CHANNEL BED SEDIMENTS AND THICNESS OF ALLUVIUM
AT THE LOWER VOLGA RIVER AND ITS DELTA
Vladislav N. KOROTAEV1, Nikolay A. RIMSKY-KORSAKOV2, and Vadim V.
IVANOV1
1
Moscow State University, Department Geography, Leninsky Hill, GSP-2, 119992, Moscow, Russia
E-mail: river@river.geogr.msu.su
2
Russian Academy Science, Institute of Oceanology, Nakhimovsky avenue 36, 117851, Moscow,
Russia
Abstract
Investigations of morphodynamics of the Lower Volga River channel from the Volgograd City to the
Astrakhan City, the Volga-Akhtuba floodplain watercourses and the main branches of the Volga River
delta have been carried out by the Faculty of Geography (Laboratory of Soil Erosion and Fluvial
Processes) of the Moscow State University (MSU) and the Oceanology Institute (Laboratory of the
Bottom Sounding) of the Russian Academy of Sciences (RAS) from 1995 to 2002. Data obtained from
seismoacoustic profiling, sonar scanning and echo-sounding of the river channel bed were coordinated
by synchronous measurements of geographical position and time from the GPS system.
Key words: channel bed sediments, alluvium, grain size of sediments, thickness of deposits, side-scan
sonar survey, seismoacoustic profile
INTRODUCTION
The Lower Volga River has been subject for significant palaeogeographic events in
the past and large-scale hydrotechnical projects at present. Both have had profound
effects on formation of the alluvial deposits. During the Caspian Sea transgressive
phases, the Lower Volga River valley filled by saline waters and marine clays.
Regressive phases, in a contrary, were dominated by the river channel incision, which
followed the sea level retreat. Such incisions reworked previously accumulated layers
of estuarine and marine deposits and redeposited those materials again, already as a
floodplain-forming alluvial facies. During the maximum Pleistocene (Makhachkala)
stage of the Late Khvalyn transgression the Caspian Sea level reached 0 m elevation
ASL. All previously formed alluvial deposits in the pre-Late Khvalyn palaeo-Volga
River valley became either buried under the Late Khvalyn ingressive marine deposits
or redeposited [11–12, 13–16, 18–20].
After the Late Khvalyn transgression, the Mangyshlak regression began. During that
period, the sea level fell to about -100 m ASL - minimal for the Late PleistoceneHolocene period. At the same time, the Volga River channel deformations were
dominated by reworking and partial removal of earlier deposits as well as infill of the
estuary by these materials. As a result, a series of internal deltas and corresponding
floodplains was formed. The next (Novocaspian) transgression began with infill of
river incisions formed during the Mangyshlak regression by the Novocaspian
deposits. At the same time, partial erosion (abrasion) of the so-called Baer mounds
took place.
The last (historical) stage of the Volga River delta development was controlled by
abrupt fall of the Caspian Sea level during the 1930-1970s. It led to increase of fluvial
activity with dominance of complex alluvial-deltaic type of sedimentation. The sea
level rise beginning since 1978 has not yet significantly affected the modern processes
of the delta formation. During the XXth century, the Lower Volga River hydrological
regime has been substantially modified by construction of a cascade of reservoirs,
dams and hydroelectric power plants located both on the main river itself and on its
2
largest tributaries. For example, interception of suspended and bedload sediment by
the Volgograd reservoir has led to two-fold decrease of sediment yield and substantial
transformation of the channel topography downstream the dam. Zone of channel
incision formed at the Volgograd dam tailrace gradually moves transgressively
downstream the Lower Volga River [8, 17, 21] (Korotaev and Ivanov, 2000). All the
above natural and anthropogenic factors determine very specific composition and
spatial distribution of the channel bed sediments along the Lower Volga River and in
its delta.
Formation of the bed sediment layer results partly from settling of dome suspended
sediment, but mainly from accumulation of sediment transported by saltation or
dragging. Grain size composition of the Volga River channel bed sediment
downstream the Volzhskaya hydroelectric power plant dam is dominated by sands
with average median diameter varying from 0,15 to 0,50 mm. Finer sediments (silty
sands, silts, clays) are accumulated during floods on inundated parts of the VolgaAkhtuba floodplain and Volga delta surfaces as overbank deposits.
