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Methology and contexts

The Danube Delta

an Overview of its Holocene Evolution
Le delta du Danube, synthèse de son évolution holocène
Nicolae Panin, Laura Tiron Duţu and Florin Duţu
p. 37-54

Abstracts

This paper presents a short synthesis on the physiography, geology, hydrology and evolution of the Danube Delta during the Holocene. Deltaic conditions were initiated during the Quaternary, when the Danube started flowing into the Black Sea basin. The Danube Delta is formed of a sequence of clastic deposits ranging from tens to 300‑400 meters thick that accumulated mainly during the Late Pleistocene and the Holocene. The Holocene evolution phases of the Danube Delta have been elucidated by corroborating geomorphological, structural, textural, geochemical, mineralogical and faunal analyses and mainly by 14C dating. Five main evolutionary phases have been elucidated: (1) the formation of the Initial Letea-Caraorman Spit, 11700‑7500 yr. BP; (2) the St. George I Delta, 9000-7200 yr. BP; (3) the Sulina Delta, 7200-2000 yr. BP; (4) the St. George II and Kilia Deltas, 2500 yr. BP to present; and (5) the Cosna-Sinoie Delta, 3500-1500 yr. BP. The paper also describes the anthropogenic changes that have occurred during the last two centuries.

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1Most of the world’s deltaic systems started forming between 7400 and 9 500 yr. BP as a result of decelerating sea-level rise (STANLEY and WARNE, 1993). Situated in the contact area between continental and marine environments, deltas serve as the primary pathway for the transport of fresh water and terrigenous sediment to the coastal ocean (MILLIMAN and MEADE, 1983). The structure and the evolution of deltaic systems are controlled by extremely complex processes and factors including variations in relative sea level, fluvial inputs, marine dynamics, morphology and tectonics. Authors have studied these processes for many of the world’s largest rivers including the Amazon (NITTROUER et al., 1986), the Yellow River (LIU et al., 2004 and 2007), the Yangtze River (CHEN et al., 2000), the Po River (CATTANEO et al., 2003), the Ganges-Brahmaputra system (KUEHL et al., 1997; GOODBRED and KUEHL, 1999 and 2000), the Mekong River (TA et al., 2002; XUE et al., 2010) and the Mississippi River (KNOX, 2006 ; BIEDENHARN et al., 2000); etc.).

2The Danube Delta (Fig. 1) is part of a large European geo-system consisting of the Danube River – Danube Delta – Black Sea.

Fig.1 – Danube Delta general view

Fig.1 – Danube Delta general view

Credit: Landsat satellite image.

3Many investigations have been carried out on this system, and especially on the Dan­ube Delta, since the middle of the 19th century, which have im­proved our understanding of the genesis, structure and evo­lution of this system. Particularly important are the studies of A. C. Hartley (1867), Gr. Antipa (1915 and 1941), C. Brătescu (1922 and 1942), G. Vâlsan (1934), I. Lepsi (1942), H. Slanar (1945), I. G. Petrescu (1957), P. CoteȚ (1960), M. Bleahu (1963), H. Grumãzescu et al. (1963), E. Liteanu et al. (1961), E. Liteanu and A. Pricãjan (1963), A. A. Almazov et al. (1963), A.C. Banu (1965), A. C. Banu and L. Rudescu (1965), N. Panin (1974, 1976, 1983, 1989, 1996, 1997, 2003 and 2009), N. Panin and D. Jipa (2002), N. Panin and I. Popescu (2004), N. Panin et al. (2005), N. Panin and W. Overmars (2012), P. ȘTESCU and B.V. Driga (1981 and 2009), C. Bondar (1972, 1992 and 1993), C. Bondar et al. (1991), L. Giosan et al. (1997, 2005 and 2006), A. Stănică and N. Panin (2009), L. Tiron (2010), L. Tiron Duțu et al. (2014) and Vespremeanu-Stroe et al. (2013).

1 - General setting and general description

1.1 - Geological context

4The Danube Delta is situated in an area of high structural mobility, repeatedly affected by strong subsidence and important sediment accumulation. Deltaic conditions developed here during the Quaternary, when the Danube started flowing into the Black Sea basin.

