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 standing water.[1][2] This occurs where a river enters an ocean, sea, estuary, lake, reservoir, or (more rarely) another river that cannot transport 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

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. Of course, 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.[6] 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 onto a new course with a shorter route to the ocean, thereby obtaining a more stable steeper slope.[7] 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.

Types of deltas

Lower Mississippi River land loss over time
Delta lobe switching in the Mississippi Delta,  4600 yrs BP,  3500 yrs BP,  2800 yrs BP,  1000 yrs BP,  300 yrs BP,  500 yrs BP,  current

Deltas are typically classified according to the main control on deposition, which is a combination of river, wave, and tidal processes,[8] depending on the strength of each.[9] 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.[10]

Wave-dominated deltas

In wave dominated deltas, wave-driven sediment transport controls the shape of the delta, and much of the sediment emanating from the river mouth is deflected along the coast line.[8] 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 the river delta, causing the delta to retreat.[5] For deltas that form further upriver in an estuary, there are complex yet quantifiable linkages between winds, tides, river discharge, and delta water levels.[11][12]

The Ganges Delta in India and Bangladesh is the largest delta in the world and it is also one of the most fertile regions in the world.

Tide-dominated deltas

Erosion is also an important control in tide dominated deltas, such as the Ganges Delta, which may be mainly submarine, with prominent sand bars and ridges. This tends to produce a "dendritic" structure.[13] Tidal deltas behave differently from river- and wave-dominated deltas, which tend to have a few main distributaries. Once a wave- or river- distributary silts up, it is abandoned, and a new channel forms elsewhere. In a tidal delta, new distributaries are formed during times when there's a lot of water around – such as floods or storm surges. These distributaries slowly silt up at a pretty constant rate until they fizzle out.[13]

Gilbert deltas

A Gilbert delta (named after Grove Karl Gilbert) is a specific type of delta formed from coarse sediments, as opposed to gently-sloping muddy deltas such as that of the Mississippi. For example, a mountain river depositing sediment into a freshwater lake would form this kind of delta.[14] [15] While some authors describe both lacustrine and marine locations of Gilbert deltas,[14] others note that their formation is more characteristic of the freshwater lakes, where it is easier for the river water to mix with 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).[16]

G.K. Gilbert himself first described this type of delta on Lake Bonneville in 1885.[16] Elsewhere, similar structures occur, for example, at the mouths of several creeks that flow into Okanagan Lake in British Columbia and forming prominent peninsulas at Naramata (49°35′30″N 119°35′30″W / 49.59167°N 119.59167°W / 49.59167; -119.59167), Summerland (49°34′23″N 119°37′45″W / 49.57306°N 119.62917°W / 49.57306; -119.62917), or Peachland (49°47′00″N 119°42′45″W / 49.78333°N 119.71250°W / 49.78333; -119.71250).

Tidal freshwater deltas

A tidal freshwater delta[17] is a sedimentary deposit formed at the boundary between an upland stream and an estuary, in the region known as the "subestuary".[18] 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 their brackish junction with the mainstem estuary up to 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[4] as well as processes that are unique to the tidal freshwater setting.[19][20] 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 the onset of or changes in historical land use, especially deforestation, intensive agriculture, and urbanization.[21] 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.[22][23][24]

Estuaries

Other rivers, particularly those on coasts with significant tidal range, do not form a delta but enter into the sea in the form of an estuary. Notable examples include the Saint Lawrence River and the Tagus estuary.

Inland deltas

Okavango Delta

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 Inner Niger Delta and Peace–Athabasca Delta are notable examples. The Amazon has also an inland delta before the island of Marajó.

In some cases, a river flowing into a flat arid area splits into channels that evaporate as it progresses into the desert. Okavango Delta in Botswana is one well-known example.

Mega deltas

The generic term mega delta can be used to describe very large Asian river deltas, such as the Changjiang (Yangtze), Pearl, Red, Mekong, Irrawaddy, Ganges-Brahmaputra, and Indus.

Sedimentary structure

The formation of a delta is complicated, multiple, and cross-cutting over time, but in 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 in small scale by crossbedding.[14][25]

Examples of deltas

The Ganges/Brahmaputra combination delta, which spans most of Bangladesh and empties into the Bay of Bengal, is the world's largest delta.

