Distributary

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A distributary, or a distributary channel, is a stream that branches off and flows away from a main stream channel. They are a common feature of river deltas. The phenomenon is known as river bifurcation. The opposite of a distributary is a tributary. Distributaries usually occur as a stream nears a lake or the ocean, but they can occur inland as well, such as on alluvial fans or when a tributary stream bifurcates as it nears its confluence with a larger stream. In some cases, a minor distributary can "steal" so much water from the main channel that it can become the main route.

Related terms[edit]

Common terms to name individual river distributaries in English-speaking countries are arm and channel. They may refer to a distributary that does not rejoin the channel it has branched from (e.g., the North, Middle, and South Arms of the Fraser River, or the West Channel of the Mackenzie River), or one that does (e.g. Annacis Channel and Annieville Channel of the Fraser River, separated by Annacis Island).

In Australia, the term anabranch is used to refer to a distributary that diverts from the main course of the river and rejoins it later. In North America an anabranch is called a braided stream.

North America[edit]

In Louisiana, the Atchafalaya River is an important distributary of the Mississippi River. Because the Atchafalaya takes a steeper route to the Gulf of Mexico than the main channel, it has captured more and more of the Mississippi's flow over several decades, after the Mississippi meandered into the Red River. The Old River Control Structure, a dam which regulates the outflow from the Mississippi into the Atchafalaya, was completed in 1963 to prevent the Atchafalaya from capturing the main flow of the Mississippi and stranding the ports of Baton Rouge and New Orleans.

In British Columbia, Canada, the Fraser River has numerous sloughs and side-channels which may be defined as distributaries, but its final stretch has three main distributaries, the North Arm and the South Arm, and a few smaller ones adjoining them.

Examples of inland distributaries:

South America[edit]

The Casiquiare is an inland distributary of the upper Orinoco, which flows southward into the Rio Negro and forms a unique natural canal between the Orinoco and Amazon river systems. It is the largest river on the planet that links two major river systems.

Europe[edit]

Satellite image of part of the Rhine–Meuse–Scheldt delta

Asia[edit]

Indian Subcontinent[edit]

Africa[edit]

  • The Nile River has two distributaries, the Rosetta and the Damietta branches. According to Pliny the Elder it had in ancient times seven distributaries (east to west):
    • The Pelusiac
    • The Tanitic
    • The Mendesian
    • The Phatnitic
    • The Sebennytic
    • The Bolbitine
    • The Canopic
See History of the Nile Delta.
  • The Okavango River ends in many distributaries in a large inland delta called the Okavango Delta. It is an example of distributaries that do not flow into any other body of water.

Oceania[edit]

Delta of Papua New Guinea's Fly River

A number of the rivers that flow inland from Australia's Great Dividing Range form distributaries, most of which flow only intermittently during times of high river levels and end in shallow lakes or simply peter out in the deserts. Yarriambiack Creek, which flows from the Wimmera River into Lake Coorong, and Tyrrell Creek, which flows from the Avoca River into Lake Tyrrell, are two distributaries in Victoria, Australia. The Narran River flows from the Balonne River in Queensland, Australia into Narran Lake in New South Wales.

Many of Papua New Guinea's major rivers flow into the Gulf of Papua through marshy, low-lying country, allowing for wide, many-branched deltas. These include the Fly River, which splits into three major and several minor rivers close to its mouth. The Bamu River splits into several channels close to its mouth, among them the Bebea, Bina, Dibiri, and Aramia. The Kikori River also splits into a multitude of channels as it crosses the plains close to the Gulf of Papua. The Purari River splits into three major channels as it approaches its mouth.

New Zealand's second-longest river, the Clutha River, splits into two arms, the Matau and the Koua, some 10 kilometres from the South Island's Pacific Coast. A large island, Inch Clutha lies between the two arms. Many of the rivers crossing the Canterbury Plains in the central South Island are braided rivers, and several of these split into separate branches before reaching the coast. Notable among these is the Rangitata River, the two arms of which are separated by the low-lying Rangitata Island.

See also[edit]