Boundaries

The ecoregion includes the whole Yenisei River drainage except for Lake Baikal and lakes of the Khantayka River headwaters (southwestern Putorana Plateau). The western border (with the Ob’ ecoregion [602]) strictly follows the watershed along the Yuribeyskaya Gryada Range, the eastern margin of Zapadno-Sibirskaya Nizmennost’ Plain (Nizhneyeniseyskaya Vozvyshennost’), and the Abakan Range. In the southwest, the Shapshal’skiy Range divides the Chulyshman drainage area [Chuya ecoregion [604]) and Yenisei headwaters (Kemchik River). In the south, the border runs along the ranges of Zapadnyy Tannu-Ola, Vostochnyy Tannu-Ola, and Sangilen. The latter divides the Tesiyn Gol drainage area (Western Mongolia ecoregion [622]) from the Malyy Yenisei [Ka Hem] drainage. Between the Bulnain Nuruu Range in the north and Tarvagatayn Nuruu [Tarbagatin Ondor Uul] Range in the south, the southern border of the ecoregion curves around the Ider Gol River drainage area (Selenga River basin), and extends southeast along the Hang Kai [Hangayn Nuruu] Range, which divides this ecoregion from the eastern part of the Western Mongolia ecoregion [622]. In the southeast the ecoregion is contiguous with the Inner Mongolia Endorheic Basin ecoregion [621] and the Amur [Heilungkiang] drainage area along the Hentiy Han [Hentey Haan Nuruu] and Yablonovyy ranges. In the east, the border of the ecoregion goes along the Yenisei and Lena [608] divide (ranges just to the east, north, and west of Lake Baikal, Limskiy Range, and eastern slopes of the Srednesibirskoye Ploskogor’ye Upland). In the southeast at about 66 °N the border turns west along the divides between the Kotuy and Ayan rivers (Khatanga River headwaters in ecoregion 607) and Nizhnyaya Tunguska and Kureika rivers. The border of the Yenisei ecoregion runs along the southern spurs of the Putorana Plateau, which lies in the extreme northwest of the Srednesibirskoye Ploskogor’ye Upland in the Taimyr ecoregion [607].  The border then turns northward along the divide of the Lower Yenisei and the Pyasina River [607] drainages.

Topography

The Yenisei River drainage is characterized by complex relief. A large central part of the territory is occupied by the Central Siberian Plateau that extends in a longitudinal direction for more than 1500 km, and from west to east along the entire width of the Yenisei. The Western Siberian Lowland to the west runs along the left coast of the Yenisei in a relatively narrow zone (100-250 km) from the northwestern spurs of the Eastern Sayan Mountains up to the coasts of the Kara Sea. The northern extremity of the Central Siberian Plateau, represented by the Putorana Mountains, ends abruptly around the North Siberian (Taimyr) Lowland. The Altai-Sayan Highland lies at the southern end of the ecoregion. This is an orographically complex region, connected in the south by the northern outskirt of the hollow of the Bol’shiye Ozera (Large Lakes). The outskirt of the hollow is represented by the Ubsu-Nur desertified hollow (ecoregion 622).

The drainage of the Yenisei River occupies the middle part of the Asian continent. It extends from north to south for 3200 km and from west to east for 100 to 1200 km. The valley of the Yenisei River separates the Middle Siberian Plateau from the eastern part of the Western Siberian Lowland, which is smaller, lower, and to a large extent, marshy. One of the characteristic features of the structure of the Yenisei’s hydrographic network is its sharply pronounced asymmetry. The right coast mountain and well-developed side is 5—6 times larger in area than the left coast section that lies in a narrow zone on the eastern outskirt of the West Siberian Lowland.

The hydrographic network was formed mostly in the remote geological epochs. The high elevations in the southern part of the ecoregion (Sayan Mountains), the Central Siberian Plateau (a mountain region occupying an extensive area in the middle part of the Yenisei drainage), and the North Siberian Lowland in the north determined the direction of flow for the majority of rivers from south to north and from southeast to northwest. The main river, Yenisei, flows in a northerly direction almost exclusively; hence it is often called the Yenisei longitude. The length of the Yenisei from the confluence of its tributaries to its estuary is 3487 km; the length from the headwaters of the Bolshoi Yenisei is 4092 km; and from headwaters of the Malyi Yenisei is 4050 km. If the Selenga River is regarded as the headwaters of the Yenisei the length of the entire system (Selenga—Angara—Yenisei) would be equal to 5940 km. The total area of the drainage is 2,580,000 km². Of this area, 1,039,000 km² falls in the Angara drainage.

