Hydrography

Hydrographic information on the Rivers of Mongolia

There are almost 4000 Rivers with a total length of 70,000 km on the territory of Mongolia. Hydro graphically Mongolia's rivers are divided among the Arctic basin, Pacific basin and the blind drainage basin of Central Asia.

Most of the large rivers belong to the Arctic basin, which spans the Khangal and Khentii mountains and the Khovsgol area (a small part of the Mongolian Altai mountains) and occupies about 25 per cent of Mongolia's territory.

Rivers of the Arctic Basin

The total length of rivers in the Arctic Ocean basin is 35.000 km or 50 per cent of the total length of all rivers. The flow in this basin accounts for 51.4 per cent of the total annual run-off. The Selenge and its tributary the Orkhon are Mongolia's major rivers in this basin.

Rivers of the Pacific Ocean Basin

The rivers in the country's east. Flowing from the slopes of the Khentii mountains and the off-spurs of the Ikh Hyangan. Come under the Pacific Ocean basin. The area of the drainage system, which gravitates towards this basin, occupies about 11 per cent of the country's territory and its flow accounts for about 15 per cent of the total run-off. The major rivers of the Pacific Ocean basin are the Onon, Ulz, Kherlen and Khalhyn Gol.

Rivers of blind drainage Central Asian

The blind drainage basin of Central Asia engages about two thirds of Mongolia's territory. The water distribution on this territory is rather uneven. More or less developed drainage systems are to be found in the Mongolian Altai. In the western parts of the Khangai mountains and on its southern slopes. The basin accounts for nearly 33 per cent of the total run-off of all the rivers of Mongolia.

Drainage Density, Flow and Temperature

The drainage density changes depending on physical and geographical conditions. It is much more dense in areas with an elevated surface flow. i.e. in the forest steppe zone and weak in the steppe and specifically in the semi-desert zones. The average drainage density throughout the whole country is 0.04 km/sq. km. The surface flow is formed by rains and snow melt water, but they are not the same everywhere. One of the special features of the rivers in Mongolia is the high level of alimentation by unconfined groundwater, the volume of which varies from 25 to 40 per cent. In the Gobi desert, brooks are fed exclusively by unconfined groundwater.

Mean annual flow

Generally, the distribution of mean annual flow mirrors the distribution of rainfall. Which in turn is determined by the topography of the land. Therefore in general, there is a combination of latitudinal with latitudinal sonality. The former is expressed in a general decrease in the size of flow from north to south, and the latter in a drop in flow with a lowering of the altitude of the land and middle elevation of a river basin. The mean annual flow is highest in Khentii at 6 1/s per sq. km. It maybe even higher in the upper reaches the Onon, Minj and Yoroo. Local surface flow is practically nil in the lowlands of Mongolia, including the Gobi, the Depression of the Great Lakes, and most of the elevated highlands of eastern Mongolia.

Yearly distribution of flow

This varies. Water discharge in rivers usually increases in the second half of April and continues up to mid-May. The maximum spring flood occurs owing to high mountain thaw water. Water discharge becomes less in the second half of May and a quite stable drought period settles in late May and early June. In July, August and occasionally in September, rivers are rain-flooded several times (2-7) the maximum discharge of which is usually 1.5-3.0 times higher than the greatest flow of spring flood in winter, major rivers in Khangai and Khentii have a low but stable flow owing to unconfined ground-water reserves.

The temperature of water and ice conditions

The change of water temperature in rivers throughout the territory and in the course of a year corresponds in general, to the changes in air temperature. All rivers, on average, begin to warm up in late April and the temperature of the water rises gradually until July. During the warm period of the year the mean water temperature changes substantially in different parts of the country. Rivers fed by glaciers have the coldest waters (6-IO C). The highest yearly and monthly temperatures of river waters have been recorded in the southern and western lowlands. The mean temperature of water in July in the lower reaches of the Zavkhan, Baidrag and other rivers are more than 18-20 C. The water begins to cool from July-August and continues on average up to the second week of November. All the rivers in Mongolia freeze up. most of them for 140-18O days. The thickness of the Ice cover reaches 8O-120 cm on average. Rivers in the plains become ice-free in April and those in the mountains remain ice-covered until mid-May.

Basin of the Arctic ocean (key to colours below)

1. Selenge moron 593 (992) 232000 (447000) Khangay Lake Baikal (Russia)
2. Ider 452 24555 Khangay Selenge moron
3. Delger moron 445 26640 Ulaan taiga Selenge moron
4. Chuluut 415 10750 Khangay Ider
5. Orkhon 1124 132835 Suvraga khairkhan Selenge moron
6.Tuul 819 50400 Khentii Orkhon
7.Kharaa 291 15000 Khentii (Tolgoit) Orkhon
8.Eoroo 323 11860 Khentii Orkhon
9. Egijn gol 475 40454 Lake Khovsgol Selenge
10. Shishkhid 298 18300 Ulaan taiga Enisay source
11.Khanul 421 14620 Khangay Selenge moron

Basin of the Pacific ocean (key to colours below)

12. Kherlen 1090 (1264) 116455 Khentiy lake Dalainuur
13. Ulz 420 (428) 27500 Khentiy lake Tari Khokh
14. Onon 296 (445) 29070 (40220) Khentiy Ingedey
15. Khalkhiin gol 233 17120 Khiyangan lake Buir

Blind drainage basin of Central Asia (key to colours below)

16. Tesiin gol 568 33358 Bulnay lake Uvs
17. Khovd 516 58OOO Altay lake Khar Us
18. Zavkhan '" 71210 Khangay lake Ayrag
19. Baydrag 310 28277 Khangay lake Boontsagaan
20. Tuin gol 243 9410 Khangay lake Orog
21. Ongiin gol 435 52920 Khangay lake Ulaan nuur
22. Bulgan 250 (310) 8750 Altai Urungi
23. Khungui 200 21693 Khangay lake Taatsiin agaan

Key to main rivers
Name of river
Length
Area sq.km
Rivers source
Flows into

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