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Physical Geography of Northern Eurasia

Climatic Change and the Development of Landscapes

The Development of the Hydrographic Network of Northern Eurasia

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The Pre-Quaternary development of the Drainage Network

On the platform plains, the present outline of drainage systems and the location of the main drainage basins were mostly determined before the neotectonic stage. On the East European plain, for example, the divide between the east- and west-flowing rivers was formed near the modern Valday upland as early as the Carboniferous. The divide between southern and northern rivers in the eastern part of the plain formed in the Mesozoic. In the Miocene, neotectonic uplifts resulted in the active incision of rivers as indicated by the deep buried valleys. On the interflueves such valleys are about 300-400 m deep and are filled with sediments, which are up to 300 m thick. Nikolaev (1988) attributes these deep valleys to the general uplift of the continent in the Miocene. Alluviation began in the late Miocene and proceeded through the Pliocene. Valleys of many large rivers, such as the valley of the Volga in its lower course and the valley of the Don in its middle and lower course, have been inherited from that time.

In Western Siberia, the drainage network is also old. In the north, valleys of the main rivers such as the Ob, Nadym, Pur, Taz, and Yenisey feature overdeepening in their lower reaches which locally can be as much as 200-290 m below the present sea level (Korzhuev, 1975). The deep incision is dated to the pre-Quaternary and can be related to the low level in the Arctic Ocean. As the position of large rivers is usually controlled by tectonics and most of them are located in the negative tectonic structures, the present-day valleys are inherited from the ancient ones. In the south of the West Siberian plain, the drainage network had been well developed by the middle Pliocene and many modern rivers have inherited the Pliocene valleys. The end of the Neogene was marked by a reversal in tectonic movements of both the land surface and the ocean floor which resulted in the Arctic transgression.

In Eastern Siberia, the drainage pattern has formed as a result of uplifts. The mountains of Southern Siberia, the Anabar massif and the Aldan shield already existed in the Paleozoic while others, for example, the Putorana plateau, formed later. A well-developed drainage system existed on the Siberian platform in the Mesozoic and in the Paleogene. Rivers flowed towards the inner parts of the platform, to the Verkhoyansk foredeep, which was subsiding while mountains were building in the

Verkhoyansk-Kolyma fold belt. Before the neotectonic stage, river valleys were predominantly of meridional and submeridional orientation. In the Neogene-Quaternary, the drainage systems were restructured as a result of neotectonic movements and a new system of latitudinal valleys developed. Many old river valleys are still well expressed on watersheds of the Central Siberian plateau. Because of the tectonic uplift in the Baikal region, the Lena lost its connection with the Baikal, leaving a wide, dry valley. The Angara, which originates in the Baikal, changed its course more than once. However, in contrast to the East European plain, there are few buried ancient valleys in Eastern Siberia with the exception of a large buried valley in the lower course of the Lena.

Old large overdeepened valleys are typical of the platforms of Northern Eurasia. The valley of the Amur, the largest river of the southern Far East, is of particular interest. The Amur flows through several climatic and tectonic regions which experienced very different neotectonic evolution. However, the river has never changed its course and its ancient Neogene valley is located close to the modern one, about 100 m above the present-day channel (Markov, 1965). Only in the lower course did the Amur deviate northwards during the Quaternary. The ancient alluvial deposits in the Ussuri and other tributaries of the Amur also reveal the relative stability of the drainage network.

In the mountains, watersheds generally coincide with the areas of most active uplift. In valley profiles, longitudinal and transverse segments (with respect to ridges) alternate. The anomalous development results from a difference in rainfall received by slopes or from a difference in position of the base level. For example, tributaries of the Lena and the Aldan have a lower base level and higher runoff. They cut through the main ridge and capture the streams belonging to the Yana and the Indigirka basins. In the mountains of Central Asia, transverse valleys drain the northern slopes of the Tien-Shan, Turkestansky, and other ridges as a result of a difference in rainfall received by the northern and the southern slopes.

In the regions of active Cenozoic volcanism, rivers were locally diverted due to the lava effusions such as, for example, the Kamchatka drainage systems at the end of the Pliocene (Melekestsev, 1980).

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