The Yenisei River is commonly divided into three parts: the Upper Yenisei – from the beginning of the river (Kyzyl) up to the estuary of the Tuba (length 1238 km); the Middle Yenisei, from the estuary of the Tuba up to the estuary of the Angara (717 km); and the Lower Yenisei, from the confluence with the Angara to the mouth (2137 km). Downstream from the confluence with the Lower Tunguska the width of the Yenisei ranges on average from 2 to 5 km. Where the river channel is split into branches its width increases up to 7-12 km.

Downstream from Krestovsky Cape the Yenisei’s estuary (marine) begins; its length is approximately 300 km. Two groups of islands occur here:  Mininskiye Islands and Brekhovskiye Islands. In this region the Yenisei River is split into numerous branches and anabranches. The total length of the river in the alignment of the islands reaches 40—50 km.

Darkhadskaya Hollow is situtated in the drainage of the Shishkhid River, a tributary of the Upper Yenisei. The absolute height of the hollow in the region of Lake Don-Tsagan is 1560 m. The mountains surrounding Darkhad Hollow are morphologically connected with the Sayan Mountains and the mountains of the Western Khubsugul region. A large part of the hollow is swampy. Permafrost is spread nearly throughout the entire territory.

The drainage area of the Selenga includes the group of Hangayn lakes, with large lakes Hovsgol Nuur [Khubsugul = Kosogol], Dood Nuur [Dood Tsagaan], and Terhiyn Tsagaan. Lake Khubsugul is a part of the system of tectonic basins of the Baikal type, constituting its southern link in northern Mongolia. It is connected hydrographically with Lake Baikal through Lake Egiin-Gol, a tributary of the Selenga River falling into Lake Baikal. The altitude of Lake Khubsugul above sea level is 1645 m, length is 134 km, maximum width is 39 km, average width is 20.4 km, area of the drainage is 5130 m², surface area is 2730 m², maximum depth is 262 m, and average depth is 138 m. The peculiarity of Lake Khubsugul as an ecosystem is determined by the nature-climatic complex characteristic of alpine areas. The sharply continental climate determines the characteristic features of biological life of water masses of the lake, in particular its oligotrophy.

Freshwater habitats

The Upper Yenisei is mountainous in character as it cuts through the Western Sayan Mountains.  Here it flows through a deep ravine, which in some areas turns into a rocky corridor-canyon with vertical slopes, and a channel that narrows to 100 m. The flow velocities during floods reach 5—7 m sec^-1. The valley of the Middle Yenisei lies in mid-altitude mountain terrain. Downstream from the village of Oznachennoye the river flows through Minusinskaya Hollow, which is of steppe character. Between Minusinsk and Krasnoyarsk the Yenisei first crosses the Batevskii Ridge and then the northwestern spurs of the Eastern Sayan Mountains. The valley here again becomes narrow and deep. Downstream from Krasnoyarsk the left slope lowers notably and the valley loses its mountain character. The Lower Yenisei is a wide, powerful flow with notable depths. Downstream from the confluence with the Lower Tunguska River the width and water volume of the river increase even more.

The main water sources of the Yenisei are melt waters and rain waters; subsoil waters are of secondary importance in the feeding of the river. Related to this, the water regime of the Yenisei is characterized by a high and long flood in spring-summer, stable mean water in autumn, and low mean water in winter. Spring flood occurs in the first half of the summer due to snowmelt that occurs up to the beginning of the summer in the upper part of the drainages. Its duration is influenced by rains falling in the same period. The maximum level is observed during the most abundant entry of melt waters from the mountainous part of the drainage. The flood wave moving downstream is enhanced by local snowmelt. Under those conditions the amplitude of fluctuation of water level may be quite high, reaching 8—20 m. In the lower reaches tidal fluctuations of the water level occur. The tide on the Yenisei is conspicuous between Gurukhanek and Kureika (i.e. at a distance of more than 800 km from Dickson).