The St. Clair River delta, between the Canadian province of Ontario and the U.S. state of Michigan, is the largest delta emptying into a body of fresh water.

Other rivers with notable deltas include the:

The Ebre River delta at the Mediterranean Sea

Ecological threats to deltas

Human activities, such as the creation of dams for hydroelectric power or to create reservoirs can radically alter delta ecosystems. Dams block sedimentation, which can cause the delta to erode away. The use of water upstream can greatly increase salinity levels as less fresh water flows to meet the salty ocean water. While nearly all deltas have been impacted to some degree by humans, the Nile Delta and Colorado River Delta are some of the most extreme examples of the ecological devastation caused to deltas by damming and diversion of water. Construction, irrigation, and land alteration have impacted delta formation. As humans have altered surface roughness, runoff, and groundwater storage, studies have shown river delta retreat. However, historical data documents show that during the Roman Empire and Little Ice Age (times where there was considerable anthropogenic pressure), there were significant sediment accumulation in deltas. The industrial revolution has only amplified the impact of humans on delta growth and retreat.[28]

Deltas in the economy

Ancient deltas are a benefit to the economy due to their well sorted sand and gravel. Sand and gravel is often quarried from these old deltas and used in concrete for highways, buildings, sidewalks, and even landscaping. More than 1 billion tons of sand and gravel are produced in the United States alone.[29] Not all sand and gravel quarries are former deltas, but for ones that are, a lot of the sorting is already done by the power of water.

As lowlands often adjacent to urban areas, deltas often comprise extensive industrial and commercial areas as well as agricultural land. These uses are often in conflict. The Fraser Delta in British Columbia, Canada, includes the Vancouver Airport and the Roberts Bank Superport and the Annacis Island industrial zone, and a mix of commercial, residential and agricultural land. Space is so limited in the Lower Mainland region, and in British Columbia in general, which is very mountainous, that the Agricultural Land Reserve was created to preserve agricultural land for food production.

Deltas on Mars

Researchers have found a number of examples of deltas that formed in Martian lakes. Finding deltas 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.[30]

