Delta construction
Delta morphology
Delta abandonment
Delta destruction
Deltas and human activity
Resources
A delta is a low-lying, almost flat landform, composed of sediments deposited where a river flows into a lake or an ocean. Deltas form when the volume of sediment deposited at a river mouth is greater than what waves, currents, and tides can erode. Deltas extend the coastline outward, forming new land along the shore. However, even as the delta is constructed, waves, currents, or tidal activity may redistribute sediment. Although they form in lakes, the largest deltas develop along seashores. Deltas are perhaps the most complex of all sedimentary environments. The term delta comes from the resemblance between the outline of some deltas and the fourth letter in the Greek alpha-bet—delta (Δ)—which is shaped like a triangle.
Some areas of the delta are influenced more by river processes, while marine (or lake) activities control other parts. Deltas do not form if wave, current, or tide activity is too intense for sediment to accumulate. The degree of influence by river, wave, current and tide activity on delta form is often used to classify deltas. Among the many factors that determine the characteristics of a delta are the volume of river flow, sediment load and type, coastal topography and subsidencerate, amount and character of wave and current activity, tidal range, storm frequency and magnitude, water depth, sea level rise or fall, and climate.
Delta construction
Delta plain
As a river flows toward the sea or a lake, it occupies a single, large, relatively straight channel known as the main distributary channel. The main distributary may soon branch off, like the roots of a tree, into many separate smaller distributaries. The number of branches formed depends on many different factors
such as river flow, sediment load, and shoreline slope. Large sand-filled distributary channels occupy the delta plain, the nearly level, landward part of the delta, which is partly subaerial (above lake or sea level).
The natural levees that flank large distributary channels are another element of the delta plain. Natural levees are mounds of sand and silt that form when floodwaters flow over the banks of the distributary and deposit their sediment load immediately adjacent to the channel. Unlike natural levees on rivers, delta levees do not grow especially large. Therefore, they are easily broken through by floodwaters— a process called avulsion. This forms small channels, or crevasses, that flow away from the distributaries, like the little rootlets from the larger roots of a tree. The fan-shaped deposits formed during breeching of the distributaries are called crevasse splays.
Between the distributary channels, a variety of shallow, quiet water environments form, including freshwater swamps and lakes, and saltwater marshes. It is in these wetland basins that large volumes of organic matter and finegrained sediment accumulate.
Delta front
When sedimentladen river water flows into the standing water at the mouth of a distributary, the river water slows and deposits its load. This forms a sediment body called a distributary mouth bar, or bar finger sand—so named because the distributary channels look a bit like the fingers on a human hand. Distributary mouth bars form on the delta front, the gently seaward sloping, marine-dominated part of the delta that is all subaqueous, or below water level.
Subaqueous levees may extend out from the natural levees of the delta front onto the delta plain. These help confine the water flow seaward of the distributary mouth to a relatively narrow path, so that the delta continues growing outward at a rapid pace. The area of the delta front between the distributary mouth bars is called the interdistributary bay; the salinity here is brackish to marine.
Water velocity slows consistently outward from the distributary mouth; the river water consequently deposits finer and finer sediment as it flows seaward. Eventually, a point is reached where the average grain size decreases to clay-sized sediment with only minor silt. This is the prodelta area, where the bottom generally has a very low slope. On occasion, large blocks of sediment break free from the delta plain, slide down the steeper delta front, and become buried in prodelta muds.
Delta morphology
Vertical character
Ideally, if a delta is growing seaward, or prograding, as deltas typically do, a thick deposit with three stacked sediment sequences develops. The lower sequence, or bottomset beds, contains flatlying silt and clay layers of the prodelta. The middle sequence, or foreset beds, contains seaward-inclined layers of sand and silt produced by distributary mouth bar sedimentation. The upper sequence, or topset beds, consists of flatlying sand deposits formed in distributary channels and natural levees, interlayered, or interbedded, with fine-grained interdistributary bay deposits. A variety of factors, such as marine influence, changes in sediment supply or sea level, tend to complicate this picture; the actual sediment distribution in a deltaic sequence is typically very complex.
Surface character
The landward to seaward transect—from dis-tributary channel sands to prodelta muds—outlined above is typical of deltas, like the Mississippi River delta, which experience minimal marine influence. These lobate, or bird’s foot deltas, migrate farther and farther out into the ocean, because of multiple distributary channels and bar sands, each building seaward-extending lobes. On coastlines where waves or currents erode and redistribute much of the delta’s sand, this lobate form becomes highly modified.
In areas where wave power significantly modifies the delta, the sands of distributary mouth bars, and to a lesser degree, distributary channels and natural levees, are reworked into shore-parallel sand bodies known as barrier islands, or shore-attached beaches. These commonly form along coasts exposed to powerful waves and where water depth rapidly increases seaward. This allows waves to erode both the delta plain and delta front, and produces a delta with a smoother, more regular, convex shape. The Niger Delta located along the west coast of Africa is an example of a wave-dominated delta.
In locations where tidal range—the difference between high and low tide—is fairly high, strong tidal currents sweep across the delta front and up the channels of the delta plain. These reversing currents erode the delta and redistribute the deposits into large sand bodies oriented perpendicular to shore. This tends to make the coastline concave and, also, gives the delta a smoother, more regular shape. The Ganges-Brahmaputra Delta at the border between India and Bangladesh is an example of a tide-dominated delta.
