|Regions||Altai Krai, Novosibirsk Oblast, Tomsk Oblast, Khantia-Mansia, Yamalia|
|- left||Katun River, Anuy River, Charysh River, Aley River, Parabel River, Vasyugan River, Irtysh River, Northern Sosva River|
|- right||Biya River, Berd River, Inya River, Tom River, Chulym River, Ket River, Tym River, Vakh River, Pim River, Kazim River|
|Cities||Biysk, Barnaul, Novosibirsk, Nizhnevartosk, Surgut|
|Primary source||Katun River|
|- location||Belukha Mountain, Altai Republic|
|- elevation||2,300 m (7,546 ft)|
|Secondary source||Biya River|
|- location||Lake Teletskoye, Altai Republic|
|- elevation||434 m (1,424 ft)|
|Source confluence||Near Biysk|
|- elevation||195 m (640 ft)|
|Mouth||Gulf of Ob|
|- location||Ob Delta, Yamalia|
|- elevation||0 m (0 ft)|
|Length||2,962 km (1,841 mi)|
|Basin||2,972,497 km2 (1,147,688 sq mi)|
|- average||12,475 m3/s (440,550 cu ft/s) |
|- max||39,747 m3/s (1,403,652 cu ft/s)|
|- min||2,360 m3/s (83,343 cu ft/s)|
Map of the Ob River watershed
It rises at the confluence of two major rivers of the Altai Krai, the Katun and Biya, 20-kilometre (12 mi) southwest of Biysk. The Katun, which springs from glaciers of Belukha Mountain deep in the Altay Mountains, is much longer and larger than the Biya, and thus is the main source. The Ob flows west through lowlands and swings north towards Barnaul. At this city the river veers northwest then northeast into Novosibirsk Reservoir, formed by a dam near the city of Novosibirsk. The river then runs northwards, and is joined by the Tom River near Tomsk. Sixty kilometres (37 mi) downstream of Krasny Yar one of the main tributaries, the Chulym River, joins from the right.
The Ob flows northwest through a broad and lightly populated valley filled with meanders, marshes, backwaters and oxbow lakes. Near Nizhnevartosk it receives the Vakh River from the north. The river then travels generally west, and near Khanty-Mansiysk receives the Irtysh River, its greatest affluent. The Irtysh, which rises far south in China and flows through Kazakhstan then Russia, is longer than the Ob above their confluence but has a lesser flow. Here the Ob turns northwest, flowing towards the Arctic. Below Oktyabrskoye the Ob curves due north, then east near Salekhard for its final run, dumping its water into the Gulf of Ob, a 800-kilometre (500 mi) estuary that connects to the Kara Sea of the Arctic Ocean.
This river's drainage basin of more than 2,970,000 square kilometres (1,150,000 sq mi) is the largest watershed flowing to the Arctic. The basin's vast boundaries span four countries – Russia, Kazakhstan, China and Mongolia, covering most of western Siberia and much of northern Central Asia. However, the entire course of the Ob, and its major headwater the Katun, lie entirely inside Russia's borders. It is the third-largest river system of Russia by discharge, after the Yenisey and Lena. The Ob's largest tributary, the Irtysh, drains over half the total area of the basin, but provides less than a fourth of the total flow. With the Irtysh, the Ob stretches 5,410 kilometres (3,360 mi), forming the longest river system in Russia.
The Ob watershed ranges from alpine, steppe and semiarid climes in the south to frozen permafrost, taiga and tundra regions near and north of the Arctic Circle. Falling less than 200 metres (660 ft) from source to mouth, the Ob is slow-flowing in most areas and thus its valley comprises large regions of boreal muskeg and marshland. The vast majority of the basin comprises plains, but high mountains lie to the south, with a maximum height of 4,506 metres (14,783 ft) at Belukha. Temperatures in the basin range from a summer high of 40 °C (104 °F) in Kazakhstan to winter lows of −60 °C (−76 °F) in the Altays. In the north it ranges from −28 °C (−18 °F) in winter to 4 °C (39 °F) in summer.
The river rises greatly in the summer, which is the period with the most rainfall (up to 1,575 millimetres (62.0 in) in the Altays) and snowmelt. There is usually an initial pulse of snow and ice melt from the headwaters in early summer, followed by rising of the middle tributaries in mid-summer after heavy rainfall. The river flow gradually decreases until the whole river is frozen, usually by November in most years. The lower river is frozen for 220 days each year and the upper for 150, until the spring thaw, in late April or early May, breaks the ice loose and sends the river rushing towards the Arctic.
