Delta Abandonment, Delta Destruction, Deltas And Human ActivityDelta construction, Delta morphology
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 alphabet—delta—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 subsidence rate, amount and character of wave and current activity, tidal range, storm frequency and magnitude, water depth, sea level rise or fall, and climate.
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 flood waters 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 flood waters—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 fine-grained sediment accumulate.
When sediment-laden 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 your 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.
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, contain flat-lying silt and clay layers of the prodelta. The middle sequence, or foreset beds, contain seaward-inclined layers of sand and silt produced by distributary mouth bar sedimentation. The upper sequence, or topset beds, consists of flat-lying 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.
The landward to seaward transect—from distributary 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, as a result 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.
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