Distributary

A distributary is a stream channel that splits off from a larger river and flows away from it. This is the opposite of a tributary, which flows into a larger stream. Distributaries form when a river bifurcates, and they are most common in river deltas, where a river meets a lake or ocean and breaks into a network of channels. They can also appear inland, on alluvial fans, or when a tributary splits near its meeting point with a bigger river. Occasionally, a small distributary may take so much water from the main channel that it eventually becomes the main route.

In English-speaking countries, individual distributaries are often called arms or channels. These terms can describe a branch that does not rejoin the main river—like the North, Middle, and South Arms of the Fraser River, or the West Channel of the Mackenzie River—or one that does, such as the Annacis Channel and Annieville Channel of the Fraser, which are separated by Annacis Island. In Australia, a distributary that leaves the main course and later rejoins it is called an anabranch. In North America, a river with such branching is known as a braided river.

In North America, the Atchafalaya River in Louisiana is a major distributary of the Mississippi River. Because the Atchafalaya follows a steeper path to the Gulf of Mexico, it has captured increasing amounts of the Mississippi's flow over decades, after the Mississippi meandered into the Red River of the South. To prevent the Atchafalaya from taking over the main flow and stranding the ports of Baton Rouge and New Orleans, the Army Corps of Engineers completed the Old River Control Structure in 1963, a dam that regulates outflow from the Mississippi into the Atchafalaya. In British Columbia, Canada, the Fraser River has many sloughs and side-channels that qualify as distributaries. Its final stretch contains three main distributaries—the North Arm and South Arm—along with several smaller ones. Inland examples include the Teton River in Idaho, a tributary of Henrys Fork that splits into the North Fork and South Fork, which rejoin Henrys Fork miles apart. At the Parting of the Waters National Landmark in Wyoming's Teton Wilderness, North Two Ocean Creek divides into Pacific Creek and Atlantic Creek, which ultimately flow to their respective oceans. The Kings River in California has built a large alluvial fan where it leaves the Sierra Nevada for the Central Valley; its distributaries flow north to the Pacific via the San Joaquin River and south into an endorheic basin around Tulare Lake. The Qu'Appelle River in Saskatchewan and Manitoba is a distributary of the South Saskatchewan River, controlled by the Qu'Appelle River Dam, which forms the southern arm of Lake Diefenbaker.

In South America, the Casiquiare canal is an inland distributary of the upper Orinoco River. It flows south into the Rio Negro, creating a natural link between the Orinoco and Amazon river systems. It is the largest river on Earth that connects two major river systems.

In Europe, the Rhine has three main distributaries: the IJssel, the Waal, and the Nederrijn (Lower Rhine), formed by two separate bifurcations in the Rhine–Meuse–Scheldt delta. The Nemunas Delta (Memeldelta) has three principal distributaries: the Gilgestrom, the Rußstrom, and the Atmath. The Akhtuba River is a major distributary of the Volga, splitting off just before the Volga Delta. The Tärendö River in northern Sweden is an inland distributary, far from the river's mouth, starting at the Torne River and ending at the Kalix River. The Little Danube in Slovakia branches off from the Danube near Bratislava, flows into the Vah, and rejoins the Danube near Komárno, enclosing the largest freshwater island in Europe. In Ireland, the Abbey River in Limerick is a distributary arm of the River Shannon that rejoins the Shannon, forming an island where King John's Castle stands.

In Eastern Asia, the Huai River in China splits into three streams. The main stream passes through the Sanhe Sluice, exits via the Sanhe River, and enters the Yangtze River through Baoying Lake and Gaoyou Lake. A second stream leaves the east bank of Hongze Lake through the Gaoliangjian Gate and flows into the Yellow Sea at Bidan port via the Subei Guan'gai Zongqu, the main irrigation channel of Northern Jiangsu, covering 168 kilometers. A third stream leaves the Erhe lock on the northeast bank of Hongze Lake, passes the Huaishuhe River north of Lianyungang city, and empties into Haizhou Bay through the Hongkou.

In Southeast Asia, the Tha Chin River and Noi River are distributaries of the Chao Phraya River in Thailand, splitting off about 200 kilometers upstream from the Bay of Bangkok. The Brantas River in East Java, Indonesia, branches into two distributaries: the Mas River (also called the Surabaya River) and the Porong River. The Hong River in northern Vietnam has notable distributaries including the Day River, Ninh Co River, and Luoc River, all of which empty into the Gulf of Tonkin.

On the Indian subcontinent, the Kollidam River is a distributary of the Kaveri River. Himalayan rivers such as the Ganges, Brahmaputra, and Indus, along with many tributaries, form inland distributaries over vast alluvial fans as they move from the mountains to the flat Indo-Gangetic Plain. These areas are highly prone to flooding, as seen in the 2008 Bihar flood on the Kosi River. The Padma River is the main distributary of the Ganges.