See also

References

  1. ↑ Miall, A. D. 1979. Deltas. in R. G. Walker (ed) Facies Models. Geological Association of Canada, Hamilton, Ontario.
  2. ↑ Elliot, T. 1986. Deltas. in H. G. Reading (ed.). Sedimentary environments and facies. Backwell Scientific Publications, Oxford.
  3. ↑ Blum, M.D. and Tornqvist, T.E. 2000. Fluvial responses to climate and sea-level change: a review and look forward. Sedimentology 47: 2–48.
  4. 1 2 Pasternack, Gregory B.; Brush, Grace S.; Hilgartner, William B. (2001-04-01). "Impact of historic land-use change on sediment delivery to a Chesapeake Bay subestuarine delta". Earth Surface Processes and Landforms. 26 (4): 409–427. ISSN 1096-9837. doi:10.1002/esp.189.
  5. 1 2 Anthony, Edward J. (2015-03-01). "Wave influence in the construction, shaping and destruction of river deltas: A review". Marine Geology. 361: 53–78. doi:10.1016/j.margeo.2014.12.004.
  6. ↑ "Dr. Gregory B. Pasternack – Watershed Hydrology, Geomorphology, and Ecohydraulics :: TFD Modeling". pasternack.ucdavis.edu. Retrieved 2017-06-12.
  7. ↑ Slingerland, R. and N. D. Smith (1998), "Necessary conditions for a meandering-river avulsion," Geology (Boulder), 26, 435–438.
  8. 1 2 Galloway, W.E., 1975, Process framework for describing the morphologic and stratigraphic evolution of deltaic depositional systems, in Brousard, M.L., ed., Deltas, Models for Exploration: Houston Geological Society, Houston, Texas, p. 87– 98.
  9. ↑ Perillo, G. M. E. 1995. Geomorphology and Sedimentology of Estuaries. Elsevier Science B.V., New York.
  10. ↑ Orton, G.J. and Reading, H.G. 1993. Variability of deltaic processes in terms of sediment supply, with particular emphasis on grain size. Sedimentology 40:475–512.
  11. ↑ "Dr. Gregory B. Pasternack – Watershed Hydrology, Geomorphology, and Ecohydraulics :: TFD Hydrometeorology". pasternack.ucdavis.edu. Retrieved 2017-06-12.
  12. ↑ Pasternack, Gregory B.; Hinnov, Linda A. (October 2003). "Hydrometeorological controls on water level in a vegetated Chesapeake Bay tidal freshwater delta". Estuarine, Coastal and Shelf Science. 58 (2): 367–387. doi:10.1016/s0272-7714(03)00106-9.
  13. 1 2 Fagherazzi S., 2008, Self-organization of tidal deltas, Proceeding of the National Academy of Sciences, vol. 105 (48): 18692–18695,
  14. 1 2 3 Characteristics of deltas. (Available archived at – checked Dec 2008.)
  15. ↑ Bernard Biju-Duval, J. Edwin Swezey. "Sedimentary Geology". Page 183. ISBN 2-7108-0802-1. Editions TECHNIP, 2002. Partial text on Google Books.
  16. 1 2 "Geological and Petrophysical Characterization of the Ferron Sandstone for 3-D Simulation of a Fluvial-deltaic Reservoir". By Thomas C. Chidsey, Thomas C. Chidsey, Jr (ed), Utah Geological Survey, 2002. ISBN 1-55791-668-3. Pages 2–17. Partial text on Google Books.
  17. ↑ "Dr. Gregory B. Pasternack – Watershed Hydrology, Geomorphology, and Ecohydraulics :: Tidal Freshwater Deltas". pasternack.ucdavis.edu. Retrieved 2017-06-12.
  18. ↑ Pasternack, G. B. 1998. Physical dynamics of tidal freshwater delta evolution. Ph. D. Dissertation. The Johns Hopkins University, 227pp, 5 appendices.
  19. ↑ Pasternack, Gregory B.; Hilgartner, William B.; Brush, Grace S. (2000-09-01). "Biogeomorphology of an upper Chesapeake Bay river-mouth tidal freshwater marsh". Wetlands. 20 (3): 520–537. ISSN 0277-5212. doi:10.1672/0277-5212(2000)020%3C0520:boaucb%3E2.0.co;2.
  20. ↑ Pasternack, Gregory B; Brush, Grace S (2002-03-01). "Biogeomorphic controls on sedimentation and substrate on a vegetated tidal freshwater delta in upper Chesapeake Bay". Geomorphology. 43 (3–4): 293–311. doi:10.1016/s0169-555x(01)00139-8.
  21. ↑ Pasternack, Gregory B.; Brush, Grace S. (1998-09-01). "Sedimentation cycles in a river-mouth tidal freshwater marsh". Estuaries. 21 (3): 407–415. ISSN 0160-8347. doi:10.2307/1352839.
  22. ↑ Gottschalk, L. C. 1945. Effects of soil erosion on navigation in upper Chesapeake Bay. Geographical Review 35:219–238.
  23. ↑ Brush, G. S. 1984. Patterns of recent sediment accumulation in Chesapeake Bay (Virginia-Maryland, U.S.A.) tributaries. Chemical Geology 44:227–242.
  24. ↑ Orson, R. A., R. L. Simpson, and R. E. Good. 1992. The paleoecological development of a late Holocene, tidal freshwater marsh of the upper Delaware River estuary. Estuaries 15:2:130-146.
  25. ↑ D.G.A Whitten, The Penguin Dictionary of Geology (1972)
  26. 1 2 Robert L. Bates, Julia A. Jackson, Dictionary of Geological Terms AGI (1984)
  27. ↑ Hori, K. and Saito, Y. Morphology and Sediments of Large River Deltas. Tokyo, Japan: Tokyo Geographical Society, 2003
  28. ↑ Maselli, Vittorio; Trincardi, Fabio (2013-05-31). "Man made deltas". Scientific Reports. 3. ISSN 2045-2322. PMC 3668317 Freely accessible. PMID 23722597. doi:10.1038/srep01926.
  29. ↑ Mineral Information Institute. 2011. Sand and Gravel. http://www.mii.org/Minerals/photosandgr.html
  30. ↑ Irwin III, R. et al. 2005. An intense terminal epoch of widespread fluvial activity on early Mars: 2. Increased runoff and paleolake development. Journal of Geophysical Research: 10. E12S15

Bibliography

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