RESEARGH METHODS
Field investigations of the Lower Volga River channel section from the Volgograd
city to the Astrakhan and deltaic distributor water divider was carried out in 1995–
2002. It involved taking bed sediment samples and continuous mapping of bed
morphology by the side-scan sonar device
River channel surveys included field investigations of the river channel bed
topography and alluvial sediment by means of depth sounding and bedload sediment
sampling. Office works involved determination of lateral and vertical channel
deformation rates from overlays of channel survey maps and pilot charts created at
different periods. For carrying out depth-sounding channel survey, a specially
designed software-hardware sounding system was employed.
Side-scan sonar devices (SSD) are at present the most widespread of hydroacoustic
equipment used for surveying bottoms of water bodies. Technique for the SSD
application for a river channel bed morphology mapping and qualitative evaluation of
bedform parameters was cooperatively developed by research groups from the
Oceanology Institute of the Russian Academy of Sciences (OI RAS) and Faculty of
Geography of the Moscow State University (FG MSU) in 1995–1999 [9, 10].
RESULTS
In the Lower Volga River upstream the delta apex (from the Volgograd Dam to the
Astrakhan city) medium-grained sands (mean particle diameter Md=0,25–0,50 mm)
represent the most widespread sediment type within the studied channel section. Fine
sands (Md=0,10–0,25 mm) and coarse silts (Md=0,05–0,10 mm) were found to be
less widespread. Increased grain size of the channel bed sediment near settlements
Cherniy Yar, Solenoe Zaimiche and Nikolskoe is believed to be associated with
formation of poorly sorted sediments as a result of erosion of the valley side bedrock
exposures located nearby.
Different types of the channel bed topography related to dune development are
formed in the Volga River channel from sandy sediments. Morphometrical
characteristics of dunes vary along the river course depending on hydrological and
morphological conditions of the flow-sediment interaction, morphodynamic channel
type and bed sediment thickness. For example, in high-sinuosity meanders (the
3
Tsagan-Amanskiy, Shaposhnikovskiy and Zamyanovskiy main channel sections) and
in relatively straight channel sections with active braid bar formation (such as the
Dembinskiy-Kapitanskiy main channel section) the entire main channel bed width is
occupied by relatively small 1,5–2,5 m high and 25–40 m long. In meander apexes
and downstream limbs, as well as in widened sections of a relatively straight channel,
sandy bedload sediment compose larger dune bedforms with length up to 130–160 m
and relative height up to 3 m.
Exposures of marine clays or floodplain loams on the channel bed without superficial
sediment cover are often observed along eroded bedrock slopes in straight channel
sections (Kopanovka, Seroglazka and Rechnoy) or in meanders with actively eroded
floodplain banks (yar Chilimniy, Tsaganskiy, Kopanovskiy, Kuznetsovskiy,
Danilovskiy, Parashkin, Arbuzniy and Shambayskiy). Their eroded surface possesses
a very characteristic striate microtopography formed by longitudinal rills and ridges.
Clay exposures represent relatively elevated zones of the channel bed (to 2–4 m)
forming local obstacles for sandy dune migration. The «sand-clay» boundary on the
channel bed is always very sharp, without any transitional zone, except for channel
sections with actively accreting point bars, where clayey bed surface is overlain by
underwater ends of sandy spits. Medium or small ripples are formed from finegrained sands at the channel bed sections where clays or loams are covered by thin
(less than 1 m) bedload sediment layer. In such cases, the bedrock surface
microtopography appears through the sandy bed microforms (Fig. 1).
Fig. 1. Hydroacoustic survey image of the Volga River channel bed obtained from the SSD:
A – the Khvalyn age clay bedrock outcrop; B – sandy dunes
The Volga River floodplain alluvium is most widely distributed between the Volga
main channel and Akhtuba branch. It is represented mainly by clays. Usually, central
parts of floodplain segments are dominated by clays, whereas zones adjacent to
channels are covered by loamy sands. Thickness of floodplain deposits does not
usually exceed 2–3 m. However, on levees along channels it can reach 5–7 m, in
central parts of floodplain – 10 m. Palaeochannel infill sediments are represented by
high-porosity oozes, clays, loams and loamy sands with thickness varying from 8–10
to 20 m.
The channel bed sediment grain size gradually decreases within the Volga River delta
from its apex to the distal seaward edge (Fig. 2). From the Astrakhan water divider
4
down to the Bahtemir deltaic arm inlet the main channel bed sediment is dominated
by medium-grained (Md=0.25-0.35 mm) and fine-grained (Md=0.15-0.20 mm) sands.
