The West Siberian Plain (Russian: -? Zapadno-Sibirskaya ravnina) is a large plain that occupies the western portion of Siberia, between the Ural Mountains in the west and the Yenisei River in the east, and by the Altay Mountains on the southeast. Much of the plain is poorly drained and consists of some of the world's largest swamps and floodplains. Important cities include Omsk, Novosibirsk, Tomsk and Chelyabinsk.
The West Siberian Plain is located east of the Ural Mountains mostly in the territory of Russia. It is one of the Great Russian Regions and has been described as the world's largest unbroken lowland—more than 50 percent is less than 100 m (328 ft) above sea level—and covers an area of about 2.6-2.7 million square kilometres (1.0 million square miles) which is about one third of Siberia, extending from north to south for 2,400 km (1,491 mi), from the Arctic Ocean to the foothills of the Altay Mountains, and from east to west for 1,900 km (1,181 mi) from the Ural Mountains to the Yenisei River.
The plain has eight distinct vegetation regions: tundra, forest-tundra, northern taiga, middle taiga, southern taiga, sub-taiga forest, forest-steppe, and steppe. The number of animal species in the West Siberian Plain ranges from at least 107 in the tundra to 278 or more in the forest-steppe region. The long Yenisei river flows broadly south to north, a distance of 3,530 km (2,193 mi) to the Arctic Ocean, where it discharges more than 20 million litres (5 million gallons) of water per second at its mouth. Together with its tributary Angara, the two rivers flow 5,530 km (3,436 mi). The valley formed by the Yenisei acts as a rough dividing line between the West Siberian Plain and the Central Siberian Plateau. Glacial deposits extend as far south as the Ob-Irtysh confluence, forming occasional low hills and ridges, but otherwise the plain is exceedingly flat and featureless.
Winters on the West Siberian Plain are harsh and long. The climate of most of the plains is either subarctic or continental. Two of the larger cities on the plain[clarification needed] are Surgut and Nizhnevartovsk.
The West Siberian Plain consists mostly of Cenozoic alluvial deposits and is extraordinarily flat. A rise of fifty metres in sea level would cause all land between the Arctic Ocean and the Ob-Irtysh confluence near Khanty-Mansiysk to be inundated (see also Turgai Straits, West Siberian Glacial Lake). It is a region of the Earth's crust that has undergone prolonged subsidence and is composed of horizontal deposits from as much as 65 million years ago. Many of the deposits on this plain result from ice dams that reversed the flow of the Ob and Yenisei rivers, redirecting them into the Caspian Sea, and perhaps the Aral Sea as well. It is very swampy and soils are mostly peaty Histosols and, in the treeless northern part, Histels.
This is one of the world's largest areas of peatlands, which are characterized by raised bogs. It is believed that Vasyugan Swamp is the world's largest single raised bog, covering approximately 51,600 square kilometres (19,900 sq mi).
In the south of the plain, where permafrost is largely absent, rich grasslands that are an extension of the Kazakh Steppe formed the original vegetation, which had almost all been cleared by the early 21st century.
Large regions of the plains are flooded in the spring, and marshlands make much of the area unsuitable for agriculture. The principal rivers in the West Siberian Plain are from west to east the Irtysh, Ob, Nadym, Pur, Taz and Yenisei. There are many lakes and swamps. This area had large petroleum and natural gas reserves. Most of Russia's oil and gas production was extracted from this area during the 1970s and 80s.