and hydrographic regime

• Distributaries: One of a number of bifurcating channels that carry sediment to the delta front.
  • distributary systems develops when channel deposition caused by decreased momentum forms distributary mouth bars that cause the channel to bifurcate
• Distributor mouth bars: Sandy deposits immediately seaward of each distributary mouth
  • In the seaward direction these deposits grade into the silty delta front and pro delta clays
• Interdistributary regions: may include bays, marsh, salt pans, dune fields, beach ridges, mangrove forests, etc. depending on hydrographic regime and climate
• Natural levees: Narrow elevated margins of channels.
• Crevasse splays
• Beach ridges
• Mud flats

Factors influencing delta morphology

Climate:

In addition to influencing reverie processes, the climactic setting of the delta plain governs vegetation 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

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

1. Water discharge
2. Sediment yield

Energy of the receiving basin

1. waves, and littoral currents: These currents redistribute sediment parallel to the coast and with the aid of wind create beach ridges.
   
   
2. Tides: Tidal currents carrying sediment perpendicular to the coast and also increase the area over which waves and do their work.

Characteristics of the receiving basin

1. Shelf width and slope: These characteristics influence how readily sediment is lost to the floor of the ocean basin. A broad shallow delta, such as the Niger River delta, could only develop on a broad stable shelf.
   
   
2. Tectonics: subsiding or stable platform. A subsiding basin (e.g. Mississippi embayment) tends to stabilize the location of the depocenter and decrease the frequency and distance of delta lobe switching., known as avulsion. The elongate birdfoot 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 Apalachicola basin. Now much of the sediment that would normally feed the delta plain is being carried beyond the continental shelf.
   
   
3. Receiving basin geometry and region of sediment input (axial for from the sides)

Delta Classifications

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

• Scott (1969)
• Fisher and others (1969)
• Galloway (1975)
• Coleman, J.M., and Prior, D.B., 1980
• 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 in figure 3 stresses the importance of waves, tides, and sediment input. The classification of Coleman and Prior, 1980 (to be discussed in class), which defines six difference delta models, the most comprehension and incorporates other variables such as climate, topography and tectonics of the receiving basin, littoral currents the other factors mentioned above.

Table 1: River deltas of the world. Compiled from Coleman 1968, Coleman and Prior 1980, Davies 1996 and other sources.