Sandy ripples developed in a thin layer of fine sandy sediment covering outcrops of
marine clays are observed along the right bedrock bank of the Volga River channel
right arm at the river reach where it erodes the so-called Baer mounds. There are
some channel sections without any bedload sediment near the Kolpakovskiy,
Durnovskiy and Jirgak yars and along the main channel rightbank from the
Streletskaya landing to the Solyanka settlement. At zones where the main Volga
River channel is separated into arms by the Gorodskoy and Ilinskiy islands, thalwegs
of active channel arms (the Trusovskoy arm) have medium- to fine-grained sands
bedload sediment. At the same time, gradually infilled inactive branches are
characterized by pure fine sands (mean particle diameter 0.11-0.15 mm). Specific
feature of the Astrakhan reach of the Volga River is a large percentage of shell
fragments in the channel-forming sediment volume (up to 10-30%).
BAKLANYA
IL. ISSKUSSTVENNY
OLYA
0,1
OPEN SEA
RASSVET
KHARBAY
IKRYANOE
ASTRAKHAN
IL.DJIRGAK
0,3
APEX DELTA
D, mm
0,4
0,2
BAKHTEMIR
0,5
SERGIEVKA
0,6
0
0
10
20
30
40
50
60
70
80
90
100 110
120 130 140
150 160
170 180
190 200
210 220
230 240
Distanсe, km
Fig. 2. Variations of the channel bedload sediment mean particle diameter within the Volga River delta
along a transect from the delta apex via the Bakhtemir arm and the Volgo-Caspian canal to the open
sea.
In the deltaic branches themselves, bedload sediment grain size and channel bed
topography are largely controlled by hydrological and morphological of flow
characteristics (substantially varying along a length of branches) as well as by the
delta geological structure. For example, at crossing the territory abundant in specific
landforms termed as Baer mounds, channel beds of the Volga River delta branches
are mainly composed of eroded marine clays, rarely covered by some shell fragments
eroded from those. Riffles composed of fine sand are observed only within a belt up
to 100 m wide along the maximum flow velocity line. In contrast, fine-grained
(Md=0.15-0.25 mm) sandy dunes often occupy entirely width of deltaic branches
outside the territory of the Baer mounds topography, especially downstream of
confluences of deltaic arms and branches. In general, bedload sediment particle size
within the delta part above the sea level rarely exceeds 0.35 mm at the Bakhtemir
direction and 0.15 mm at the Belinskiy direction. Locally increased particle size of
bedload sediment (up to 0.42 mm) is observed at places of abundant accumulation of
5
shell fragments (from 15 to 42% of total bedload sediment in some pools) and
downstream confluences with large tributaries (e.g. mouth of the Baklaniya branch in
the Bakhtemir arm).
Artificial navigation canals beds (Volgo-Caspian, Belinskiy, etc.) built outside a
seaward margin of the Volga River delta within the coastal waters of the Caspian Sea
are covered by fine to very fine sands and silts. There are no mobile bedforms in
artificial canals. However, echo sounding survey has allowed us to discover local bed
scours up to 15-20 m deep.
Results of the bedload sediment survey and echo sounding-based mapping have
shown that the majority of secondary deltaic branches formed within the territory of
the dominant Baer mounds topography (the so-called mound area) experience a lack
of bedload sediment. As a result, it is reflected by a widespread occurrence of
sections with bare channel bed composed of bedrock clays. For example, the
Shmagin branch channel has practically no bedload sediment from its inlet to the
outlet (25 km), its bed being composed of plastic clays. Within the Kizan deltaic arm
dominant type of bedload sediment is fine sand (mean particle diameter 0.17-0.20
mm). However, from the Bezymyanniy island upper end down to zone where the
Kizan arm separates onto widened outlets of a few smaller branches (the Rytiy,
Kulaginskiy and Nikitinskiy branches), for about 11 km, its channel bed is
represented by uneven clayey surface, only at places covered by thin layer of sandy
sediment. At the same time, most of smaller deltaic watercourses within the coastal
zone are filled by clayey and silty sediment (the Bushminskiy canal, Nikitinskiy and
other branches).
