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Dynamics of the Selenga river network and delta structure
Geography and Natural Resources 29 (2008) 343–347
Dynamics of the Selenga river network and delta structure E. A. Ilyicheva * Institute of Geography SB RAS, Irkutsk Received 19 May 2008
Abstract The results of investigations for the Selenga river basin and delta are presented. The causes for the changes in the channel network structure are considered. The study revealed tendencies of erosion activity and plane deformations within the delta. An analysis is made of the distribution of water flow rate and sediment loads for the channel network, based on field measurements and existing research material. Keywords: channel network, dynamics of the river system, delta, sediment loads, lateral erosion, plane deformation, distribution of water flow rate, channelforming flow rate, granulometric composition of bottom sediments.
Introductio roduction The estuarine region of the Selenga is the most dynamical and ecologically vulnerable natural complex forming part of the drainage system of a unique freshwater water body of the globe, Lake Baikal. In addition to having an important biosphere value, the delta has the functions of a barrier filter with respect to the flows of various substances arriving with the river waters from the huge territory of the basin. The river waters bring mechanical material, and the chemical, biological and thermal runoff to the head of the delta. The high rates of deformation of the channel network are responsible for the variability of landscapes and have a substantial effect on the ecological state of the territory. The flows of material undergo transformation in the water streams of the forked delta as they advance toward the estuarine shore along a longitudinal and transverse direction. Structural and dynamical characteristics of the channel network, and the conditions of the dispersal of the runoff of water and sediments are necessary for the study of the complex transformation processes of the flows of material. In this connection, our (ongoing since 2003) investigations into the regime of the Selenga itself as well as into the dynamics and ecological conditions in its estuarine region are becoming increasingly important. Objects and methods of investigations Our investigations encompass the channel network of the Selenga basin and its delta. Cartographic methods were used
* Corresponding author. E-mail address: [email protected] (E. A. Ilyicheva)
to determine the structural dynamics of the river network in the Selenga middle mountains, and plan migrations of the branches and channels of the delta. The channel-forming water flow rates and the dynamics of their changes for different years were calculated. The field observations in the estuarine region of the Selenga were made during the summer-autumn low water period (2003-2007). The observations included measurements of the depths, flow velocities, water and sediment flow rates as well as sampling of bottom sediments at a temporal observing network. Leveling surveys of the water surface and the bottom were carried out for all main water streams from the head of the delta and forking nodes to the mouth. Increases or decreases of the elevations of the bottoms, and also the caving rates of the shores were recorded in permanent reference areas. Results and discussion The Selenga river basin. The channel network of the drainage basin, performing its natural functions of transformation of atmospheric precipitation to the river runoff and draining subterranean water, serves also as the indicator of anthropogenic interference in the natural balance of formation of physical, chemical and biological processes. The time interval from the early 20th century to 1960-1970 saw some change sin the structure of the network of water linearerosion features, which was caused by an enhancement of anthropogenic pressure on the Selenga basin’s ecosystems. The relationships between the dynamics of anthropogenic impacts and the accompanying processes of response of the channel network were studied through a combined treatment of the evolution of the layers of the natural envelope and nature management.
Copyright © 2008 IG SB, Siberian Branch of RAS. Published by Elsevier B.V. All rights reserved doi:10.1016/j.gnr.2008.10.011
344
E. A. Ilyicheva / Geography and Natural Resources 29 (2008) 343–347
Analysis of the dynamics of the river systems within the Selenga basin, relying on structural regularities and characteristics of river network structure, revealed an increase in diversity in the hydronetwork pattern. The study determined the main factors of a natural and anthropogenic character causing an intensification of the erosional processes commonly accompanied by the emergence of new erosion forms. The group of natural factors includes; the torrential character of rainfall; lithological composition of underlying rock (sands, and loamy sand and light-loamy loessal sediments occurring along the valleys and their tributaries), and aeolian processes promoting active surface reconfiguration. The anthropogenic component largely includes extensive (sometimes injudicious) ploughing-up) of virgin and fallow lands at the period of intensive development of the territory. The share of some ploughed-up river basins was as much as 50% of the area (most arable lands were located on surfaces of up to 6° or sometimes up to 12° steepness. Lands of light mechanical compositions were mainly used as agricultural lands. Equally important anthropogenic factors acting to change the river water regime within the basin, followed by response of the channel network, were forest felling and an intensification of cattle husbandry in Soviet times. Disafforestation in the catchment area caused an increase in the layer and modulus of the spring-time runoff and, hence, an enhancement in erosional processes. Ploughing furrows and cattle trodden paths acting as the vehicles for the runoff of rain water and melt water, also intensify the water-erosion