The Development of the Hydrographic Network of Northern Eurasia
The drainage network in Northern Eurasia was influenced by various factors, which could
be roughly categorized into two groups: tectonic and climatic. The general drainage
pattern, as well as the location of principal river valleys, is controlled by large
orographic features, which in turn are controlled tectonically. Climate controls the
formation of drainage systems through rainfall, which should be sufficient to generate
runoff. The relative importance of these factors and relationships between them have
varied since the Cenozoic.
The first Paleogene-Miocene stage featured a stable tectonic regime with slightly
differentiated movements and a relatively wet climate which favoured the development of a
dense drainage system across most of the area. The next stage (between the end of the
Miocene and the Pliocene) was marked by active orogenesis and the differentiation of
topography. Spatial climatic differences increased. Most importantly, the plains of
Central Asia and Kazakhstan became extremely dry and since then have been developing as
internal drainage basins. Finally, during the Quaternary, the drainage network experienced
climatic fluctuations of much greater amplitude than previously and differentiated
neotectonic movements. Climatic change affected drainage systems in different ways. In the
north, repeated glaciations and changes in ocean level were of primary importance. In the
continental southern areas, fluctuations in moisture supply resulted in the atrophy of
parts of the drainage network, the disintegration of river systems, and changing levels of
inner lakes.
