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Bifurcation of Rivers

The Bifurcation of Rivers Into Distributaries

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Distributary River

The distributary of the Casiquiare River comes from the upper Orinoco River and joins the Rio Negro River as a main left tributary of the Amazon River.

Image Courtesy of Dimitar Zhelev
While most are familiar with the concept of river tributaries, the streams that feed into a larger river, there is a less-common situation where a river divides into smaller streams. This is known as bifurcation and the resulting streams are known as distributaries.

Bifurcation in Hydrology

In science, many specific processes and phenomena are named with Greek or Latin word which makes the terms internationally understandable. One example is bifurcation (from the Latin words for double and fork). In the field of physical geography, this term names a hydrological point in which a single river stream is separated into two or more distributaries.

Types of Bifurcation

There are three main types of river bifurcation:

1) In the first type, after a bifurcation, the streams merge again in one main river course. An example could be the situation with the inland delta of the Niger River in Africa or other braided rivers. Some waterfalls are also short-period bifurcating objects like the Niagara Falls where the Goat Island divides the water stream.

2) When the distributaries inflow in one drainage basin in a short distance. All coastal deltas are examples of this type. In this case the slow velocity of the river stream and the great amount of sediments cause bifurcation and create many alluvial islands.

3) The third type is a rare natural phenomenon. The separated streams runoff into different drainage basins and they do not 'meet' each other again. This is the most curious type.

This hydrological 'curiosity' has a short geological time of existence. Usually, it happens when the river stream reaches watershed areas with flattened topography. Sooner or later, the equilibrium is disturbed by erosion or tectonic displacement. Then, one of the streams disappears. That is why current and historical bifurcations could be distinguished. In addition, some of the cases appear only seasonally or accidentally when the river discharge rate is higher and the main river bed cannot 'hold' the water. The annals of history can also show evidences for man-made bifurcations.

The Great Expedition Discovers a Distributary

The German polymath and great geographer Alexander von Humboldt, together with the French botanist Aimé Jacques Alexandre Bonpland, explored the watershed between the catchments of the Orinoco and the Amazon rivers in 1800. The task was part of a bigger scientific work that took place in the Guiana Highlands.

Having some tips by the local tribes, they were the first to 'discover' and describe the Casiquiare river as a natural connection between two river systems. The distributary comes from the upper Orinoco River and joins the Rio Negro River as a main left tributary of the Amazon River. This is likely the most quoted case of bifurcation in the scientific articles.

Distributaries Around the world

There are some other examples around the world. The Rosson River in the European part of Russia naturally 'merges' the catchments of the Narva River and the Luga River. The Echimamish River in Manitoba, Canada, divides and 'joins' the Hayes River and the Hudson River. The Torne River on the border between Finland and Sweden has four bifurcations in its watershed area. Generally, many smaller examples can be found in Scandinavia, Karelia, Siberia, and Canada.

Localizing them on the map, it would be easy to deduce that the landscapes in these regions had been covered by ice sheet during the last continental glaciation. As a result of this massive ice pressure, the terrain is flattened and the areas are rich in water, therefore, there are more hydrological and topographic 'chances' for bifurcation forming.

Another 'appropriate' place for this phenomenon to occur would be the floor of former inland seas or lakes, as in the case of Central Asia. Nowadays, some of the largest endorheic basins* are situated in this part of the world. The rivers flowing on the flat clay floor in these areas can easily change their river beds and in cases of big rainfall they can overflow and create short-existing bifurcations.

Bifurcation in the Past

In the past, cases of bifurcation had been recorded in the region around the Caspian Sea and the Aral Sea. There the Amudarya River used to bifurcate seasonally and to spread its waters into both inland seas. No such kind of event has been recorder after the environmental disaster with the Aral Sea.

Also in this part of Eurasia, the border Chu river between Kazakhstan and Kyrgyzstan divided its waters into the Boom Gorge and the Issyk Kul Lake. The later connection has already vanished in history due to the excessive use of water for irrigation and the accumulation of sediments that blocked the tiny stream.

North of the Caucasian Mountains in Russia, the small Kalaus River used to connect the river systems of East Manych River (Caspian Sea basin) and West Manych River (Sea of Azov basin). The bifurcation was seasonal but it totally disappeared when the Kalaus River was dammed.

Human-Caused Bifurcations

In the far north of Russia, the Kuloy River once distributed its waters between the Mezen River and the Northern Dvina River. The bifurcation was recorded in the past but later it disappeared. The topographic depression of the former distributary was used for the construction of the Kuloy-Pinega Canal in 1928. The Serbian king Milutin in 14th century ordered the Nerodimka River to be connected to the Sazlija pond by digging a canal. Nowadays, the river still bifurcates in the Ibar River (Black Sea drainage basin) and in the Lepanec River (Aegean Sea drainage basin). Even artificial, the bifurcation is declared as a strict wildlife conservation zone. After the collapse of Yugoslavia it is in the current territory of Kosovo.

In the 1930s in Soviet Russia a strange idea of Siberian river reversal was developed. Some politicians and engineers introduced the concept for reversing the 'uselessly' flowing Siberian rivers into the Arctic Ocean. They wanted to make these rivers flow to the populated areas of Central Asia by system of canals and man-made bifurcations for the purpose of providing additional irrigation supplies. Fortunately, the idea failed and another environmental disaster was prevented.

Bifurcation of Lakes

If a lake inflows in two different river systems or drainage basins this could be lake bifurcation. In this case, there is also a chance for seasonal or man-made bifurcation. Sometimes it could be caused by the standing waves (a.k.a. seiches**) in some lakes.

The same postglacial regions in Siberia, Scandinavia, and Canada provide the appropriate conditions for the formation of this type of lakes. Also in the coastal areas many lakes (lagoons or limans) could reach the sea by more than one outflow. Generally, these objects are small and not that hydrologically significant.

On the border between Argentina and Chile in Patagonia, there is a giant lake that inflows both in the Atlantic Ocean and in the Pacific Ocean. This is the lake named Buenos Aires (in Argentina) or General Carrera (in Chile). The lake drains to the Pacific Ocean on the west through the Baker River. However, there is also a sporadic stream from the same lake that heads east, called Fénix Chico. It reaches the Deseado River and eventually joins the Atlantic Ocean.

Future Bifurcations

It is hard to predict where and when a river bifurcation may appear. The ongoing process of climate change and the thawing of the ice sheets, caps and glaciers will uncover some 'new' places for potential bifurcations. The diversity of our dynamic Earth can always surprise us with some drastic geological events that may either 'destroy' or 'create' new bifurcations.

* endorheic basin - a drainage basin with no connection to the World Ocean, an isolated drainage system. The desert parts of Central Asia, Australia and Africa are classical examples.
** seiches - a standing wave in an enclosed or partially enclosed body of water. Seiches and seiche-related phenomena have been observed on lakes, reservoirs, bays, harbors and seas. The key prerequisite for the formation of a seiche is for a body of water to be at least partially bounded, so that it creates a standing wave.

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