activity. Research results indicate that the stream-order composition of small river systems has changed dramatically. All river basins are characterized by the network development through an increase of the number of order I and II water streams by a factor of 2 or 3 in the general case. The river network response to externally driven changes manifested itself quantitatively in the dynamics of the water stream lengths as well. The total length of the river systems increases several times in some cases. The increase in total length varies from 10 to 60 km (Kunalei river – 12 km, and Bichura river – 22 km). The time interval 1895-1973 saw the emergence of a large number of newly formed water streams of a short length (0.5-2.0 km), and an increase in the river network density. There were also taking place the processes of significant development of the gully network, especially within the basins of the Selenga middle mountains. An increase in the river network density resulting from intense development of lands is also possible where gullies open subterranean aquifers. Such processes, as a rule, change a river network for a short period of time. No trend of climatic changes were found to occur within the Selenga basin during the first two-thirds of the 20th century, and, as our investigations showed, the main effects are caused by structural changes in agricultural land use. At the end of the 20th century, however (1984-1995 – a period of high water level), no extreme development of erosion was recorded – this was due to the abrupt decrease in economic
activities in the catchment area, and this is reflected in the decrease in the volumes of sediments in the channel network [1]. This, in turn, led to its deficit in the river flow thus causing an enhancement of the channel processes in the estuarine region of the Selenga. The channel processes, and the hydrographic network of the delta. The estuarine region of the Selenga river belongs to the open type with the estuarine coastal region, and with a protruding lobate delta. The Selenga delta is the only inland freshwater delta generated by a long-lasting interaction of the Lake Baikal – Selenga river interaction and represents a unique accumulative plain having a number of specific natural characteristics. The regime of the delta clearly shows a season of its significant inundation at the period of high water level from April to October caused by a rise of Lake Baikal’s water level, and by runoff of the Selenga river during floods and freshets. Another characteristic of the modern processes of flooding is the general rise of Baikal’s level after the construction of the Irkutsk Hydro at the end of the 1950s. The outer boundary of the delta is configured by a chain of long sand spits (sand bars), with the river waters flowing into Lake Baikal through the gaps in them. The water masses having significant speed bring abundant river sediments whose occurrence areas are clearly seen in space-acquired images. The estuarine sand bars are located on the northern and northwestern sides, i.e. where the edge of the delta undergoes the effects of agitation caused by winds with the highest recurrence rate. The protrusion of the lacustrine edge of the delta is not uniform. Two dynamical regions can be identified. The marginal portions of the delta protruding into shallow water bays (Proval, and Posolsky Sor) do nearly not experience any wave effects and are constantly growing in size. The growth zone encompasses the northern and southern portions of the delta. The average rate of their protrusion can reach 50 m per year, which is confirmed by the analysis of modern maps. The other zone, the region of dynamical equilibrium, is characterized by relative stability of the position of the lake’s boundary. Furthermore, seasonal inundation of the territory is possible, which is associated with the operating conditions of the Irkutsk Hydro. Geographically, this area lies at the estuaries of the central channels. Dynamical regions are outlined by clearly pronounced bays [2]. The natural processes of delta formation are manifested as fast and significant changes of the delta’s channel network. The system of delta water streams has two important functions, i.e. it constitutes the framework, on the one hand, and a powerful transport structure of the delta, on the other, where there are taking place physical, chemical and biological processes of transformation of matter. The territory under investigation is characterized by complex, dynamical hydrographic structure. The head of the delta is in the area of the village of Zhilino, where the river splits to form two main branches: the northern (Krasnoyarsk) branch, and the southern (Tvorogovsky) branch. Subsequently, chan-
O. I. Kalnaya and V. I. Zabelin / Geography and Natural Resources 29 (2008) 343–347
nels of a next, lower order separate from the main branches that diverge along their edges. All network is divided into three groups according to their hydromorphological features. The northern group includes the Lobanovskaya, Manzar and Dologan branches whose branches flow into the Proval Bay; the central group consists of the Kolpinnaya, Sredneustie and Galutai branches which flow into the lake in the middle sector of the delta, and the southern group includes the Levoberezhnaya, Kharauz and Shamanka branches which carry the runoff of water and sediments to the lake in the southern sector of the delta (Fig. 1).
Fig. 1. Schematic map of the delta of the Selenga river and hydrometric sites. Hydrometric sites: 1 – Selenga – Maloye Kolesovo – head of the delta; branches: 2 – Leveberezhnaya, 3 – Main channel (Kharauz), 4 – Galutai, 5 – Sredneustie, 6 – Kolpinnaya, 7 – Lobanovskaya, 8 – Dologan.
To determine the form of the channel, its stability and the type of channel process we calculated the hydromorphological parameters. Most of the branches are characterized by a transitional state of the channel varying from U-shaped to parabolic. A most striking example of a U-shaped channel is the relatively young Levoberezhnaya branch.