In the lower reaches the Yenisei is covered by ice approximately ten days later than neighboring local rivers. This is accounted for by the fact that the Yenisei carries a large amount of heat to the estuary. Therefore, a relatively long period of time is needed for the cooling of the water mass. In the period of freezing a large amount of bottom ice is formed, which is called “shaksha”by the local population. In spring when the river opens up heavy ice jams are observed. The water level during the ice jams rises to 20 m.

In terms of water volume, the Yenisei ranks first among the rivers of the former USSR; its average annual water flow is equal to 17,400 m³ sec^-1.

Terrestrial habitats

The ecoregion is comprised of Sayan montane forests, Sayan intermontane steppe, Sayan alpine meadows and tundra, and South Siberian forest-steppe in the south; East and West Siberian taiga throughout its middle range; and Yamal-Gydan tundra and Taimyr-Central Siberian Tundra in the north.

Description of endemic fishes

The ecoregion contains two endemics in its headwaters: Gobio sibiricus and Kosogol grayling (Thymallus nigrescens).

Kosogol [Hovsgol Nuur] grayling (Thymallus nigrescens) is an endemic species of Lake Hovsgol Nuur [Kosogol = Khubsugul] and its inflowing rivers. It is a common fish of the littoral, down to about 25 m in depth. In summer (July, August) part of the fish stock consisting mainly of immature fish remains in the pelagic waters, while the other part of the fish stock migrates down to 60-80 m. There are two ecological forms of the grayling – one spawning in lower reaches of rivers flowing into the lake, and one that spawns in the lale littoral zone. Those forms are litophilous. The former spawns from mid-May until mid-June. The spawning takes place in the evening at a water temperature of 6.8-10 ^oC in rivers. The late spawners lay the eggs in littoral down to 5 m depth in July-August. The water temperature required for spawning of the late-spawning Kosogol grayling is 7-14^oC. The fish reaches a maximum weight of 350 g at 9-11 years old and a body length of 310-350 mm. The Kosogol grayling is an omnivore, feeding predominantly on plankton.

It should be emphasized here that the headwaters of all great Siberian rivers – Ob’, Yenisei, and Lena – may still contain pre-glacial relicts of different age, and this phenomenon needs special studies.

Other noteworthy fishes

There ecoregion contains three endemic subspecies that have limited distributions connected to the Selenga River (Yenisei headwaters); however, they are of doubtful status and need to be studied further.

Leocottus kesslerii gussinensis is known from the Selenga River headwaters (Baikal-Yenissei system): Lake Gusinoye (Russia); and lakes Tsagaan Gol, Tsagaan Nur, Bain [Bayan] Gol, Tsaidam (Mongolia). The nominative subspecies occurs in Lake Baikal and lower reaches of its tributaries.

Leocottus kesslerii arachlensis is only known from the Arakhley Lakes in the upper Khilok River, a tributary of the Selenga River. However, these lakes are considered by some geographers to belong partly to the Vitim system (a Lena tributary). This case needs special phylogeographic studies because of its affinities with Baikalian Leocottus kesslerii.

The Siberian Baikal sturgeon (Acipenser baerii baicalensis) inhabits Lake Baikal throughout most of its life cycle. The Selenga River is the major spawning ground for Baikal sturgeon, which undertakes spawning migrations in this river. In Mongolia it enters the rivers Orhon, Tula, and Dergen Moron. There are two migrations during the warmer part of the year. One starts in the second half of April at a water temperature of 3-5 ^oC. This migration ends in mid-June. The second migration, which is the major migration of this sturgeon, starts at the end of July and ends in mid-September. This migration coincides with summer floods in the Selenga. The migration stops with declining water temperature. Sturgeons have often been observed to overwinter in deep pools of the Selenga and its tributaries. In both migration periods the migrants include sexually mature individuals as well as younger fish. Apart from the migrating sturgeon there is a non-migrating sturgeon population in the rivers Orhon and Tula of the Selenga catchment. The Baikal sturgeon’s taxonomic relationship to Acipenser baerii stenorrhynchus, which inhabits the lower reaches of Yenisei, are not clear. Some authors consider Acipenser baerii as a monotypic species.

Justification for delineation

All large rivers in Siberia flowing into the Polar (Arctic) 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, and a high variety of Coregonus species) and close hydrological features. The Yenisei is the 6^th longest river in the world, and its drainage basin is the 10^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’ [602], Yenisei, and Lena [608] basins are quite similar, we delineate them here as separate ecoregions to create tractable conservation units.