Estuary and delta
At the end of the Ob River delta, which comprises a pair of main channels through which the river diverges before flowing over an immense sandbar built of sediments carried downstream from Siberia, lies an immense estuary called the Gulf of Ob (Obskaya Guba). A branch of the Kara Sea, in turn part of the Arctic Ocean, the bay is some 800 kilometres (500 mi) long and 30 to 100 kilometres (19 to 62 mi) wide. It is also relatively shallow with an average depth of 11 metres (36 ft). This estuary is the largest in the world and is known for its abundant oil resources. Although the Ob is the main inflow, the Taz, another major river, enters an arm of the gulf from the east. From autumn to mid-summer, the Gulf completely freezes over, with ice up to 2 metres (6.6 ft) thick; the ice-free season comprises only August and September.
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The Ob river travels generally northwest through the middle of the huge physiographic province known as the West Siberian Plain or Lowland. This region is one of the largest areas of continuous plains on Earth, covering more than 3,000,000 square kilometres (1,200,000 sq mi). The basin of the Ob River began forming more than 540 million years ago when the ancient surface of northern Asia began a steady subsidence that continues to this day. Constant sediment deposits from rivers and glaciers kept the land no more than 100 metres (330 ft) above sea level. About 1,000 metres (3,300 ft) of Tertiary-era (65-1.8 MYA) rock underlie the surface.
During the Pleistocene, large continental ice sheets or glaciers advanced southwards from the Arctic, covering northern Siberia with impassable walls of ice and bulldozing the land into long, narrow basins divided by small but long series of hills. The glaciers blocked the Ob River, causing it to back up into huge Lake Mansi, that covered much of western Siberia. During high points the lake overflowed into the Caspian Sea through the Volga River and southwards into the Aral Sea as well – the outflow might have been so great that the water eventually flowed into the Black Sea, then the Mediterranean Sea. These prehistoric lakes formed and re-formed several times before the glaciers retreated at the end of the Ice Age, allowing the Ob to once again flow to the sea.
Physically the plains comprise vast areas of marshes and swamps, although the low northeast-southwest trending hills created by glacial moraine deposits cross much of the basin. Much of the Ob basin is poorly drained, resulting in large lakes and frequent flooding as high flows cannot escape quickly enough. For much of its length the Ob is a great braided stream that meanders and branches across the entire width of its valley. The river channel is surrounded by numerous sloughs, peat bogs and oxbow lakes created where old streambeds have been cut off and new ones have formed.
The southern outline of the Ob watershed began forming as early as five hundred million years ago, when geologic upheaval caused by the collision of the Indo-Australian Plate and the Eurasian Plate caused the upthrust of the great mountain ranges of central Asia – the Himalayas, Pamir Mountains, Tian Shan, and even the Altai Mountains, far north of the plate boundary. The most recent orogeny or mountain-building episode began about 1.6 million years ago, and continues to this day.
Rocks underlying the Altai have been shaped, torn and twisted by tectonic and volcanic activity constantly, and thus exhibit a variety of rare minerals such as gold and marble. The Ob's headwaters, the Biya and Katun, as well as the Kara-Irtysh (Black Irtysh), Chulym, and many other smaller streams, have incised deep valleys into the range creating huge vertical relief of up to 2,400 metres (7,900 ft).
During the Pleistocene, valley glaciers scoured out U-shaped gorges, and in some instances dammed rivers to create long and narrow lakes. One such glacier impounded the Chuya River, a tributary of the Katun, to create a huge lake holding 3,500 cubic kilometres (2.8×109 acre·ft) of water. Around 12,000 years ago the ice dam catastrophically failed, sending almost 18,000,000 cubic metres per second (640,000,000 cu ft/s) churning down the Katun towards the Ob. This event which may have occurred more than once, may be the largest flood ever known to occur on Earth, rivaling the Bonneville Flood and Missoula Floods which happened during the same period in western North America.
As a transportation artery
Human use and impact
Most of the upper Ob River flows through a heavily farmed region and significant amounts of water are drawn from the river to provide irrigation to the surrounding fields. From the Ob's headwaters at the confluence of Katun and Biya to Novosibirsk, discharge steadily decreases according to streamflow gauges along the route during the growing season. Further north, the climate becomes too cold for significant agrarian development.
The Ob's main tributary, the Irtysh, is currently threatened by developments in China and Kazakhstan which propose to greatly increase the irrigated acreage along the upper reaches of the river. In the late 20th century China began serious studies on a Black Irtysh-Karamay canal which would remove up to 40% of the water from the upper Irtysh. Kazakhstan has similar proposals for the Irtysh, albeit on a smaller scale. It is feared that with intensive irrigation, parts of the Ob basin could become desert with the loss of water from the Irtysh. Pollution would become more concentrated, and fish species in the lower Irtysh and the Ob downstream from the Irtysh confluence would be affected. In the long term this would result in a severe reduction of flow in the lower Ob. Currently, of this river's discharge – 2,113 cubic metres per second (74,600 cu ft/s) at Tobolsk – 6% is already diverted.
