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River bifurcation (from (ラテン語:furca), fork) occurs when a river flowing in a single stream separates into two or more separate streams (called distributaries) which continue downstream. Some rivers form complex networks of distributaries, especially 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 which separates the distributaries. For example, a mid-stream island of soil or silt in a delta is most likely temporary. A location where a river divides around a rock fin, e.g. a volcanically formed dike, or a mountain, may be more lasting. A bifurcation may also be man-made, for example when two streams are separated by a long bridge pier. == Examples == *The largest natural bifurcation is the Casiquiare canal, which provides a navigable channel between the Amazon and the Orinoco River in Venezuela. *Canada's aptly-named Divide Creek splits into two branches near Kicking Horse Pass on the Alberta-British Columbia border. One branch flows west to the Pacific Ocean; the other flows east and eventually reaches the Atlantic Ocean via Hudson Bay. * Similarly, at Two Ocean Pass in Wyoming, USA, North Two Ocean Creek splits at the Parting of the Waters into Atlantic Creek, which flows east to the Gulf of Mexico via the Yellowstone, Missouri and Mississippi Rivers and Pacific Creek, which flows west to the Pacific Ocean via the Snake and Columbia Rivers. *A bifurcation of the Nerodimka River in (in Albanian ''Nerodimja'') (in the city of Uroševac/Ferizaj, Kosovo, Serbia; (at ) was a hydrological curiosity as separate streams flowed into the Aegean and the Black Sea. The Nerodimka bifurcation was the first hydrological protected object in the former Yugoslavia (1979). The Nerodimka bifurcation is a strict wildlife sanctuary, category I according to IUCN, with an area of 13.0 ha (about 32 acres). This bifurcation is considered to be an artificial phenomenon, but created under extremely favorable natural conditions. *In the past, the small Kalaus River in south-western Russia, when reaching the thalweg of the Kuma-Manych Depression at , would split, the two distributaries becoming the headwaters of the West and East Manych Rivers. The former flows west into the Don River and eventually into the Sea of Azov, while the latter flows east, and is lost in the steppe before ever reaching the Caspian Sea. However a dam was built, preventing water from flowing from the Kalaus into the East Manych; thus the Kalaus is now the source of the West Manych only.〔Alexander Anatolievich Bazelyuk (Базелюк Александр Анатольевич), "(АНТРОПОГЕННОЕ ИЗМЕНЕНИЕ ГИДРОГРАФИЧЕСКОЙ СЕТИ КУМО-МАНЫЧСКОЙ ВПАДИНЫ )" (Anthropogenic changes in the Hydrographic Network of the Kuma-Manych Depression), summary of the Cand. Sci. dissertation. Rostov-on-Don, 2007. Includes maps.〕 *The Bahr Yussef is a channel which splits off the west side of the Nile and drains into the Birket Qarun, an inland sea in the Fayum Depression. Originally a natural bifurcation for flood waters, its flow was increased by canalisation in the 12th Dynasty (about 1900 BC). Around 230BC, the channel of the Nile from which it came (itself a bifurcation) dried up, but has since been fed by a new canal to allow water again to make it from the Nile to Al Fayyum. The entire waterway is over 300 km long, consisting of modern canals taking Nile water from Asyut to Dairut,〔http://maps.google.co.uk/maps?f=q&source=s_q&hl=en&geocode=&q=dairut&aq=&sll=27.556982,30.836563&sspn=0.14549,0.264187&ie=UTF8&hq=&hnear=Dairut,+Assiut,+Egypt&ll=27.561966,30.809011&spn=0.036219,0.066047&t=h&z=15〕 the old Nile channel then runs alongside the Nile for over 150 km to Lahun, then the Ancient Egyptian canal carries the water into the Fayum Depression. *In Suriname, the Wayombo and Arrawarra split, the first flowing into the Coppename, the second into the Nickerie. *The Swedish side of Torne River has a distributary called the Tärendö River, which on average transports 57% of the water of the Torne River into the Kalix River.〔http://www.pajala.se/mun/pajala/www.nsf/English/764B6DC8BFD894E2C1256FB30024F22D? 〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「River bifurcation」の詳細全文を読む スポンサード リンク
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