River bifurcation (from Latin : furca, fork) occurs when a river (a bifurcating river) flowing in a single channel separates into two or more separate streams (called distributaries) which then continue downstream. Some rivers form complex networks of distributaries, typically in their deltas. If the streams eventually merge again or empty into the same body of water, then the bifurcation forms a river island.
River bifurcation may be temporary or semi-permanent, depending on the strength of the material that is dividing the two distributaries. For example, a mid-stream island of soil or silt in a delta is most likely temporary, due to low material strength. A location where a river divides around a rock fin, e.g. a volcanically formed dike, or a mountain, may be more lasting as a result of higher material strength and resistance to weathering and erosion. A bifurcation may also be man-made, for example when two streams are separated by a long bridge pier.
River bifurcation occurs in many types of rivers. It is common in meandering and braided rivers. In meandering rivers, bifurcations are often unstable in their configuration, and usually result in channel avulsion. [1] The stability of bifurcation is dependent on the rate of flow of the river upstream as well as the sediment transport of the upper reaches of the branches just after bifurcation occurs. [2] The evolution of bifurcation is highly dependent on the discharge of the river upstream of the bifurcation. [3] Unstable bifurcations are bifurcations in which only one channel receives water. Within deltas, these typically create channels with relatively large widths, and are also known as channel avulsions. Stable bifurcations are bifurcations in which both channels receive water. [4]
In deltas, the directions of distributaries resulting from bifurcation are easily changeable by processes like aggradation, or differential subsidence and compaction. [5] The number of distributaries that are present is in part determined by the rate of sediment discharge, [6] and increased sediment discharge leads to more river bifurcation. This then leads to increased numbers of distributaries in deltas.
Delta bifurcation has a typical angle at which it is observed, with a critical angle of approximately 72º. [7] However, observations and experiments show that many distributary channel bifurcations do not actually exhibit a bifurcation angle of 72º, but rather grow towards this angle over time after initiation of bifurcation. [8] This implies that bifurcations that occur in deltas are semi-permanent, as many observed channels do not exhibit this angle due to their relatively recent initiation, or because some of the channels that do reach this bifurcation angle did not last to be observed.
As is the case with river confluence, bifurcation is important in dividing land and morphological areas. Rivers are abundantly used as political boundaries, marking borders between regions of opposing countries, states and peoples, among other things. Sudden river bifurcation, even temporary, can disturb terranes that would otherwise be considered the same region. Bifurcations are different from confluences in that many confluences are considered important sites for cities and trade. But due to the semi-permanence of most bifurcated rivers, and their uncommon occurrences, the concept of construction is not largely exhibited at sites of river bifurcation.
Distributaries are common components of deltas, and are the opposite of tributaries. These distributaries, that are a result of river bifurcation, are important for the deposition and movement of water, sediment and nutrients from farther inland to the larger body of water that it empties into. [9] Deltas are very important to humans, as the delta distributary regions provide homes to roughly half a billion people, and are exceptionally biologically rich. [10]
Bifurcated rivers are largely semi-permanent, and are subject to constant change in their configuration from evolving terranes and flow rates. As a result of this, observation of the process by which rivers bifurcate and then gradually deteriorate has been poorly documented. The evolution of river bifurcations from single channel to multi-channeled and back again is largely dependent on discharge rate from the backwater regions of the channel. [11] The bifurcation of channel systems begins when a single channel is forced to split when a bar of sediment causes initiation of the two channel system, however, this does not always result in a system in which both channels receive flow. In braided systems, evolution of bifurcate systems is largely determined by the water level of adjacent branches of the system. [12] The water level differences in braided systems are themselves caused by closure of branch entrances as a result of bar growth. [13] In addition to bar growth, differences in direction of bifurcated river flows from compound bar shapes and backwater effects also influence the evolution of the braided system.
Bifurcations move largely as a result of migration of the upstream channel. [14] The configuration of the bifurcated system is also modified by the migration of bars within the system. [15] This can cause sudden variations in channel widths, as well as width asymmetry in the system. [16] Over time, the stable channel system will eventually deteriorate until only one channel receives flow from upstream, this then creates an unstable channel, one in which no flow passes through.
River bifurcations impact the surrounding area in a plethora of ways, namely, redistributing flow of water, sediment and nutrients throughout a watershed and delta. In addition to this, migrating bifurcations and landforms can alter the terranes in a given region affected by this process. Sudden bifurcation initiation can cause small scale flooding of the surrounding area. The opposite, deterioration of a stable bifurcation to an unstable one, can have similar effects, as flow that was split through two channels now being directed through one can cause the stable channel to surpass bank-full stage, or the point at which the water level is above the river bank. This can also cause flooding, and is a prominent issue in regions where levees are in use. Bifurcations are a major distributor of nutrients and mineral particulates to biologically rich areas in deltas. Sudden deterioration or initiation of bifurcated systems can disrupt the deposition of material required for various organisms to live, and thus has an indirect impact on surrounding ecosystems via flow patterns.
A distributary, or a distributary channel is a stream channel that branches off and flows away from a main stream channel. It is the opposite of a tributary, a stream that flows towards and into another stream or river. Distributaries are a result of river bifurcation and are often found where a river approaches a lake or an ocean and divides into distributary networks; as such they are a common feature of river deltas. They can also occur inland, on alluvial fans, or where a tributary stream bifurcates as it nears its confluence with a larger stream. In some cases, a minor distributary can divert so much water from the main channel that it can later become the main route.