Dams and hydropower
Northern river reversal
In the 1950s the Soviet government promoted heavy agricultural development along several Central Asian rivers including the Volga River, Amu Darya and Syr Darya. This resulted in desiccation of large areas of non-irrigated land and the severe shrinkage of various bodies of water, specifically the Aral Sea, once the world's fourth-largest lake. Although the idea of diverting the flows of Siberian rivers to the arid plains of Central Asia was not new, having first arisen as early as the 1830s, the first serious proposals were made in the late 1960s. One plan involved diverting 6-7% of the Ob's water through a 2,550-kilometre (1,580 mi)-long canal towards the Aral Sea. Other proposals called for rerouting other watercourses such as the Yenisei, the Pechora or the Northern Dvina.
These grand plans almost immediately ran into controversy over political, economical and ecological consequences. Scientists feared that reduction of freshwater flow to the Arctic could affect levels of sea ice leading to disruptions of climate trends in northern Siberia and possible negative effects on ocean currents. It was also suggested that after a period of time in which benefiting countries such as Kazakhstan and Uzbekistan had adjusted to the new influx of water, Russia could use water supply to increase its political power in the region – which was part of the Soviet Union until its collapse in 1991.
- "Ob River at Salekhard". River Discharge Database. Center for Sustainability and the Global Environment. 1930-present. Retrieved 2010-11-06. Check date values in:
- Vogtmann, Dobretsove and Mittlestaedt, p. 261
- ACME Mapper (Map). Cartography by NAVTEQ. ACME Mapper. Retrieved 2010-11-07.
- Vasiliev, Oleg F. "Water Resources Management in the Ob-Irtysh River Basin and Related Environmental Problems". International Association of Hydro-Environmental Engineering. Retrieved 2010-11-06.
- "Climate and Hydrology of the Ob basin". Encyclopaedia Britannica. 1995. Retrieved 2010-11-07.
- "Ob, Gulf of". Encyclopaedia Britannica. 1995. Retrieved 2010-11-07.
- Wilkman, Göran; Okko, Olli (2001-08-17). "Transportation system for the export of gas-Condesate from Ob Bay, western Siberia" (PDF). Technical Research Center of Finland. Aker Arctic. Retrieved 2010-11-07. More than one of
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- "West Siberian Plain". Encyclopaedia Britannica. 1995. Retrieved 2010-11-11.
- "Lakes of the Ob River Valley, Siberia". Geomorphology from Space. National Aeronautics and Space Administration. 2009-09-09. Retrieved 2010-11-11.
- "The Altai Flood" (PDF). Geology and Geological Engineering. Colorado School of Mines. Retrieved 2010-11-11.
- Vajda, Edward J. "Geological Chronology". Western Washington University. Retrieved 2010-11-07.
- "Altai mountains". Siberian Institute for Ecological Initiatives. 2006-02-23. Retrieved 2010-11-11.
- Yashina, T. (2008-03-31). "Protection of geological heritage in the Altai Mountains (Russia)". Global Network of National Geoparks. Retrieved 2010-11-11.
- Herget, Jürgen (2005). "Reconstruction of Pleistocene ice-dammed lake outburst floods in the Altai Mountains, Siberia". Geological Society of America. p. 19.
- Jones, Vardanian and Hakopian, p. 343
- "Chapter 4: Water Pollution/Quality". Central Asia Preparation of Regional Environmental Action Manual of Guidelines. AIT-UNEP Reginal Resource Centre for Asia and the Pacific. Retrieved 2010-11-12.
- "Irtysh discharge at Tobolsk". Soviet Union Hydro-Station archive. UNESCO. 1936–1990. Retrieved 2010-11-12.
- Schafarenko, Sergej (1999-11). "Black Irtysh river endangered by irrigation channel". Cultural and Ecological Associan Belovodje. RiverNet. Retrieved 2010-11-12. Check date values in:
- "The Aral Sea". Visualizing Earth. University of California, San Diego. Retrieved 2010-11-10.
- Nicholson, Alex (2002-12-10). "Luzhkov Wants to Reverse a River". The Moscow Times.
- Prugh, Tom (2004-07-01). "Heroic river diversion plan resurrected". World Watch. The Free Library. Retrieved 2010-11-10.
- "Russian Politicians Use Water Resources to Assert Influence". Strategy Page. 2008-09-14. Retrieved 2010-11-10.
- Vogtmann, Hartmut; Dobretsov, Nikolai; Mittelstaedt, Astrid (2006). Environmental security and sustainable land use: with special reference to Central Asia. Springer. ISBN 1-40204-492-5.
- Jones, J. Anthony A.; Vardanian, Trahel G.; Hakopian, Christina. Threats to Global Water Security. NATO Science for Peace and Security. シュプリンガー・ジャパン株式会社. ISBN 9-04812-343-7.