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Physical Geography of Northern Eurasia

Rivers, Lakes, Inland Seas, and Wetlands

<<< Suspended Sediment and Mineralization | Physical Geography Index | Anthropogenic Modifications of River Runoff >>>

The Thermal and Ice Regime

Water temperature in the majority of rivers is low or at best moderate as a result of the early and severe winters and widespread permafrost. In the European north, Siberia and the Far East, the annual water temperature is less than 5°C and only in the comparatively small area in steppes and semi-deserts does it exceed 9°C. Even the mean temperature of July, which is the warmest month, seldom exceeds 16°C in Siberia and the Far East, and is less than 10°C in the northern part. In the European territory it is close to 20°C and only in the southernmost regions is the mean July temperature higher than 24°C (Sokolova, 1951). Because most rivers flow in a meridional direction, the thermal regime of the large rivers is azonal. In northern Siberia, such rivers are an important factor in the amelioration of local climate.

Many rivers, draining plains, freeze each year. The formation of ice begins in late September in the extreme north-east of Siberia while in the European south-west it does not occur until late December. The northern Siberian rivers remain frozen for 7-8 months until May (in north-eastern Siberia until June) and ice can be 1.5-2 m thick. In the European south-west and in the southern Maritime Province of the Far East, ice cover stays for a short period of 1-2 months and its thickness usually does not exceed 30 cm. However, even in the northern areas, where thermal ground waters are close or where the warmer deep waters of lakes influence the temperature regime of rivers, polynyas (open unfrozen areas) often develop. Similarly to the large polynyas of the Arctic Ocean, they play an important role in the supply of oxygen to the aquatic ecosystems.

Ice cover and numerous ice blocks, which form in the shallow sectors, cause narrowing of river channels. Water either piles up behind the ice blocks and on freezing forms ice mounds, or spreads over the ice, forming surface icings. Usually, these are not large but in Siberia ice mounds and icings can develop on a very large scale. Thus, the area covered by ice mounds and icings on the river Moma, a tributary to the Indigirka, annually builds to 160-180 km2 with a water volume of 500-600 million m3 (Sokolov, 1964). In spring and early summer, these ice reservoirs provide additional water discharge, which in the case of the river Moma averages 15-20 m3 s-1. On the north-flowing rivers, ice cover in the lower course serves as a barrier for the melt water coming from the south. Substantial kinetic energy is required for the destruction of ice. Often ice break-up is accompanied by the submergence of ice and formation of ice jams. Water levels under such conditions are extremely high, which enhances spring flooding. Heavy ice jams are particularly common on the Northern Dvina, Ob, Yenisey, and Lena.

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