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All channels of the delta are meanders. This is due to the presence of space for wandering, and of readily washed off earth materials. The width of the meandering belt near the head is 10 km toward Ulan-Burgasy Range (along a northeastward direction), and up to 5 km along a southwestern direction from the contemporaneous channel. In the northern sector, in the meandering belt there remain false river channels (the old Kharauz, in the area of the settlement of Baikalo-Kudara), and small lakes. In this part of the delta, the largest width of the walking belt varies from 7.5 to 10 km. In the southern sector, the meandering belt is 2-3.5 km. The process of erosion and walking was confirmed by Glushkov’s and Grishanin’s calculated parameter values (G = 5.3-9.0) and (M = 0.9-1.1), respectively. The intensity of subsidence of the bed and of sediment accumulation in the channel is readily illustrated by the transverse profiles of the channels obtained by leveling for the sites of a temporal observing network (Table 1). Combined cartographic data from different time intervals, and field data point to substantial plan migrations and hydronetwork reconfigurations. The crawling velocity of the meanders averages 1-1.5 m/year, and the rate of lateral erosion, according to field observations, averages 3-4 m/year. The rate of plan deformation in the case of a transverse displacement of the meanders on the site of separation into two main branches is about 1 m/year. Furthermore, the leading edge of caving encompasses the entire near-head portion of the meander and partly washes it off. The forking node moved 1.5 km downstream. The processes of lateral erosion at the delta head now have a disastrous character. As a result, there is taking place caving of the portion of the Selenga bed along its left bank. This area is home to human settlements, pastures, and hayfields. Work is currently underway on deepening the river bottom, and the new straitening channel has been created, which transport the main runoff, and the left bank is being strengthened by means of a dam of rocky material. The velocity field of the delta runoff is different for each sector. For the northern branches, the flow velocity distribution has a rather uniform character, namely, a gradual decrease from 1.4 m/s at the point of forking to 0.2-0.4 m/s in the estuaries. The flow velocities in the central sector are not high – in Table 1
Elevations of the water surface and bottom of the branches of the temporal network Branch of river
В
Нsur
Нbot
ΔНsur
ΔНbot
455.09
+0.68
–0.72
455.54
455.50
+0.77
–0.04
456.22
456.40
+1.68
+0.18
455.96
456.21
454.86
–1.75
–1.35
458.038
453.21
453.34
+1.128
+0.13
2003
2005
2003
2005
2003
2005
Dologan
150
192
457.01
457.69
455.81
Lobanovskaya
125
144
457.03
457.80
Kolpinskaya
28
33
456.22
457.90
Sredneustie
51
32
457.71
Galutai
110
85
456.91
Note. В – channel width, m; Н – absolute elevations of the water surface and bottom, m, above sea level; ΔН – change in the elevations of the water surface and bottom.
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E. A. Ilyicheva / Geography and Natural Resources 29 (2008) 343–347
the sources and in the middle part, they are 0.5-1 m/s, while along a greater length the velocity is as high as 0.3-0.4 m/ s. The meanders in the southern sector and in the estuary, respectively, have the velocities of 0.8-1.4 m/s, and up to 0.3-0.4 m/s. The channel flows under the water surface in the southern group are seen in space-acquired images as far as the estuarine sand bar of the delta. The runoff and sediments. At the point of entry into the estuarine region (post raziezd Mostovoi) a maximum water abundance for the period of instrumental observations was recorded in 1973, with the water flow rate being 1472 m3/s. The maximum and minimum yearly mean flow rates of sediments were 130 kg/s (1941) and 12 kg/s (1972), respectively. Typically the Selenga river undergoes time-coincident fluctuations of the runoff of water and sediments (Fig. 2). The seasonal distribution of the sediment runoff is in excellent agreement with that of the water runoff. The water regime of the river, the processes of channel erosion, and rain-induced erosion loss on the slopes of the basin. An increase of the water and sediment runoff starts in April in time coincidence with the thawing of snow within the basin; in May there occurs an abrupt increase in the water and sediment runoff in connection with the freshet, and in June there takes place a decrease in water abundance, and a more abrupt decrease of the sediment runoff, which is explained by there being no surface erosion loss. At the time of floods (in July) there is an increase of the sediment runoff with a peak of turbidity, and abundance is peaked in August. Subsequently, the decrease in sediment flow rates keeps ahead of that of the water flow rates. In the time interval from April to September the channel network transport as much as 96% of the annual runoff of sediments. A part of the sediments entering into the delta precipitates in the branches and on the delta surface, and some of them are carried away beyond the delta. The Selenga supplies to the delta 2079 thou t of suspended sediments and carries away 852 thou t. The amount of sediments remaining in the delta constitutes 54% of the sediment runoff for all tributar-
Fig. 2. Long-term fluctuations of the runoff of water (Q) and sediments (R). Selenga river (post raziezd Mostovoi).
ies. The structure and hydromorphological parameters of the hydrographic network of the delta, and also the presence of aerial-aquatic vegetation are favorable for the retention of sediments, and for sedimentation. In the delta there remain from 44 to 74% of the sediments in the years with the least and largest sediment runoff, averaging 59% [3]. Based on the data for the flow rates of sediments in the “working” meanders of the delta network (at the head, in the center, and in the estuarine portions), it can be concluded that at the period of low water level in the summer 50-60% of sediments arriving at the delta head accumulate in its upper part, and 20-30% precipitate along the stretch from the posts of the temporal network to the estuaries of the branches. Some amount of the remaining 15% precipitates along the stretch from the estuary of the branches to the sand bar. At this stage it is not possible to estimate the share of the sediments arriving at the water body of Lake Baikal. Such a distribution pattern promotes a relatively rapid protrusion of the delta adjacent to the lake. In comparing the hypsometric elevations on the maps for different time intervals, we found that during the past 20 years in the northern sector the rise of the surface in the estuarine portion of the meanders averaged 0.3-0.6 m, and it was 0.2 m in the zone of the middle flow of the central sector for the same period. The most significant rise of the elevations of relief was recorded in the southern sector of the delta, nearer to the lake’s boundary, from 0.3 to 1 m. The distribution of sediment runoff in the branches of the delta depends largely on the water abundance of a river, the river channel parameters, and on granulometric composition of suspended wash and bottom sediments and in general corresponds to the water runoff distribution in the case of increased and decreased water abundance. As far as a water-abundance average year is concerned, however, the picture of the sediment distribution is as follows: 55% for the southern meanders, 10% for the central meanders, and 35% for the northern meanders. At the period of low water level in th summer-autumn season, the structure of sediment distribution is somewhat different. The overwhelming part (оver 60%), 5% and up to 30% of sediments are transported, respectively, by the meanders of the northern, central and southern sectors (Fig. 3). Significant levels of suspended sediments are observed in the water of the Lobanovskaya meander, and of its component meanders (the Manzar and Dologan meanders). The bottom sediments of the Lobanovskaya meander, independently of its water abundance, show a predominance of particles of sand fraction [4]. This evidence can be explained by the influence of winds. The northern sector is dominated by northeasterly winds (Barguzin) with a recurrence rate of 18%, which creates a hydrostatic upthrust on the side of the Proval Bay. In such cases there is a decrease in the water flow velocities to minimum values. This is followed by intense precipitation of sediments with a subsequent growth of the delta in the estuary of the Lobanovskaya meander further out into the Proval Bay.
