Aleksey Yu. Sidorchuk1 1Faculty of Geography, Lomonosov Moscow State University, Moscow, Russia * Corresponding author: [email protected] THE Fluvial system ON THE EAST 03|2018 GES European PLAIN: SEDIMENT source 5 AND SINK Abstract. 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. Key WORDS: The fluvial system, delivery ratio, recent accelerated erosion, sediment deposition, Late Glacial catastrophic erosion event, East European Plain Citation: Aleksey Yu. Sidorchuk (2018) The fluvial system on the East European plain: sediment source and sink. Geography, Environment, Sustainability, Vol.11, No 3, p. 5-20 DOI-10.24057/2071-9388-2018-11-3-05-20 Introduction etc.). Erosion by water produces sediments – particles of different size and weight. A fluvial system is the cascade of connected These sediments can be transported or disconnected linear depressions through the fluvial system only when and (channels) with catchments, formed by where transport capacity of the flow is modern and ancient water flows, having higher than sediment load. The ability of long history of morphological evolution the fluvial system to deliver sediments to (Makkaveev 1955; Schumm 1977). These the lowermost point is well described by flows, generally directed by the gravity force the ratio between the amount of sediments from upper to lower reaches, interact with arriving at the outlet of a fluvial system the surface layer of the lithosphere, being and the total eroded sediments within the one of the main denudation agents. catchment, i.e. delivery ratio Dr. The fluvial system can be of erosive type, when delivery The erosion and deposition in the fluvial ratio is high and of depositional type, with systems are well investigated (Gregory and low delivery ratio. The boundary can be Walling 1976; Schumm 1977; Meade 1996 appointed at Dr=50% (Sidorchuk 2015). GEOGRAPHY, ENVIRONMENT, SUSTAINABILITY 03 (11) 2018 The modern global fluvial system delivers typically less than 7 with 80% in the range annually to ocean and internal seas about of 0.1-1.0 (see Fig. 2.14 in Litvin 2002). 19 109 ton of sediment particles (Milliman and Farnsworth 2011). This is only a part The drainage density of the modern of the global relief destruction by erosion. permanent rivers and streams with the Annual soil erosion by water only from the length more than 1 km on the southern global farmland (about 10% of the global megaslope of the East European plain is terrestrial area) is estimated in a broad range 0.36 km/km2: 150717 rivers totally 574414 GEOGRAPHY from 172.1 109 (Ito 2007) to 22.2 109 ton (van km long belong to the Volga River basin 2 Oost et al. 2007). In any case, the modern (1360000 km ), 13012 rivers totally 90416 6 global delivery ratio is lower than one, and km long belong to the Don River basin sediment sink in the fluvial systems (mostly (422000 km2) and 31954 rivers totally at the lowlands) is one of the significant 163467 km long belong to the Dnieper geomorphological processes. The alluvial River basin (504000 km2) (Domanitskiy et and non-alluvial sediments, deposited in al. 1971). This is only the part of the fluvial the fluvial systems, together with channel system within the study area. The net of morphology, represent the memory of the dry valleys with a density of 0.4-0.6 km/ system (Knighton 1984). km2 dominates the physiography of the erosion landscape (Fig. 1). These dry valleys The Late Glacial period was characterized (called “balka” or “sukhodol”) typically have by extremal surface runoff, typical at least lengths of 1-10 km. These systems drain for the Northern Hemisphere (Dury 1965; water mostly during snow thaw period Volkov 1963; Sidorchuk 2003). However, for or after intensive rain. Dense vegetation this period the type of the fluvial systems usually cover their bottoms and slopes, so on the lowlands, erosional or depositional. the modern erosion here is negligible in the is not well investigated. The main goal of pristine conditions. The bottom width of this paper is to show the differences, as dry valleys is much larger than the width of well as the similarities, in the processes of the flows in these valleys, and the ratio of the fluvial systems on the East European these two width increases with basin area plain during these two highly contrasting decrease. The same effect is typical for small periods of erosion: the period, dominated and median river valleys. Dokuchaev (1878) by human-induced accelerated erosion, was the first describing this phenomenon lasted about 300 years, and the Late-glacial and addressing it to the history of fluvial period about 2-3 thousand years long, with system evolution. the natural extremal erosion. The Late-glacial period of extreme erosion The rivers of the East European Plain Climate-induced episodes of accelerated The East European plain is a large lowland erosion occurred repeatedly during covering an area of about 4.0 106 km2 periods of deglaciation of the continental (Alayev et al. 1990). The largest European ice-sheets (Vandenberghe 1995). The rivers: Volga, Don, Dnieper, drain its large rivers of the Late Glacial time were southern megaslopes. The headwaters of formed during these episodes of high these rivers on the main water-divide of surface runoff and were first described by the plain have maximum altitudes of about G. Dury (1965) and I. Volkov (1963). The 250-300 m. Therefore, the inclinations of reconstructed mean annual surface runoff the main rivers 1900-2700 km long are not was 2-4 times greater on the East European more than 0.13-0.14%0. The inclinations plain for the Late Glacial time about 16- and lengths of river basin slopes vary in 18 ka ago, and the maximum discharges the broad range with highly negatively were 5-6 times greater (Sidorchuk et al. asymmetrical distributions. The multiple 2001, 2008). The river channels were then values of the LS factor of USLE (Wischmeier wider than the modern ones, long and and Smith 1978), calculated on the field deep gullies dissected slopes of the valleys, scale using slope length and inclination, are and the permanent watercourse net was Aleksey Yu. Sidorchuk THE Fluvial system ON THE EAST EUROPEA ... GEOGRAPHY 7 Fig. 1. The dry valley net at the Khoper River basin. The part of SPOT image ID 5 119-246 07/07/30 08:26:01 2 J, central point lat. 51.5o N, lon. 41.92o E (provided by MSU Geoportal) much denser. The volume of the linear 2 (1) erosion during this geologically rather short Vb = 0.007H0 L episode (2-3 thousand years) was extremely H0 vary in the Khoper River basin in the range high. of 20 to 85 m with a mean value of 45 m. For example, the Khoper River basin (the The number of dry valleys with the length tributary of the Don River) with an area of 2 less than 10 km can be estimated from the 61,100 km has a recent total length of the data on the structure of the fluvial network. permanent flows and dry valleys of 36,000 This structure is well described by the km. The permanent river net (267 rivers magnitude – frequency function (Fig. 2). It with the total length of 9000 km) consists is convenient to use the length of the dry of the streams equal and more than 10 km valley L as the magnitude, and the sum of long. The dry valleys (some of them with the lengths of the valleys ΣL with the length permanent springs) are shorter than app. greater than L as the frequency: 10 km length and form the erosion net with a total length of 27000 km, which dissects L = ML1 D (2) the basin slopes. The variables in formula (2) correspond to the The “negative” volume of these dry valleys terms of the fractal approach. The exponent can be estimated from the valley volumes D can be called the fractal dimension of with known number of the valleys. The the valley network, and the coefficient M is negative volume of a dry valley V (km3) b a measure of the structure of the network, depends on its length L (km) and the equal to the total length of all valleys with difference between the upper and lower the length more than 1 km. The formula (2) points of its longitudinal profile H (km). 0 is empirical and dimensional. therefore all Measurements in the Khoper River basin lengths in formulas (1)-(6) are in the same led to the relationship: units (km). GEOGRAPHY, ENVIRONMENT, SUSTAINABILITY 03 (11) 2018 GEOGRAPHY 8 Fig.