Thickness of channel bed sediment layers was studied by means of coring and
seismoacoustic profiling. First reliable information about the modern Volga River
alluvium thickness infilling the Late Khvalyn incisions of the main channel and major
deltaic arms were obtained from the State Oceanographic Institute coring program
carried out during 1937-1939. Altogether there were 50 deep cores taken. Of those,
largest thickness of the modern channel alluvium up to 21 m was observed at
relatively depressed central part of the delta where the palaeo-Volga flow is believed
to be concentrated. At slightly more elevated western and eastern parts of the Volga
River delta there are systems of the deltaic watercourses (the Bakhtemir, Staraya
Volga, Kizan and Kigach arms) incised to 5-11 m into bedrock represented by the
Late Khvalyn marine deposits. At the coastal part of delta (outside zone of the Baer
mounds development) and at its submerged part, uneven roof of the Khvalyn age
marine deposits is observed at depth from 7 m (western part of the delta) to 18 m
(central part) and 13 m (eastern part) from the deltaic plain surface. Thus, only central
part of the delta surface is composed from its apex to distal seaward edge by the
modern alluvium infilling the deep and wide incision formed after the Khvalyn age
marine deposition. At most of the other parts of delta, the Late Khvalyn marine
deposits are exposed on surface in form of the Baer mounds. At places where the
latter are absent, the Late Khvalyn marine deposit roof sharply sinks and becomes
overlain by the Holocene alluvial and alluvial-marine deposits.
Thickness of channel sediment in deltaic watercourses generally varies from 0.5 m at
crossings with the Baer mounds or in deep pools to 5-12 m at riffle zones. Most of the
Bakhtemir direction (the Astrakhan city - the Bakhtemir arm - the Volgo-Caspian
canal) is characterized by a high variability of the eroded Khvalyn clay roof elevation
(Fig. 3). Outside the delta distal coast there is a tendency of increasing elevation of
the clay roof and decreasing thickness of the overlying alluvial sediment to 1-2 m. It
6
increases back again gradually to 2-4 m at the submerged distal end of the delta in the
open sea.
H, M
-25,50
-25
ALLUVIUM
-27,37
ALLUVIUM
ALLUVIUM
-30
-50
0
20
40
60
70
CLAY
80
90
100
110
120
CLAY
CLAY
OPEN SEA
CLAY
-45
BAKLANYA
KHARBAY
-40
ISKUSSTVENNY
ALLUVIUM
-35
130
140
KM
150
Fig. 3. The Khvalyn clay roof elevation and the modern alluvial sediment thickness within the
Bakhtemir deltaic arm channel from its inlet to the open sea.
Continuous seismoacoustic profiling carried out in the Volga River main channel
from the Akhtubinsk to Astrakhan city, in the Volga delta branches Buzan,
Bakhtemir, Bushma, Krivaya Bolda and in some other branches has allowed to trace
variations of the marine deposit roof elevation along the studied channels and
determine precisely a real thickness of the modern channel bed sediment layer.
Results of the seismoacoustic diagram interpretation and analysis has shown that bed
sediment thickness in the Volga River main channel between the Volgograd city and
the Verkhnee Lebyazhee settlement does not exceed 6-8 m on average. Along the
eroded bedrock escarpments of the right valley side, where roof of marine clays lies
close to the surface, the bed sediment layer thickness does not exceed 1–2 m. Some
sections of the Volga River main channel bed are almost devoid of bedload sediment,
and up to 1/3 of the bed surface area at such sections is represented by exposures of
bedrock clay (yar Cherniy, Nikolskiy, Vetlyanskiy, Pechinistiy, Kopanovskiy,
Seroglazovskiy, Zamyanovskiy) or floodplain loams – pechina – (yar Soleniy,
Gerasimovckiy, Bolhunskiy, Chilimniy, Tsaganskiy, Nizhnekopanovskiy,
Kuznetsovskiy, Danilovskiy, Enotaevskiy, Parashkin, Arbuzniy, Shambaiskiy).
CONCLUSIONS
Continuous seismoacoustic profile of the Lower Volga River channel bed and sidescan sonar images of channel bed sections of the Volga River delta arms and branches
have been acquired for the first time. On basis of the extensive field data set, thickness
and stratigraphy of the modern channel bed sediment have been evaluated. Analysis
of side-scan sonar images and echo depth-sounding records have allowed us to study
channel bed morphology and obtain quantitative parameters of dune bedforms.
REFERENCES
1. Galaktionov, V.D. (1960) Engineering-geological conditions of the Volga-Don water
transportation system construction area. In: Geology of the Volga-Don water transportation
system construction area. Moscow-Leningrad, Gosenergoizdat Publ. (in Russian).
