Boundaries

The ecoregion includes the Yamal Peninsula and rivers of the Obskaya Guba Bay basin. The western border [with ecoregions 407 and 410] goes along the Ural Range down to Turgai Tableland [Turghay Ustirti], which divides the headwaters of the Igriz and Turgai [ecoregion 601] from the Tobol (Irtysh tributary) headwaters. The border crosses the Turgai Dolina Valley [Torghay Zhylghasy], which includes the Ubagan River (Tobol tributary), and runs southward along the western slopes of the Kazakhskiy Melkosopochnik upland [Qazaqtyng Usaqshoqylyghy]. It then turns east to divide the Sarysu River basin [ecoregion 626] from the Nura River system and tributaries of Lake Tengiz that belong to this ecoregion. The border travels further east around 49° N, with the Lake Balkhash tributaries [ecoregion 624] to the south, and then travels along the Chingistau Range [Shinggystau Zhotasy] to include the Ashchysu River drainage area. Here, this ecoregion is contiguous with the Upper Irtysh ecoregion [603] along the slopes of the Kalbinskiy and Ul’binskiy ranges. The border crosses the Irtysh River at Ust’ Kamennogorsk and extends eastward along the Ivanovskiy Range, with Ul’ba River basin [603] to the south. The border then curves to the north along the Konsuyskiy Range, which divides the Koksa River headwaters [604] and Uba River headwaters [602]. It travels further east and southeast along the Terektinskiy Range to the confluence of the Chuya and Katun’ rivers (below this point, the Katun’ basin belongs to the Ob’ ecoregion). The border next extends east along the Saldzhar Range, and then north along the Sumul’tinskiy Range, excluding the Chebdar River basin [Chulyshman tributary, 604]. The border reaches the southernmost extremity of Lake Teletskoye [in 602]. Finally, the eastern border of the ecoregion extends northward along the Abakan Range, which forms the border with the Yenisei ecoregion [605], and along the eastern margin of the Zapadno-Sibirskaya Nizmennost’ Plain (along the western margin of the Yenisei ecoregion [605]).

Freshwater habitats

Throughout most of its length, except for the upper reaches, the Ob’ is a typical plain river. Downstream from the confluence of the Biya and Katun rivers it flows in an undulating forest – steppe plain. The Ob’ enters the taiga zone around the confluence with one of its large tributaries, the Tom’ River. Here, the width of the valley is 20 km, the floodplain width is 1-5 km, the depth during the mean water period is 2-6 m, and flow velocity is 0.3-0.5 m s^-1 (during flood, up to 2 m s^-1). Downstream from the estuary of the Tom’ River, it flows among the swampy taiga plain. Coniferous forests and swamps occupy the wide, flat interfluves of the Ob’ and Irtysh. The width of the valley increases up to 30-50 km, and in the floodplain, up to 20-30 km. A large number of lakes and bayous occur within the floodplain, covered by meadows and forests. The river channel is split into an elaborate network of branches and anabranches. The depth in the mean water period is 4-8 m, flow velocities are 0.2-0.5 m s^-1, and maximum flow velocities are up to 1.8 m s ^-1. In the lower part, downstream from the confluence with the Irtysh, the Ob’ becomes a powerful water stream. During the spring tide, flooded areas may extend 40-50 km. The maximum depths are 15-20 m. Flow velocities vary from 0.2 to 0.5 m s^-1, and in the flood period can reach 1.6 m s^-1. The largest tributary of the Lower Ob’ is the Northern Sos’va.

The delta of the Ob’ River begins at the Bolshiye Yary Island. There, the river is split into a number of branches, the largest of which are Khamanelskaya Ob and Nadymskaya Ob (the width of the latter is 30 km). Ob’ Bay is a typical estuary formed as a result of flooding of the river valley.

The amplitude of the water level fluctuation over most of its flow is 9—10 m. In the flood period overflows are observed, attaining tens of kilometers in width in the lower reaches. The Ob’ River freezes first in the lower reaches, in early November. In the upper reaches, the river freezes around mid-November. It is typical that the river freezes in the lower reaches about 10 days later than the rivers contiguous with it, although it is characterized by lower amounts of water. This is accounted for by the large amount of heat that is carried north by the Ob’ from the southern latitudes.

With the exceptionally plain nature of the terrain and, respectively, very small slopes, the river network on the whole cannot drain rapidly melting waters and rain waters. Its drainage is weak and does not spread far beyond the limits of the valley. This has contributed to the extensive marshes and swamplands covering even flat watershed areas of the taiga zone.  Around half of the total area of marshland and swampland in Russia occurs in the Ob’ drainage. The well-known Vasyuganskiye marshes extend for hundreds of kilometers. 