E. A. Ilyicheva / Geography and Natural Resources 29 (2008) 343–347
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Table 2 Distribution of measured water flow rates (%) in sectors for different water abundance at the head of the Selenga delta (2003-2007) Water abundance at the head of the delta (Q), m3/s 409.1 462.2 472.45 848.9 850 855.53 882.25 1009.9 1130.1 3093.17 3843.25 Fig. 3. Distribution of suspended sediment runoff in the branches of the delta, %. Branches: 1 – Dologan, 2 – Manzar, 3 – Lobanovskaya, 4 – Kolpinnaya, 5 – Sredneustie, 6 – Galutai, 7 – Levoberezhnaya, 8 – Kharauz, 9 – Shamanka.
The runoff distribution is also changeable. Material is transported mainly by lateral meanders. Changes of the “working” regime of the meanders occur on a long-term scale as well as according to water abundance phases. Recent years have witnessed a tendency for an increase in water abundance at the delta head as well as in the main branches and meanders. It can be suggested that the redistribution is caused by an increase in water abundance in general, and by an increase in the runoff at the period of low water level in particular, because the runoff at the delta head increased more than twice from 2003 to 2007. According to the data reported in [5], with an increase in the runoff at the delta head, there was a decrease in the contribution of the runoff for the southern sector of the delta. Analysis of the runoff distribution spanning the period of our observations showed that the contribution of the runoff for the northern sector is high and relatively constant (37-53%). The contribution of the runoff through the central meanders varies from 15 to 21%; it reaches 45% for the southern meanders. During the period of low water of the open channel the distribution of the runoff in the sectors depends on the water abundance at the head of the delta (Table 2). With a rise of the water level in it (H > 400 cm, Q > 3000 m3/s), there is an increase of the runoff in the branches of the southern group. Conclusion The river system of the Selenga river is characterized by its rapid response to changes of the naturally occurring set of factors that are responsible for the formation of the hydronetwork and anthropogenic component within the river basin. The estuarine region of the river system serves as the indica-
Sectors northern
central
southern
41.7 35.74 40.17 37.66 37.6 45.54 42.21 37.68 37.43 34.33 37.8
14.9 18.11 14.9 18.11 20.1 19.76 18.1 27.02 11.71 14.15 13.01
43.4 46.15 44.93 42.24 42.2 34.7 39.71 35.3 50.86 51.21 49.18
tor of the ecological state across the drainage area. Our research showed that the changes of the delta’s channel network for the last 50 years of the 20th century implied a dynamical change in the delta’s morphostructure and microrelief. The morphogenesis of the estuarine region was accompanied by changes of the channel network parameters, the emergence of new branches of the river and silting of the pre-existing branches, and by a redistribution of the runoff of water as well as of sediments. The processes of lateral erosion are recorded during floods and high water periods and are characteristic for branches having high-water level; they manifest themselves most clearly on the branches in the northern sector. Plan channel deformations have a directional-intermittent character. Displacements of the channels in the southern and northern sectors are substantial and are accompanied by active formation of modern alluvial features. The branches of the delta in the central sector are intensively washed out; this is particularly true of the points of bifurcation. References 1. Bazhenova O. I. and Martyanova G. N. Formation of extreme morphoclimatic situations in southern Siberia. Geografiya i prirod. resursy, 2004, No. 4, pp. 87-94. 2. Ilyichyova E. A. Structure and dynamics research on the delta of the Selenga river. Proc. 10th Intern. Symp. On River Sedimentation. Moscow, 2007, v. 4, pp. 72-80. 3. Potemkina T. G. Distribution of the runoff and sediments in water streams within the delta of the Selenga river. Geografiya i prirod. resursy, 1995, No. 1, pp. 75-78. 4. Khazheyeva Z. I. and Tulokhonov A. K. Mineral composition of bottom sediments in the branches within the delta of the Selenga river. Proc. Scient. Conf. “Main Factors and Regularities in Different landscape Zones”. Ulan-Ude, 2005, pp. 143-146. 5. Ainbund M. M., Davtyan N. A., Sudolsky A. S., and Fialkov V. A. A study on the dynamics of river mouths and near-delta portions of water bodies as exemplified by the Selenga river and Lake Baikal. Proc. IV All-Union Hydrol. Congress. Leningrad, 1975, v. 5, pp. 356-365.