2. Galaktionov, V.D. (1960) Alluvium as a foundation for building hydrotechnical
constructions. In: The Gidroproekt Institute Proceedings, Vol. 3, Applied geology. Moscow,
the Gidroproekt Institute Publ. (in Russian).
3. Geology of the Volga River delta. (1951) Edited by M.V. Klenova. The GOIN
7
Proceedings, Vol. 18(30). Leningrad, Gidrometeoizdat Publ. (in Russian).
4. Geology of the Volga-Don water transportation system construction area. (1960) MoscowLeningrad, Gosenergoizdat Publ. (in Russian).
5. Goretskiy, G.I. (1966) The Volga River valley formation in lower and middle Pleistocene.
Moscow, Nauka Publ. (in Russian).
6. Korotaev, V.N. (1999) Granulometric composition and spatial distribution of bedload
sediment of the Lower Volga River. In: Dynamic and termic characteristics of rivers,
reservoirs and sea coastal zones. Proceedings of the Vth conference. Moscow, IVP RAN
Publ. (in Russian).
7. Korotaev, V.N., Zaitsev, A.A., Rimskiy-Korsakov, N.A., and Sychev, V.A. (1996) The
channel morphology and sediment stratigraphy in the western subsistem of the Volga River
delta distributaries. Vestnik MSU. Series 5. Geography. Vol. 5 (in Russian).
8. Korotaev, V.N., and Ivanov, V.V. (2000) Channel deformations of the Lower Volga River.
Vestnik MSU. Series 5. Geography. Vol. 6 (in Russian).
9. Korotaev, V.N., and Rimskiy-Korsakov, N.A. (2000) Investigation of the river channel
relief and sediment by means of hydroacoustic sounding techniques. In: Geomorphology on te
XXIst century frontier. Proceedings of the Chukin readings. Moscow, Moscow State
University Publ. (in Russian).
10. Korotaev, V.N., Rimskiy-Korsakov, N.A. and Sychev, V.A. (1997) Application of
hydroacoustic sounding techniques for investigation of bottom topography in estuaries of
large rivers. In: Modern methods and techniques of oceanological investigations. Proceedings
of the conference. Moscow, IO RAN Publ. (in Russian).
11. Korotaev, V.N., and Chernov, A.V. (2000) Morphology and dynamics of the VolgaAkhtuba floodplain. Geomorfologiya, 3 (in Russian).
12. Korotaev, V.N., and Chernov, A.V. (2001) Formation of the Volga-Akhtuba valley
floodplain and the Volga River palaeodeltas during the late Pleistocene and Holocene. In: Soil
erosion and river channel processes. Vol. 13. Moscow, Moscow State University Publ. (in
Russian).
13. Kronenberg S.B., Rusakov G.V., Svitoch A.A. (1997.) The wandering of the Volga delta:
a response to rapid Caspian sea-level change. In.: J. Sedimentary Geology,107, pp. 189–209
(in English).
14. Lohin, M.Yu., and Maev, E.G. (1990) The Middle Pleistocene age palaeodeltas on shelf
of the Middle Caspian Sea northern part. Vestnik MSU. Series 5. Geography. Vol. 3 (in
Russian).
15. The Lower Volga River: geomorphology, palaeogeography and channel morphodynamics.
(2002) Ed. By G.I. Rychagov and V.N. Korotaev. Moscow, GEOS Publ. (in Russian).
16. Nikolaev, V.A. (1962) Geological evolution, relief and alluvial deposits of the VolgaAkhtuba valley and the Volga River delta. In: Natural environment and agriculture of the
Volga-Akhtuba valley and the Volga River delta. Moscow, Moscow State University Publ. (in
Russian).
17. Popov, I.V. (1963) The Volga River channel deformations at its reach between the
Volgograd and Astrahan cities. In: Proceedings of the State Hydrological Institute, Vol. 108
(in Russian).
18. Rychagov, G.I. (1977) The Caspian Sea Pleistocene history. Moscow, Moscow State
University Publ. (in Russian).
19. Svitoch, A.A., and Yanina, T.A. (1994) Structure and development of the Volga River
delta. Geomorfologiya, 4 (in Russian).
20. Svitoch, A.A., and Yanina, T.A. (1997) Quaternary deposits of the Caspian Sea coastal
zones. Moscow, Moscow State University Publ. (in Russian).
21. The Volga River deltaic zone: hydrological and morphological processes, redistribution of
pollutants and the Caspian Sea level change influence. Moscow, GEOS Publ. (in Russian).