The peculiar nature of taiga rivers is also attributed to the plain character of the terrain. They are characterized by very small slopes (less than 0.1 m/km). Vasyugan, for instance, has a mean slope of approximately 0.02 m/km. The river current is weak, coasts are low, floodplains are wide with numerous lakes, and river channels are overgrown by dense vegetation. In some areas it is difficult to distinguish where the river ends and the swamp begins. Rivers of the taiga zone have strong, undulating channels that wander through the wide floodplains. During spring tide they flood extensively, impounding the adjoining swamps.

The Western Siberian Lowland is characterized by a large number of lakes of various types, including glacial, floodplain, intra-marsh, termokarst, lake hollows, etc. The largest is Lake Chany.

Terrestrial habitats

The ecoregion lies within the Western Siberian Lowland, which is one of the largest lowlands on earth. It extends 2500 km from north to south, and 1500 km from west to east. It is an extensive swampy plain, with absolute heights up to 80-120 m. The Ob’ River crosses the entire lowland from south to north, from Novosibirsk up to the estuary (approximately 3000 km). It is weakly inclined toward the north, with a slope of only 94 m, or on average a little less than 3 cm per 1 km. In the Western Siberian Lowland zonal dispersal landscapes are pronounced. The extreme northern part of the Ob’ River drainage near Salekhad lies in the tundra zone. Taiga extends from the southern boundary of the tundra zone, up to Ekaterinburg – Tomsk, with coniferous and mixed forests. The southern part of the drainage lies in a forest-steppe zone that gradually grades into a steppe zone.

Description of endemic fishes

The ecoregion has one strict endemic species. Pravdin’s dwarf whitefish (Coregonus pravdinellus) is a small-sized whitefish (up to 11-14 cm length in adults) with autumn spawning. It is endemic to Lake Teletskoye, and spawns in the lower reaches of its tributaries, or enters the source of the Biya River (that flows from the lake). It is exclusively pelagic, feeding on zooplankton in the water layer between 0-50 m.  It is not abundant, but is also not rare.

Other noteworthy fishes

Ignatov’s minnow (Phoxinus czekanowskii ignatowi) inhabits small left bank tributaries of the valley part of the Irtysh River (e.g. Selety, Olenty, and other rivers that do not have connection to Irtysh at present). This rare species is spread mosaically in small springs. These peculiar small water bodies have been partially destroyed and included into the irrigation system of the Irtysh – Karaganda Channel.

Teletskoye Lake whitefish (Coregonus smitti) (listed in Fishbase as a synonym of the more widespread Coregonus pidschian) is the other endemic whitefish of Lake Teletskoye. It is a larger-sized lacustrine whitefish (up to over 40 cm total length) that is rather strictly lacustrine. It spawns in the lake, and feeds on benthos.

A local form of burbot (Lota lota), called "natio profunda," occurs in Lake Teletskoye. The Lake Teletskoye burbot differs by some morphological characters from the burbot of its main range, but mostly by peculiarities of biology – it is a deepwater dweller, inhabits depths of 20-30 m in winter, and migrates further down, up to 100-120 m in summer.

Ecological phenomena

A characteristic feature of the Lower Ob’ is the extremely elaborate system of sors — shallow floodplain water bodies that serve as the major areas of feeding migrations of coregonid fishes. Sardine cisco (Coregonus sardinella), Arctic rainbow smelt (Osmerus mordax), and humpback whitefish (C. pidschian) migrate upstream from Ob’ Bay, but do not occur further upstream from the Irtysh estuary. In the left bank tributaries of the Lower Ob’, peled (C. peled), and inconnu (Stenodus leucichthys) spawn. The Severnaya Sos’va is the only river where tugun (C. tugun) occurs (a distinct small-sized form called Sos’va herring).

Justification for delineation

All large rivers in Siberia flowing into the Polar Ocean from the Ob’ to the Lena and Yana are characterized by a pronounced similarity of the fauna (absence of Salmo, presence of Acipenser and Brachymystax, a wide variety of Coregonus species) and close hydrological features. The Ob’, Yenisei, and Lena rivers are the 5^th, 6^th, and 11^th longest rivers in the world, respectively, and their drainage basins are the 5^th, 10^th, and 7^th largest. They all include several typical zones or sections from piedmont drainages to lowland deltas. Lake Baikal, as well as the upper drainages of the Ob’ and its tributary the Irtysh, are separated out as distinct ecoregions due to faunal differences. Although the fish faunas of the remaining portions of the Ob’, Yenisei [605], and Lena [608] basins are quite similar, we delineate them here as separate ecoregions to create tractable conservation units.

Level of taxonomic exploration

Good