Delta abandonment
As the river that formed a delta inevitably goes through changes upstream, a particular lobe may be abandoned. This usually occurs because a crevasse forms upstream by avulsion, and provides a more direct route or a steeper slope by which water can reach the sea or lake. As a result, the crevasse evolves into a new distributary channel and builds up a new delta lobe, a process called delta switching. The old distributary channel downstream is filled in by fine-grained sediment and, then, abandoned. Over the last 5, 000 years, the Mississippi River has abandoned at least six major lobes by avulsion.
An even larger-scale redirection of flow threatens to trigger abandonment of the entire Mississippi River delta. The Atchafalaya River, which follows an old course of the Mississippi, has a steeper slope than the modern Mississippi. At a point where the Atchafalaya River flows directly adjacent to the Mississippi, it is possible for the Atchafalaya to capture, or pirate, the flow of the Mississippi. This could permanently redirect the Mississippi away from New Orleans, Louisiana, and into Atchafalya Bay to the west. Since the 1950s, the U.S. Army Corps of Engineers has controlled the flow of the Mississippi in this area. In the 1980s, when the Mississippi River had several seasons of unusually high water levels, additional efforts were necessary to avert this disaster. What was called The Great Flood of 1993 once again threatened the surrounding areas of the Mississippi River, especially the area where it met with the Ohio River near Cairo, Illinois. No doubt, such flooding will threaten again in the future.
Delta destruction
Abandoned delta lobes experience rapid subsidence primarily due to sediment compaction. As water depth increases accordingly, enhanced wave and current attack contribute to rapid erosion of old delta deposits—a process called delta retreat—and the loss of vast tracts of ecologically important wetlands and barrier islands. Modern flood controls, such as channelization and levee construction, sharply reduce avulsion and delta switching. As a result, little or no new sediment is contributed to the delta plain outside of the large channels. Consequently, compaction, subsidence, and erosion continue unabated. This enhanced effect results in the loss of up to 15, 000 acres of wetlands per year in inactive areas of the Mississippi River delta plain. Global warming could accelerate this effect by triggering higher rates of global sea level rise and resulting in more rapid increases in water depth. Increased hurricane incidence in the 1990s, 2000s, and beyond also takes a toll. In August 2005, New Orleans experienced a gigantic natural disaster in the form of Hurricane Katrina, a category five hurricane, causing flooding in about 80% of the city. At the time of the hurricane, the U.S. Army Corps of Engineers said that the Mississippi River delta was receding faster than anyplace in the country, causing continuing problems for the residents of New Orleans.
Construction of dams upstream also impacts deltas. Dams not only trap water, they trap sediment as well. This sediment load would normally contribute to delta progradation, or at least help stabilize delta lobes. Construction of dams instead results in significant delta retreat. For instance, construction of Egypt’s Aswan High Dam on the Nile River in 1964 lead to rapid erosion of the delta plain with loss of both wetlands and agricultural lands.
Deltas and human activity
Deltas have been important centers of human activity throughout history, in part because of the fertility of the land and easy access to transportation. Many early human civilizations developed on deltas. For instance, the Nile River delta has hosted Egyptian cultures for over seven thousand years.
Deltas contain large expanses of wetlands where organic matter rapidly accumulates. Consequently,
KEY TERMS
Delta front— The seaward, gently sloping part of a delta, which is below water level.
Delta plain— The landward, nearly level part of a delta, some of which is below sea or lake level and some above.
Delta retreat— Landward migration of a delta due to erosion of older delta deposits.
Distributary channel— A large channel within a delta, which delivers water and sediment into an ocean or a lake.
Grain size— The size of a sediment particle; for example, gravel (greater than 2mm), sand (2mm to 1/16 mm), silt (1/16 mm to 1/256 mm) and clay (less than 1/256 mm).
Sediment load— The amount of sediment transported by wind, water, or ice.
Sedimentary environment— An area on the Earth’s surface, such as a lake or stream, where large volumes of sediment accumulate.
Tidal range— Vertical distance between high tide and low tide during a single tidal cycle.
delta muds are very rich in organic materials and make good hydrocarbon source rocks when buried to appropriate depths. Not surprisingly, deltaic deposits contain extensive supplies of coal, oil, and gas. Deltaic sand bodies are also excellent reservoir rocks for mobile hydrocarbons. This combination of factors makes deltas perhaps the most important hydrocarbon-bearing environment on Earth. Due to this economic bonanza, modern and ancient deltas have probably been more thoroughly studied than any other sedimentary environment.
Deltas are very low relief; most areas are rarely more than a few feet above sea level. Therefore, they contain freshwater, brackish, and saltwater basins with correspondingly diverse, complex ecologies. Minor changes in the elevation of the delta surface can flood areas with water of much higher or lower salinity, so delta ecology is easily impacted by human activities. As indicated above, humans have significantly altered deltas and will continue to do so in hopes of curbing flooding. As a result, accelerated delta retreat will continue, as well as wetlands destruction, unless humans develop new flood control technologies or new methods for wetlands protection.
Resources
BOOKS
Editors of Prentice Hall Science Explorer series. Earth’s Changing Surface. Needham, MA: Pearson Prentice Hall, 2005.
Giosan, Liviu, and Janok P. Bhattacharya, eds. River Deltas: Concepts, Models, and Examples. Tulsa, OK: Society for Sedimentary Geology, 2005.
Hsu, Kenneth Jinghwa. Physics of Sedimentology: Textbook and Reference. Berlin, Germany, and New York: Springer, 2004.
Skinner, Brian J., and Stephen C. Porter. The Dynamic Earth: An Introduction to Physical Geology. Hoboken, NJ: John Wiley & Sons, 2004.
Thurman, Harold V., and Alan P. Trujillo. The Dynamic Earth: An Introduction to Physical Geology. Upper Saddle River, NJ: Pearson Prentice Hall, 2005.
Clay Harris