Dynamics of the Selenga river network and delta structure E. A. Ilyicheva * Institute of Geography SB RAS, Irkutsk Received 19 May 2008
Abstract The results of investigations for the Selenga river basin and delta are presented. The causes for the changes in the channel network structure are considered. The study revealed tendencies of erosion activity and plane deformations within the delta. An analysis is made of the distribution of water flow rate and sediment loads for the channel network, based on field measurements and existing research material. Keywords: channel network, dynamics of the river system, delta, sediment loads, lateral erosion, plane deformation, distribution of water flow rate, channelforming flow rate, granulometric composition of bottom sediments.
Introductio roduction The estuarine region of the Selenga is the most dynamical and ecologically vulnerable natural complex forming part of the drainage system of a unique freshwater water body of the globe, Lake Baikal. In addition to having an important biosphere value, the delta has the functions of a barrier filter with respect to the flows of various substances arriving with the river waters from the huge territory of the basin. The river waters bring mechanical material, and the chemical, biological and thermal runoff to the head of the delta. The high rates of deformation of the channel network are responsible for the variability of landscapes and have a substantial effect on the ecological state of the territory. The flows of material undergo transformation in the water streams of the forked delta as they advance toward the estuarine shore along a longitudinal and transverse direction. Structural and dynamical characteristics of the channel network, and the conditions of the dispersal of the runoff of water and sediments are necessary for the study of the complex transformation processes of the flows of material. In this connection, our (ongoing since 2003) investigations into the regime of the Selenga itself as well as into the dynamics and ecological conditions in its estuarine region are becoming increasingly important. Objects and methods of investigations Our investigations encompass the channel network of the Selenga basin and its delta. Cartographic methods were used
* Corresponding author. E-mail address: [email protected] (E. A. Ilyicheva)
to determine the structural dynamics of the river network in the Selenga middle mountains, and plan migrations of the branches and channels of the delta. The channel-forming water flow rates and the dynamics of their changes for different years were calculated. The field observations in the estuarine region of the Selenga were made during the summer-autumn low water period (2003-2007). The observations included measurements of the depths, flow velocities, water and sediment flow rates as well as sampling of bottom sediments at a temporal observing network. Leveling surveys of the water surface and the bottom were carried out for all main water streams from the head of the delta and forking nodes to the mouth. Increases or decreases of the elevations of the bottoms, and also the caving rates of the shores were recorded in permanent reference areas. Results and discussion The Selenga river basin. The channel network of the drainage basin, performing its natural functions of transformation of atmospheric precipitation to the river runoff and draining subterranean water, serves also as the indicator of anthropogenic interference in the natural balance of formation of physical, chemical and biological processes. The time interval from the early 20th century to 1960-1970 saw some change sin the structure of the network of water linearerosion features, which was caused by an enhancement of anthropogenic pressure on the Selenga basin’s ecosystems. The relationships between the dynamics of anthropogenic impacts and the accompanying processes of response of the channel network were studied through a combined treatment of the evolution of the layers of the natural envelope and nature management.
Copyright © 2008 IG SB, Siberian Branch of RAS. Published by Elsevier B.V. All rights reserved doi:10.1016/j.gnr.2008.10.011
344
E. A. Ilyicheva / Geography and Natural Resources 29 (2008) 343–347
Analysis of the dynamics of the river systems within the Selenga basin, relying on structural regularities and characteristics of river network structure, revealed an increase in diversity in the hydronetwork pattern. The study determined the main factors of a natural and anthropogenic character causing an intensification of the erosional processes commonly accompanied by the emergence of new erosion forms. The group of natural factors includes; the torrential character of rainfall; lithological composition of underlying rock (sands, and loamy sand and light-loamy loessal sediments occurring along the valleys and their tributaries), and aeolian processes promoting active surface reconfiguration. The anthropogenic component largely includes extensive (sometimes injudicious) ploughing-up) of virgin and fallow lands at the period of intensive development of the territory. The share of some ploughed-up river basins was as much as 50% of the area (most arable lands were located on surfaces of up to 6° or sometimes up to 12° steepness. Lands of light mechanical compositions were mainly used as agricultural lands. Equally important anthropogenic factors acting to change the river water regime within the basin, followed by response of the channel network, were forest felling and an intensification of cattle husbandry in Soviet times. Disafforestation in the catchment area caused an increase in the layer and modulus of the spring-time runoff and, hence, an enhancement in erosional processes. Ploughing furrows and cattle trodden paths acting as the vehicles for the runoff of rain water and melt water, also intensify the water-erosion activity. Research results indicate that the stream-order composition of small river systems