A braided river consists of a network of river channels separated by small, often temporary, islands called braid bars or, in British English usage, aits or eyots.
A river delta is a triangular landform created by the deposition of the sediments that are carried by the waters of a river, where the river merges with a body of slow-moving water or with a body of stagnant water. The creation of a river delta occurs at the river mouth, where the river merges into an ocean, a sea, or an estuary, into a lake, a reservoir, or into another river that cannot carry away the sediment supplied by the feeding river. Etymologically, the term river delta derives from the triangular shape (Δ) of the uppercase Greek letter delta. In hydrology, the dimensions of a river delta are determined by the balance between the watershed processes that supply sediment and the watershed processes that redistribute, sequester, and export the supplied sediment into the receiving basin.
The Atchafalaya River is a 137-mile-long (220 km) distributary of the Mississippi River and Red River in south central Louisiana in the United States. It flows south, just west of the Mississippi River, and is the fifth largest river in North America, by discharge. The name Atchafalaya comes from Choctaw for 'long river', from hachcha, 'river', and falaya, 'long'.
In physical geography and hydrology, a channel is a landform on which a relatively narrow body of water is situated, such as a river, river delta or strait. While channel typically refers to a natural formation, the cognate term canal denotes a similar artificial structure.
The Mahakam River is third longest and volume discharge river in Borneo after Kapuas River and Barito River, it is located in Kalimantan, Indonesia. It flows 980 kilometers from the district of Long Apari in the highlands of Borneo, to its mouth at the Makassar Strait.
The Manych is a river in the Black Sea–Caspian Steppe of Southern Russia. It flows through the western and central part of the Kuma–Manych Depression. In ancient times, it was known as the Lik.
The Old River Control Structure is a floodgate system in a branch of the Mississippi River in central Louisiana. It regulates the flow of water from the Mississippi into the Atchafalaya River, thereby preventing the Mississippi River from changing course. Completed in 1963, the complex was built by the U.S. Army Corps of Engineers in a side channel of the Mississippi known as "Old River", between the Mississippi's current channel and the Atchafalaya Basin, a former channel of the Mississippi.
The Atchafalaya Basin, or Atchafalaya Swamp, is the largest wetland and swamp in the United States. Located in south central Louisiana, it is a combination of wetlands and river delta area where the Atchafalaya River and the Gulf of Mexico converge. The river stretches from near Simmesport in the north through parts of eight parishes to the Morgan City southern area.
The Kalahari Basin, also known as the Kalahari Depression, Okavango Basin or the Makgadikgadi Basin, is an endorheic basin and large lowland area covering approximately 725,293 km2 (280,037 sq mi) — mostly within Botswana and Namibia, but also parts of Angola, South Africa, Zambia, and Zimbabwe. The outstanding physical feature in the basin, and occupying the centre, is the large Kalahari Desert.
An anabranch is a section of a river or stream that diverts from the main channel or stem of the watercourse and rejoins the main stem downstream. Local anabranches can be the result of small islands in the watercourse. In larger anabranches, the flow can diverge for a distance of several or even hundreds of kilometers before rejoining the main channel.
A stream is a continuous body of surface water flowing within the bed and banks of a channel. Depending on its location or certain characteristics, a stream may be referred to by a variety of local or regional names. Long, large streams are usually called rivers, while smaller, less voluminous and more intermittent streams are known as streamlets, brooks or creeks.
In sedimentary geology and fluvial geomorphology, avulsion is the rapid abandonment of a river channel and the formation of a new river channel. Avulsions occur as a result of channel slopes that are much less steep than the slope that the river could travel if it took a new course.
Wax Lake was a lake in St. Mary Parish, Louisiana that was converted into an outlet channel, the Wax Lake outlet, to divert water from the Atchafalaya River to the Gulf of Mexico.
A bar in a river is an elevated region of sediment that has been deposited by the flow. Types of bars include mid-channel bars, point bars, and mouth bars. The locations of bars are determined by the geometry of the river and the flow through it. Bars reflect sediment supply conditions, and can show where sediment supply rate is greater than the transport capacity.
A mouth bar is an element of a deltaic system, which refers to the typically mid-channel deposition of the sediment transported by the river channel at the river mouth.
An alluvial river is one in which the bed and banks are made up of mobile sediment and/or soil. Alluvial rivers are self-formed, meaning that their channels are shaped by the magnitude and frequency of the floods that they experience, and the ability of these floods to erode, deposit, and transport sediment. For this reason, alluvial rivers can assume a number of forms based on the properties of their banks; the flows they experience; the local riparian ecology; and the amount, size, and type of sediment that they carry.
East Manych is a river in the eastern and central part of the Kuma–Manych Depression in southern Russia.
A river fork is where a river is connected to two or more clearly and equally distinct branches. It describes both tributaries and distributaries.
A deltaic lobe is a wetland formation that forms as a river empties water and sediment into other bodies of water. As the sediment builds up from this delta, the river will break away from its single channel and the mouth will be pushed outwards, forming a deltaic lobe.