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.