has changed dramatically. All river basins are characterized by the network development through an increase of the number of order I and II water streams by a factor of 2 or 3 in the general case. The river network response to externally driven changes manifested itself quantitatively in the dynamics of the water stream lengths as well. The total length of the river systems increases several times in some cases. The increase in total length varies from 10 to 60 km (Kunalei river – 12 km, and Bichura river – 22 km). The time interval 1895-1973 saw the emergence of a large number of newly formed water streams of a short length (0.5-2.0 km), and an increase in the river network density. There were also taking place the processes of significant development of the gully network, especially within the basins of the Selenga middle mountains. An increase in the river network density resulting from intense development of lands is also possible where gullies open subterranean aquifers. Such processes, as a rule, change a river network for a short period of time. No trend of climatic changes were found to occur within the Selenga basin during the first two-thirds of the 20th century, and, as our investigations showed, the main effects are caused by structural changes in agricultural land use. At the end of the 20th century, however (1984-1995 – a period of high water level), no extreme development of erosion was recorded – this was due to the abrupt decrease in economic
activities in the catchment area, and this is reflected in the decrease in the volumes of sediments in the channel network [1]. This, in turn, led to its deficit in the river flow thus causing an enhancement of the channel processes in the estuarine region of the Selenga. The channel processes, and the hydrographic network of the delta. The estuarine region of the Selenga river belongs to the open type with the estuarine coastal region, and with a protruding lobate delta. The Selenga delta is the only inland freshwater delta generated by a long-lasting interaction of the Lake Baikal – Selenga river interaction and represents a unique accumulative plain having a number of specific natural characteristics. The regime of the delta clearly shows a season of its significant inundation at the period of high water level from April to October caused by a rise of Lake Baikal’s water level, and by runoff of the Selenga river during floods and freshets. Another characteristic of the modern processes of flooding is the general rise of Baikal’s level after the construction of the Irkutsk Hydro at the end of the 1950s. The outer boundary of the delta is configured by a chain of long sand spits (sand bars), with the river waters flowing into Lake Baikal through the gaps in them. The water masses having significant speed bring abundant river sediments whose occurrence areas are clearly seen in space-acquired images. The estuarine sand bars are located on the northern and northwestern sides, i.e. where the edge of the delta undergoes the effects of agitation caused by winds with the highest recurrence rate. The protrusion of the lacustrine edge of the delta is not uniform. Two dynamical regions can be identified. The marginal portions of the delta protruding into shallow water bays (Proval, and Posolsky Sor) do nearly not experience any wave effects and are constantly growing in size. The growth zone encompasses the northern and southern portions of the delta. The average rate of their protrusion can reach 50 m per year, which is confirmed by the analysis of modern maps. The other zone, the region of dynamical equilibrium, is characterized by relative stability of the position of the lake’s boundary. Furthermore, seasonal inundation of the territory is possible, which is associated with the operating conditions of the Irkutsk Hydro. Geographically, this area lies at the estuaries of the central channels. Dynamical regions are outlined by clearly pronounced bays [2]. The natural processes of delta formation are manifested as fast and significant changes of the delta’s channel network. The system of delta water streams has two important functions, i.e. it constitutes the framework, on the one hand, and a powerful transport structure of the delta, on the other, where there are taking place physical, chemical and biological processes of transformation of matter. The territory under investigation is characterized by complex, dynamical hydrographic structure. The head of the delta is in the area of the village of Zhilino, where the river splits to form two main branches: the northern (Krasnoyarsk) branch, and the southern (Tvorogovsky) branch. Subsequently, chan-
O. I. Kalnaya and V. I. Zabelin / Geography and Natural Resources 29 (2008) 343–347
nels of a next, lower order separate from the main branches that diverge along their edges. All network is divided into three groups according to their hydromorphological features. The northern group includes the Lobanovskaya, Manzar and Dologan branches whose branches flow into the Proval Bay; the central group consists of the Kolpinnaya, Sredneustie and Galutai branches which flow into the lake in the middle sector of the delta, and the southern group includes the Levoberezhnaya, Kharauz and Shamanka branches which carry the runoff of water and sediments to the lake in the southern sector of the delta (Fig. 1).
Fig. 1. Schematic map of the delta of the Selenga river and hydrometric sites. Hydrometric sites: 1 – Selenga – Maloye Kolesovo – head of the delta; branches: 2 – Leveberezhnaya, 3 – Main channel (Kharauz), 4 – Galutai, 5 – Sredneustie, 6 – Kolpinnaya, 7 – Lobanovskaya, 8 – Dologan.
To determine the form of the channel, its stability and the type of channel process we calculated the hydromorphological parameters. Most of the branches are characterized by a transitional state of the channel varying from U-shaped to parabolic. A most striking example of a U-shaped channel is the relatively young Levoberezhnaya branch.
