A. Yu. Sidorchuk

Method of paleogeographical analogues in paleohydrological reconstructions

Abstract One of the main problems of quantitative paleohydrology is a discrepancy between very high (even catastrophic) reconstructed discharges in the paleorivers and the results of the majority of precipitation reconstructions in the same territory. To resolve the problem it is necessary to find the closest recent analogue to the hydrological regime of a paleoriver and to calculate the main hydrological and climatic parameters of the former flow with the help of this analogue. This approach to paleohydrological reconstructions is the method of paleogeographical analogues based on two assumptions: (1) similar hydrological regimes were characteristic for the paleorivers in similar paleolandscapes; (2) the hydrological regime of a paleoriver within some paleolandscape would be similar to that of a present-day river in the same type of landscape. Quantitative paleohydrological reconstruction by paleogeographical analogy calculates a wide range of paleohydrological and paleoclimatic parameters, such as maximum discharge and its return period; mean maximum discharge; mean annual discharge; volume of the floodwave; winter and annual precipitations. A study of the Khoper River paleochannel with a discharge 7 times exceeding the modern one indicates that the paleochannel formation was caused mainly by periglacial conditions with continuous permafrost and very sparse vegetation, while the rainfall increase was only two-fold. The relative errors in calculations of hydrological parameters for the present-day rivers using their modern analogues are mainly within ±10%, and up to 40%. The relative errors of palaeohydrological reconstructions are probably closer to the latter value.

River runoff decrease in North-Eurasian plains during the Holocene optimum

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.

Climate-induced changes in surface runoff on the North-Eurasian plains during the late glacial and Holocene

Abandoned rivers (large paleochannels and meanders) are common on river floodplains and low terraces on the East European and West Siberian plains. They are 10–15 times greater in size than the present-day river channels. The large paleochannels are dated back to 11–15 thousand radiocarbon years B.P. (the Late Glacial period). Based on the hydraulic and morphometric relationships for present-day rivers and the method of paleogeographic analogs, the surface runoff during the Late Glacial period was quantitatively reconstructed by the morphometric parameters of large paleochannels. The reconstructed surface runoff exceeded the present values by 1.4 times on the northern mega-slope of the East European Plain (the Northern Dvina, Mezen, and Pechora river basins), by 2.3 times on its southern mega-slope (the Volga, Don, and Dnepr basins), and twofold in West Siberia (the Ob basin). The large surface runoff volumes can be explained by the landscape and climate conditions, including the high coefficients of runoff (due to the permafrost), the increased proportion (and, conceivably, the amount) of snowfall, and, hence, the respective increased intensity of spring floods. The transformation of large Late-Glacial paleorivers due to climate warming at the beginning of the Holocene is a likely scenario of the surface runoff development within the present-day permafrost zone at the ongoing human-induced climate warming. A general decrease in surface runoff and its more uniform intra-annual distribution would result in the reduced size of rivers in the middle Siberia, Yakutia, and northeastern Russia.

High-Frequency Variability of Aggregate Transport under Water Erosion of Well-Structured Soils

The transport of soil aggregates from plots of 7.3 and 1.5 m2 caused by the erosion of typical rendzina soil by a shallow supercritical laminar overland flow was studied using a high-speed digital video camera. The diameters of the removed aggregates had a lognormal distribution, and the time intervals between the passages of two successive aggregates had an exponential distribution. Therefore, transport of soil aggregates is a compound Poisson random process, which is formed by summing local independent random erosion processes.

Evaluating bedload transport rate in a river bed taking into account data on the active and passive bed-forms dynamics

Bed-form motion in river bed can be active or passive. The active motion is governed by wave-like structures of flow velocity field-the flow controls bed transformation. In this case, the bed-forms are three-dimensional and they move without unidirectional changes in their shape. The passive motion occurs when the flow structure becomes simpler, such that it is controlled only by the presence of a bed-form relief. The bed-forms become two-dimensional and asymmetric, and in their motion, their crests move faster than troughs, thus making the bed-form to skew. To assess bedload transport rate in the case of active motion of a hierarchic system of bed-forms, it is necessary to sum bedload transported in the form of bed-forms of all orders and to take into account the transit of bedload in the bed-form-free form. In the case of passive motion, it is enough to evaluate the bedload transported as bed-forms of a single order plus transit of bedload in the free form. Theoretical studies based on Exner’s approach show that an estimate of the transit transport of bedload in the free form can be derived from the velocities of passive motion of bed-forms.

THE FLUVIAL SYSTEM ON THE EAST EUROPEAN PLAIN: SEDIMENT SOURCE AND SINk

The modern fluvial system on the lowland East European Plain is of depositional type. Sediment transport to the seas is only a few percent of the total erosion, and the main part of eroded material is accumulated in the channels. The recent deposition of suspended sediments is caused by accelerated soil erosion on the arable slopes, which led to a high rate of lateral sediment input and deposition at the river headwaters and on the floodplains. The process of accumulation is facilitated by the unfilled “negative” volume of the net of dry valleys formed during the Late Glacial catastrophic erosion event. Such events of catastrophic erosion of the sediments deposited in the lowland fluvial systems occur with a frequency of 100-120 thousand years. In the conditions of both scarce vegetation and extremal surface runoff, the entire fluvial systems become the area of intensive erosion, with the deep incision of gullies and of the river channels. Therefore, despite the modern intensive deposition, delivery ratio for the fluvial systems on this lowland territory is close to one in the long-term perspective.

Morphology and dynamics of active bed forms in the Terek R. Channel

Active bed forms of three major classes are formed in Terek lower reaches during summer floods. They include ripples, dunes of the first order, and dunes of the second order (from smaller to larger), which commonly form an incomplete hierarchy. The morphology of the bed forms is essentially stochastic and can be adequately described by probability distribution functions of bed form characteristics for some narrow ranges of hydraulic flow characteristics. At the same time, the mean values of bed form morphometric characteristics (length, height, and asymmetry) show stable relationships with flow velocity and depth. The celerity of active dunes can be adequately described by a modified Snishchenko–Kopaliani formula. The channelforming sediments that move as bed forms in Terek lower reaches account for 7% of sediment transport rate of all channel-forming sediments, a value near the lower limit for rivers with sand alluvium.