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Physical Geography of Northern Eurasia

Rivers, Lakes, Inland Seas, and Wetlands

<<< Anthropogenic Modifications of River Runoff | Physical Geography Index | The Caspian Sea >>>

Lakes

There are nearly three million lakes in Northern Eurasia — 98 per cent of those are small lakes (i.e., lakes with an area less than 1 km2). The geography of these lakes is controlled by two factors: water availability and topography. Lakes are most abundant in the north of the European territory, and in Western and northern Central Siberia, where there is ample moisture supply and widespread topographic depressions associated with periglacial landforms. Another region of widespread lake occurrence is Kazakhstan and Central Asia where lakes develop in tectonic depressions. The diversity of lakes is enormous. Lakes of tectonic origin occur mainly in the mountainous regions but also on the plains; Lakes Baikal and Issyk-Kul belong to this type. Lakes of the Kola peninsula and Karelia were originally created by tectonic activity and later modified by ice sheets. Moraine lakes develop in periglacial areas, such as the north of the East European plain. The lakes of Kamchatka have been created by volcanic action. Karst and karst-suffosion lakes are common in the regions of karst occurrence, such as the southern East European plain and the forest-steppes and steppes of Western Siberia. In Northern Siberia and Sakha-Yakutia, the development of lakes is often associated with thermokarst (see below). There is a debate on the origin of some West Siberian lakes (e.g., Lake Chany) which is attributed by some authors to relict thermokarst. Cirque and dammed lakes occur in the mountains. A typical example of a dammed lake is the Sarezskoe Lake, which formed in 1911 in the Pamir. This list does not exhaust the whole diversity of types. It is often difficult to classify lakes into a single category because various processes may be involved in their formation.

Characteristics of the largest lakes, including the Caspian and the Aral, are listed in Table 5.6.

The largest lakes of Northern Eurasia. All characteristics refer to the natural condition of the lakes

Table 5.6 The largest lakes of Northern Eurasia. All characteristics refer to the natural condition of the lakes.
* m.s.l. mean sea level. Source: Sokolov (1964), Borodavchenko (1988).

The Caspian and the Aral are not connected directly to the World Ocean. However, in the Russian-language literature they are called 'seas' by tradition because they possess many marine characteristics such as a large surface area and volume of water, a specific hydrochemical regime, complex water currents, and heavy storms. The total volume of water stored in these lakes exceeds 100 000 km3, including approximately 25 000 km3 of fresh water. Small lakes store about 3000 km3 of fresh water. Apart from the Caspian and the Aral, large saline lakes include the Issyk-Kul and Alakol. In the western part of Lake Balkhash, into which the river Ili discharges, water is fresh while in the eastern part it is brackish. The chemical composition of water is controlled by geology, precipitation chemistry, vegetation, and drainage rate. In most of the drainage lakes of the humid zone, natural water mineralization is low and these lakes belong to the carbonate type. Their natural water quality is high and those lakes, which do not experience pollution, can serve as reservoirs of drinking water (e.g., Lakes Baikal and Onega). Even those drainage lakes which are located in the south (e.g., the Zaisan) have fresh water. By contrast, the enclosed lakes of the arid zone accumulate salts brought by river runoff and their water is mineralized. The Caspian and Aral Seas are typical examples.

The regime of a lake depends on the regime of its tributaries. Lakes experience much smoother seasonal changes in water levels than rivers. In most lakes higher levels are observed when water levels in the inflowing rivers peak. In turn, lakes regulate river discharge and coefficients of runoff variation in rivers flowing through lakes (e.g., the Neva which originates in Lake Ladoga) are usually small. The thermal regime of lakes is controlled primarily by climate. Annual variability in water temperature is reduced in comparison with rivers and a lake usually freezes and is released from its ice cover later than the rivers of its catchment.

Many lakes, particularly those located in the arid zone, have been transformed by human activity. The most dramatic changes have occurred in the lakes of Central Asia and Kazakhstan and particularly in the Aral Sea (see below). In this region, most lakes experience strong natural fluctuation in level and area and the current trend is that of regression. It is an extremely unfavourable combination of the natural decline of the lakes and heavy water abstractions that is responsible for the critical condition of the lakes. Central Asia is not the only example. For several decades, the water of Lake Sevan in Armenia has been extensively used both for irrigation and for the development of hydropower. Since the 1930s, the lake level has been systematically lowered through the annual diversion of 1.2-1.5 km3 of water into the river Razdan. Water was used to irrigate 80 000 hectares of land in the Ararat Valley (the main agricultural area of Armenia) and for the power generation by the Razdan cascade. By 1957, the lake level had dropped by 10 m and its ecological condition had notably deteriorated. The original project, which would lead to the planned decrease of the Sevan's level by 50 m, had to be rejected. Planned reduction was limited to 18 m, which meant that the lake volume was reduced to 59 per cent of the original volume (Avakyan and Shirokov, 1994). The project was completed in 1970 and in the following years the water temperature increased by 8-9°C with a subsequent increase in evaporation. Oxygen concentration declined by 5-6 ml l-1, causing the development of algae and a strong decline in the population of valuable fish species, particularly the Sevan trout. Research has shown that in order to preserve the ichthyofauna of the Sevan and satisfactory water quality, the lake level should be raised by at least 6 m. This can be achieved through the reduction in water abstraction and water diversion from the river Arpa. However, Armenia is both a water- and electricity- deficient area and its economy, which at present experiences problems that are even more severe than in the other countries of the FSU, is strongly dependent on both irrigation and energy supply by the Razdan hydropower stations. The quality of the Sevan-Arpa project, however, is such that much of the diverted water is likely to be lost.

It is impossible to deal comprehensively with the characteristics of individual lakes within the constraints of a single chapter and only the largest lakes such as the Caspian, Balkhash, Issyk-Kul, Ladoga, and Onega will be treated in some detail. The Aral and the Baikal are discussed in the Regional Section. Numerous articles have recently appeared dealing with aspects of Caspian, Aral, and Baikal hydrology, and the book, Enclosed Seas and Large Lakes of Eastern Europe and Middle Asia (Mandych, 1995) contains a wealth of information about the largest lakes of Northern Eurasia.

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