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All channels of the delta are meanders. This is due to the presence of space for wandering, and of readily washed off earth materials. The width of the meandering belt near the head is 10 km toward Ulan-Burgasy Range (along a northeastward direction), and up to 5 km along a southwestern direction from the contemporaneous channel. In the northern sector, in the meandering belt there remain false river channels (the old Kharauz, in the area of the settlement of Baikalo-Kudara), and small lakes. In this part of the delta, the largest width of the walking belt varies from 7.5 to 10 km. In the southern sector, the meandering belt is 2-3.5 km. The process of erosion and walking was confirmed by Glushkov’s and Grishanin’s calculated parameter values (G = 5.3-9.0) and (M = 0.9-1.1), respectively. The intensity of subsidence of the bed and of sediment accumulation in the channel is readily illustrated by the transverse profiles of the channels obtained by leveling for the sites of a temporal observing network (Table 1). Combined cartographic data from different time intervals, and field data point to substantial plan migrations and hydronetwork reconfigurations. The crawling velocity of the meanders averages 1-1.5 m/year, and the rate of lateral erosion, according to field observations, averages 3-4 m/year. The rate of plan deformation in the case of a transverse displacement of the meanders on the site of separation into two main branches is about 1 m/year. Furthermore, the leading edge of caving encompasses the entire near-head portion of the meander and partly washes it off. The forking node moved 1.5 km downstream. The processes of lateral erosion at the delta head now have a disastrous character. As a result, there is taking place caving of the portion of the Selenga bed along its left bank. This area is home to human settlements, pastures, and hayfields. Work is currently underway on deepening the river bottom, and the new straitening channel has been created, which transport the main runoff, and the left bank is being strengthened by means of a dam of rocky material. The velocity field of the delta runoff is different for each sector. For the northern branches, the flow velocity distribution has a rather uniform character, namely, a gradual decrease from 1.4 m/s at the point of forking to 0.2-0.4 m/s in the estuaries. The flow velocities in the central sector are not high – in Table 1
Elevations of the water surface and bottom of the branches of the temporal network Branch of river
В
Нsur
Нbot
ΔНsur
ΔНbot
455.09
+0.68
–0.72
455.54
455.50
+0.77
–0.04
456.22
456.40
+1.68
+0.18
455.96
456.21
454.86
–1.75
–1.35
458.038
453.21
453.34
+1.128
+0.13
2003
2005
2003
2005
2003
2005
Dologan
150
192
457.01
457.69
455.81
Lobanovskaya
125
144
457.03
457.80
Kolpinskaya
28
33
456.22
457.90
Sredneustie
51
32
457.71
Galutai
110
85
456.91
Note. В – channel width, m; Н – absolute elevations of the water surface and bottom, m, above sea level; ΔН – change in the elevations of the water surface and bottom.
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E. A. Ilyicheva / Geography and Natural Resources 29 (2008) 343–347
the sources and in the middle part, they are 0.5-1 m/s, while along a greater length the velocity is as high as 0.3-0.4 m/ s. The meanders in the southern sector and in the estuary, respectively, have the velocities of 0.8-1.4 m/s, and up to 0.3-0.4 m/s. The channel flows under the water surface in the southern group are seen in space-acquired images as far as the estuarine sand bar of the delta. The runoff and sediments. At the point of entry into the estuarine region (post raziezd Mostovoi) a maximum water abundance for the period of instrumental observations was recorded in 1973, with the water flow rate being 1472 m3/s. The maximum and minimum yearly mean flow rates of sediments were 130 kg/s (1941) and 12 kg/s (1972), respectively. Typically the Selenga river undergoes time-coincident fluctuations of the runoff of water and sediments (Fig. 2). The seasonal distribution of the sediment runoff is in excellent agreement with that of the water runoff. The water regime of the river, the processes of channel erosion, and rain-induced erosion loss on the slopes of the basin. An increase of the water and sediment runoff starts in April in time coincidence with the thawing of snow within the basin; in May there occurs an abrupt increase in the water and sediment runoff in connection with the freshet, and in June there takes place a decrease in water abundance, and a more abrupt decrease of the sediment runoff, which is explained by there being no surface erosion loss. At the time of floods (in July) there is an increase of the sediment runoff with a peak of turbidity, and abundance is peaked in August. Subsequently, the decrease in sediment flow rates keeps ahead of that of the water flow rates. In the time interval from April to September the channel network transport as much as 96% of the annual runoff of sediments. A part of the sediments entering into the delta precipitates in the branches and on the delta surface, and some of them are carried away beyond the delta. The Selenga supplies to the delta 2079 thou t of suspended sediments and carries away 852 thou t. The amount of sediments remaining in the delta constitutes 54% of the sediment runoff for all tributar-
Fig. 2. Long-term fluctuations of the runoff of water (Q) and sediments (R). Selenga river (post raziezd Mostovoi).
ies. The structure and hydromorphological parameters of the hydrographic network of the delta, and also the presence of aerial-aquatic vegetation are favorable for the retention of sediments, and for sedimentation. In the delta there remain from 44 to 74% of the sediments in the years with the least and largest sediment runoff, averaging 59% [3]. Based on the data for the flow rates of sediments in the “working” meanders of the delta network (at the head, in the center, and in the estuarine portions), it can be concluded that at the period of low water level in the summer 50-60% of sediments arriving at the delta head accumulate in its upper part, and 20-30% precipitate along the stretch from the posts of the temporal network to the estuaries of the branches. Some amount of the remaining 15% precipitates along the stretch from the estuary of the branches to the sand bar. At this stage it is not possible to estimate the share of the sediments arriving at the water body of Lake Baikal. Such a distribution pattern promotes a relatively rapid protrusion of the delta adjacent to the lake. In comparing the hypsometric elevations on the maps for different time intervals, we found that during the past 20 years in the northern sector the rise of the surface in the estuarine portion of the meanders averaged 0.3-0.6 m, and it was 0.2 m in the zone of the middle flow of the central sector for the same period. The most significant rise of the elevations of relief was recorded in the southern sector of the delta, nearer to the lake’s boundary, from 0.3 to 1 m. The distribution of sediment runoff in the branches of the delta depends largely on the water abundance of a river, the river channel parameters, and on granulometric composition of suspended wash and bottom sediments and in general corresponds to the water runoff distribution in the case of increased and decreased water abundance. As far as a water-abundance average year is concerned, however, the picture of the sediment distribution is as follows: 55% for the southern meanders, 10% for the central meanders, and 35% for the northern meanders. At the period of low water level in th summer-autumn season, the structure of sediment distribution is somewhat different. The overwhelming part (оver 60%), 5% and up to 30% of sediments are transported, respectively, by the meanders of the northern, central and southern sectors (Fig. 3). Significant levels of suspended sediments are observed in the water of the Lobanovskaya meander, and of its component meanders (the Manzar and Dologan meanders). The bottom sediments of the Lobanovskaya meander, independently of its water abundance, show a predominance of particles of sand fraction [4]. This evidence can be explained by the influence of winds. The northern sector is dominated by northeasterly winds (Barguzin) with a recurrence rate of 18%, which creates a hydrostatic upthrust on the side of the Proval Bay. In such cases there is a decrease in the water flow velocities to minimum values. This is followed by intense precipitation of sediments with a subsequent growth of the delta in the estuary of the Lobanovskaya meander further out into the Proval Bay.