River Country Ad (103km) Delta area (km2) Q mean (cms) Sed Q x106 metric tons/yr Tidal range (m) Wave Power x107ergs climate of delta area Receiving basin (Coast) Amazon Brazil 5,877.5 467,078 149,736 12001 4.90 .1930 humid tropical Atlantic (Am-TEC) Orinoco Venezuela 951 5,440 12,658 210 1.8   humid tropical (Am-TEC) Rio de la Plata-Parana Argentina 4,144 14,245 22,000 150 1.00   humid subtropical Atlantic (Am-TEC) Sao Francisco Brazil 602 734 3,420 6 1.86 30.415 humid tropical Atlantic (Am-TEC) Parana Brazil 2,872 5,440 12,658   .64   humid tropical Atlantic (Am-TEC) Magdalena Columbia 252 1,689 7,500 -- 1.1 206.250 tropical steppe Caribbean (MSC) Congo Zaire 3,457 2,072 41300 71 1.70   humid tropical Atlantic (Af-TEC) Nile Egypt 2,716 12,512 1,480 past>50 now 0 0.50 10.250 subtropical desert Mediterranean Niger Nigeria 1,113 19,135 8,769 5 1.4 2.007 humid tropical Atlantic (Af-TEC) Senegal Senegal 196.4 4300 868   1.22 112.43 dry subtropical Atlantic (Af-TEC) Tana Kenya 63.5 3,659 172   2.9   subtropical   Shatt al-Arab (Tigris-Euphrates) Iraq 4,62 18,497 1,300 62 2.5   dry subtropical Persian Gulf Mississippi U.S. 3,344 28,568 15,631 495 0.50 .034 temperate marine Gulf of Mexico (MSC) Colorado Mexico   7770     >10       Rio Grande Mexico /U.S.   8000   17     semiarid midlatitude Gulf of Mexico (MSC) Yukon U.S. Alaska 828 5,802 5900 --- 1.20   subarctic Bering Sea (Am-TEC) St. Lawrence Canada 1463 estuary 10200 --- ---   subarctic-humid mid latitude Gulf of St. Lawrence/Atlantic Mackenzie Canada 1,448 8,506 8,583 --- 0.34   tundra/arctic Beaufort Sea/Arctic Ocean Copper U.S. Alaska   584 mi2             Nelson Canada 1,072 --- 2300 --- 5.20   subarctic Hudson Bay (Am-TEC) Amur Russia 1,855 estuary 12400 20 2.30   subarctic Sea of Okhotsk (MSC) Ob-Urtysh Russia 2,975 2,849 15800 20 0.70   subarctic Obskaka Guba/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   dry subtropic Caspian Sea Dnepr Russia 801 640 1370 9 0.09 -- dry subtropical   Amu Darya (Oxus) River Russia     1500 5     dry subtropical Aral Sea Pearl (Hsi)                   Yangtze * China 1,959 estuary 34000 478 4.20   humid subtropical East China Sea (MSC) Huang Ho (Yellow River) China 865 36,272 2,571 2,080 1.13 .218 dry subtropical Yellow Sea (MSC) Mekong * * S. Vietnam 516 93,781 14,168 187 2.6   humid tropical South China Sea (MSC) Red N. Vietnam 144 11,908 3,913   1.9   humid subtropical   Chao Phraya Thailand 92 11,329 831   2.4 .736     Klang Malaysia .9 .1,817 1,100   4.2 .218 humid tropical   Indus * Rann of Kutch W. Pakistan 888 29.524 4,274 480 2.6 14.15     Ganges-Brahmaputra* Bangladesh 1,597 105,641 34,500 1,670 3.60 .586 humid tropical Bay of Bengal/Indian Ocean Irrawaddy Burma 342 21,00 12,558 285 2.7 .193 humid tropical Bay of Bengal/Indian Ocean (MSC) Mahakam Borneo   5000   8       Magassar Straights Murray-Darling Australia 1,057 ---   35 2.80   dry subtropical Great Australian Bight/Indian Oceab 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 Cambridge Gulf/Timor Sea Danube Romania 713 2740 6250 80 0.09 .034 humid continental Black Sea                     Rhine Netherlands 160 --- 2200 --- 5.50   marine west coast North Sea (Am-TEC) Rhone France 96 1,683 1700 --- 0.20   Mediterranean, subtropical Mediterranean Sea Po Italy 847 13,398 1,484 61 .73   Dry subtropical Adriatic Sea Ebro Spain 90 624 552   0 1.55 Dry subtropical Mediterranean Sea 1 Davis, R.A, Jr. 1997

Figure 2. Ternary diagram classifying deltas using the relative influence of wave, tide and reverie processes.From Seybold and others (2007) modified after Galloway, 1975)

Distribution of deltas 90% of major rivers (Ad>105km2) debauch along: Amero-trailing edge coasts (46.6%) Marginal coasts (34.5%) Afro-trailing edge coasts (8.6%)  Modern Challenges Wetland loss Agricultural development Urbanization Erosion related to subsidence (fluid withdrawal) climate change (rising SL, decreased  precipitation) decrease in sediment supply (levees, dams, etc.)
	Figure 3. Deltas of the world. Sediment discharge values from Davis, 1980.

  Recommended Reading

• Galloway,W.E., 1975, Process framework for describing the morphological and stratigraphic evolution of deltaic depositional systems: in Broussard, M.L., ed., Deltas, 2nd. ed: Houston Geol. Soc., Houston, Texas, p. 87-98.
• Coleman, J.M., 1981, Deltas: Processes of deposition and models for exploration (2nd ed.): Minneapolis, MN, Burgess Publishing Company 124 p.
• Coleman, J.M., and Prior, D.B., 1980, Deltaic Sand Bodies: Amer. Assoc. Petrol. Geol., Continuing Education Course Note Series No. 15, 171 p.
• Coleman,, J.M., and Wright, L.D., 1971, Analysis of major river systems and their deltas, procedures and rationale, with two examples: Louisiana State Univer., Coastal Studies Inst. Tech. Rept. 95, 125 p.
• Wright, L.D., 1985, River deltas, in Davis, Jr., R. A., ed., Coastal sedimentary environments (2nd ed.): New York, NY, Springer Verlag, p. 1-76.
• Wright, L.D., and Coleman, J. M., 1973, Variations in morphology of major river deltas as a functuion of ocean wave and river discharge regimes: Amer. Assoc. Petrol. Geol. Bull., v. 57, n. 2, p. 177-205.

Internet:

• World Deltas Network (WDN)
• Geomorphology from Space: Deltaic Landforms
• Coleman and Huh, Major world deltas, a perspective from space (pdf)