River delta - Wikipedia
Deltas are wetlands that form as rivers empty their water and sediment into another body of water, such as an ocean, lake, or another river. [Relationships between typical vegetations, soil salinity, and groundwater depth in the Yellow River Delta of China]. [Article in Chinese]. Ma YL(1), Wang D(2). The Mississippi River delta is a rich ecosystem of barrier islands, While this relationship between rivers and ocean waves is generally.
However, it's unclear how such protective measures will affect the shape of the river delta, and its communities, over time.
Depending on the balance of the two, the coastline of a river delta may take on a smooth "cuspate" shape, or a more pointed "crenulated" outline, resembling a bird's foot. The new metric may help engineers determine how the shape of a delta, such as the Mississippi's, may shift in response to engineered structures such as dams and levees, and environmental changes, such as hurricane activity and sea-level rise.
Jaap Nienhuis, a graduate student in the MIT-WHOI Joint Program in Marine Geology and Geophysics, says the effects of climate change, and the human efforts to combat these effects, are already making an impact on river deltas around the world. But because there are a lot of dams on the Mississippi nowadays, there is not as much sand coming down the river, so people are very worried about how this delta will evolve, especially with sea-level rise, over the coming centuries.
Shaping a shoreline Over hundreds of thousands of years, a river's sand and silt flow toward the coast, ultimately piling up at a river's mouth in the form of a low-lying delta.
Predicting the shape of river deltas
A delta's coastline can be relatively smooth, with most sand depositing from the main river, or it can fan out in the shape of a bird's foot, as the river bifurcates into tributaries and channels, each of which deposits sand in finger-like projections.
Scientists often characterize a delta as either river-dominated or wave-dominated.
In a wave-dominated delta, such as the Nile River delta in Egypt, incoming ocean waves are stronger than the river's flow. As a result, waves push outflowing sediment back along the coast, effectively smoothing the coastline. By contrast, a river-dominated delta, such as the Mississippi's, is shaped by a stronger river, which deposits sand faster than ocean waves can push back, creating a crenulated coastline. While this relationship between rivers and ocean waves is generally understood, Nienhuis says there is no formal way to determine when a delta will tip toward a smooth or pointy shape.
The researchers came up with a simple ratio to predict a delta's shape, based on a river's sediment flux, or the flow rate of sediment through a river, and the strength of ocean waves, determined by a wave's height, frequency, and angle of approach. Alluvial fan deltas, as seen by their name, avulse frequently and more closely approximate an ideal fan shape. Most large river deltas discharge to intra-cratonic basins on the trailing edges of passive margins due to the majority of large rivers such as the Mississippi, Nile, Amazon, Ganges, Indus, and Yangtzee discharging along passive continental margins.
Topography along active margins tend to be steeper and less widespread, which results in sediments not having the ability to pile up and accumulate due to the sediment traveling into a steep subduction trench rather than a shallow continental shelf.
There are many other smaller factors that could explain why the majority of river deltas form along passive margins rather than active margins.
Similarities and Difference Between Delta and Estuary
Along active margins, orogenic sequences cause tectonic activity to form over-steepened slopes, brecciated rocks, and volcanic activity resulting in delta formation to exist closer to the sediment source. Tectonic activity on active margins causes the formation of river deltas to form closer to the sediment source which may affect channel avulsion, delta lobe switching, and auto cyclicity.
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. Tide-dominated deltas[ edit ] Erosion is also an important control in tide-dominated deltas, such as the Ganges Deltawhich may be mainly submarine, with prominent sandbars and ridges. This tends to produce a "dendritic" structure. Once a wave- or river-dominated distributary silts up, it is abandoned, and a new channel forms elsewhere.
In a tidal delta, new distributaries are formed during times when there is a lot of water around — such as floods or storm surges. These distributaries slowly silt up at a more or less constant rate until they fizzle out.
For example, a mountain river depositing sediment into a freshwater lake would form this kind of delta.
Tidal freshwater deltas[ edit ] A tidal freshwater delta  is a sedimentary deposit formed at the boundary between an upland stream and an estuary, in the region known as the "subestuary".
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  as well as processes that are unique to the tidal freshwater setting.
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.
- River delta
Research has demonstrated that the accumulating sediments in this estuary derive from post-European settlement deforestation, agriculture, and urban development.
Notable examples include the Gulf of Saint Lawrence and the Tagus estuary.
Inland deltas[ edit ] 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.
Similarities and Difference Between Delta and Estuary
In some cases, a river flowing into a flat arid area splits into channels that evaporate as it progresses into the desert. The Okavango Delta in Botswana is one example. Mega deltas[ edit ] The generic term mega delta can be used to describe very large Asian river deltas, such as the YangtzePearlRedMekongIrrawaddyGanges-Brahmaputraand Indus. Sedimentary structure[ edit ] Delta on Kachemak Bay at low tide 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: This three part structure may be seen in small scale by crossbedding.
This suspended load is deposited by sediment gravity flowcreating a turbidite. These beds are laid down in horizontal layers and consist of the finest grain sizes. The foreset beds in turn are deposited in inclined layers over the bottomset beds as the active lobe advances.
Foreset beds form the greater part of the bulk of a delta, and also occur on the lee side of sand dunes.