E. A. Ilyicheva / Geography and Natural Resources 29 (2008) 343–347
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Table 2 Distribution of measured water flow rates (%) in sectors for different water abundance at the head of the Selenga delta (2003-2007) Water abundance at the head of the delta (Q), m3/s 409.1 462.2 472.45 848.9 850 855.53 882.25 1009.9 1130.1 3093.17 3843.25 Fig. 3. Distribution of suspended sediment runoff in the branches of the delta, %. Branches: 1 – Dologan, 2 – Manzar, 3 – Lobanovskaya, 4 – Kolpinnaya, 5 – Sredneustie, 6 – Galutai, 7 – Levoberezhnaya, 8 – Kharauz, 9 – Shamanka.
The runoff distribution is also changeable. Material is transported mainly by lateral meanders. Changes of the “working” regime of the meanders occur on a long-term scale as well as according to water abundance phases. Recent years have witnessed a tendency for an increase in water abundance at the delta head as well as in the main branches and meanders. It can be suggested that the redistribution is caused by an increase in water abundance in general, and by an increase in the runoff at the period of low water level in particular, because the runoff at the delta head increased more than twice from 2003 to 2007. According to the data reported in [5], with an increase in the runoff at the delta head, there was a decrease in the contribution of the runoff for the southern sector of the delta. Analysis of the runoff distribution spanning the period of our observations showed that the contribution of the runoff for the northern sector is high and relatively constant (37-53%). The contribution of the runoff through the central meanders varies from 15 to 21%; it reaches 45% for the southern meanders. During the period of low water of the open channel the distribution of the runoff in the sectors depends on the water abundance at the head of the delta (Table 2). With a rise of the water level in it (H > 400 cm, Q > 3000 m3/s), there is an increase of the runoff in the branches of the southern group. Conclusion The river system of the Selenga river is characterized by its rapid response to changes of the naturally occurring set of factors that are responsible for the formation of the hydronetwork and anthropogenic component within the river basin. The estuarine region of the river system serves as the indica-
Sectors northern
central
southern
41.7 35.74 40.17 37.66 37.6 45.54 42.21 37.68 37.43 34.33 37.8
14.9 18.11 14.9 18.11 20.1 19.76 18.1 27.02 11.71 14.15 13.01
43.4 46.15 44.93 42.24 42.2 34.7 39.71 35.3 50.86 51.21 49.18
tor of the ecological state across the drainage area. Our research showed that the changes of the delta’s channel network for the last 50 years of the 20th century implied a dynamical change in the delta’s morphostructure and microrelief. The morphogenesis of the estuarine region was accompanied by changes of the channel network parameters, the emergence of new branches of the river and silting of the pre-existing branches, and by a redistribution of the runoff of water as well as of sediments. The processes of lateral erosion are recorded during floods and high water periods and are characteristic for branches having high-water level; they manifest themselves most clearly on the branches in the northern sector. Plan channel deformations have a directional-intermittent character. Displacements of the channels in the southern and northern sectors are substantial and are accompanied by active formation of modern alluvial features. The branches of the delta in the central sector are intensively washed out; this is particularly true of the points of bifurcation. References 1. Bazhenova O. I. and Martyanova G. N. Formation of extreme morphoclimatic situations in southern Siberia. Geografiya i prirod. resursy, 2004, No. 4, pp. 87-94. 2. Ilyichyova E. A. Structure and dynamics research on the delta of the Selenga river. Proc. 10th Intern. Symp. On River Sedimentation. Moscow, 2007, v. 4, pp. 72-80. 3. Potemkina T. G. Distribution of the runoff and sediments in water streams within the delta of the Selenga river. Geografiya i prirod. resursy, 1995, No. 1, pp. 75-78. 4. Khazheyeva Z. I. and Tulokhonov A. K. Mineral composition of bottom sediments in the branches within the delta of the Selenga river. Proc. Scient. Conf. “Main Factors and Regularities in Different landscape Zones”. Ulan-Ude, 2005, pp. 143-146. 5. Ainbund M. M., Davtyan N. A., Sudolsky A. S., and Fialkov V. A. A study on the dynamics of river mouths and near-delta portions of water bodies as exemplified by the Selenga river and Lake Baikal. Proc. IV All-Union Hydrol. Congress. Leningrad, 1975, v. 5, pp. 356-365.