River runoff decrease in North-Eurasian plains during the Holocene optimum |
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| Authors: | A Yu Sidorchuk A V Panin O K Borisova |
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| Institution: | 1.Moscow State University,Moscow,Russia;2.Institute of Geography,Russian Academy of Sciences,Moscow,Russia |
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| Abstract: | Three stages were identified in the development of meandering rivers and the formation of floodplains with natural levees
in Northern Eurasia: the development of rivers with size larger than that of the modern ones; the development of rivers smaller
than the modern ones; and the development of rivers of the present-day morphodynamic type. Small oxbows of the second stage
are widespread in the floodplains of lowland rivers in Northern Eurasia. The largest amount of floodplain segments with such
oxbows can be seen in the forest zone, mostly in the coniferous forests of northeastern European Russia. The available radiocarbon
datings show that river channel were significantly decreasing in size and the steepness of meanders was increasing during
the Atlantic period of the Holocene. Data on changes in the size of river channels were used to evaluate the ratios between
paleo- and modern discharges and to construct a map of difference between runoff depths in the Holocene optimum and in the
present and assess changes in water runoff volume. The discharges in the basins of the Vyatka and middle Irtysh accounted
for as little as 40–50% of their current values. North, east, and west from those basins, the ratio of ancient and present-day
discharges increases. During the Holocene optimum, water runoff from the northern megaslope of the East European Plain was
∼180 km3/year, which is 30% less than the present runoff from the same drainage area. The annual runoff in Volga basin was ∼134 km3, which is almost half as large as the present value. The runoff in Don and Dnieper basins during the Holocene optimum was
40% less, and that in the Ob and Irtysh basin was 30% less than the present one. If we accept the hypothesis that the Holocene
optimum was a climate analogue of global anthropogenic warming of the mid-XXI century, the obtained estimates of the state
of water resources in Northern Eurasia acquire great prognostic importance. |
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