5The Danube Delta overlaps the Pre-Dobrogean Depression, which, in turn, lies mainly on the Scythian Platform. The sequence of the Scythian Platform deposits, which constitute the fill material of the Pre-Dobrogean Depression, displays six sedimentation cycles: Palaeozoic calcareous–dolomitic; Lower Triassic of considerable thickness (400-2,500 m), slightly unconformable over‑subjacent deposits consisting of red continental detrital deposits with interlayered volcanic rocks; Middle-Upper Triassic transgressive, marine, comprising carbonate rocks in the lower part (350-450 m limestones, and 500-600 m dolomites) and of detrital rocks (450 m) in the upper part; Jurassic transgressive marine, consisting of detrital deposits at the base (Middle Jurassic, 500‑1,700 m thick) and carbonate ones at the top (Upper Jurassic, 1,000 m thick in the southern area); Lower Cretaceous overlying Jurassic deposits, consisting of red continental deposits of varying thickness (ca. 500 m) and Sarmatian-Pliocene overlying different Mesozoic deposits and consisting of alternating clay, sand and sandstone (200‑350 m thick) (Panin, 2003).

1.2 - The Danube River Danube Delta – Black Sea System

6The Danube River is one of Europe’s most important waterways, flowing over 2,860 km across the continent from the Schwarzwald Massif down to the Black Sea. The Danube is listed (after the river Volga) as the second largest river in Europe. Its drainage basin extends over 817,000 km2 and more than 15 countries share the Danube catchment area, which is inhabited by more than 80 million people.

7The average annual water discharge of the Danube River at the delta apex is 6,550 m3.s-1. According to N. Panin (2003), N. Panin and D. Jipa (2002), M. Meybeck et al. (2003) and D.E. Walling (2006), the present sediment discharge was modified by the building of the Iron Gates dams that induced a critical decrease in the sediment discharge from ≈ 67 million t.yr-1 to ≈ 30‑40 million t.yr-1.

8The Danube Delta, the largest delta in the European Union, is located at the mouth of the Danube River, where it flows into the Black Sea. Situated between 44°25’ and 45°30’ N and between 28°45’ and 29°46’ E (Fig. 1), the Danube Delta is bordered by the Bugeac Plateau to the north and by the Dobrogean Orogenic Unit to the south.

1.3 - The Danube Delta Hydrographical Network

9The delta starts (the delta apex) at the first bifurcation of the Danube, named Ceatal Izmail (Mile 43 from the mouth zone, measured along the Sulina distributary), into the Kilia distributary to the North and to Tulcea distributary in the South.

10At present the Kilia distributary, the largest of the delta system, is 117 km long and forms the border between Ukraine and Romania. At its mouth Kilia, forms a secondary lobate delta of 24,400 ha, with numerous distributaries (the main ones are the Oceakov flowing to the NE, and Stary Stambul oriented towards the S‑SE); this secondary delta lies within Ukrainian territory and, in 1998, was declared the “Ukrainian Danube Biosphere Reserve”, a fully protected ecological zone, under UNESCO jurisdiction.

11The Tulcea distributary stretches from the Ceatal Izmail 17 km to the east to a new bifurcation, Ceatal Sfântu Gheorghe (St. George) at Mile 33.84 (km 62.2), where it divides into two main distributaries: Sulina on the left and Sfântu Gheorghe (St. George) on the right.

12From the Ceatal St. George, the Sulina distributary flows eastward 71.7 km (present-day length, including the 8 km of dykes at the mouth of the arm) towards the Black Sea. The Sulina distributary’s present-day physiography results from a large diversion programme carried out between 1868 and 1902 by the European Danube Commission. This project shortened the branch by 24 % (83.8 km before the cut-offs, and now only 63.7 km), and induced a deepening of the river channel in time, from less than 2.5 m in 1857 to at least 9.5 m in 1959. The shortening and deepening of the river channel radically changed the hydrological regime of the Delta by increasing the water discharge of the Sulina distributary from 7 – 9 % to about 19 % of total Danube discharge.

13The St. George distributary, starting from the hydrographic knot at Ceatal Sfântu Gheorghe is 108.8 km long until the sea. The distributary flows some 15-20 km across the North Dobrogean unit, formed of old geological formations that are difficult to erode, and this influences the river’s physiography. The course of the St. George branch can be subdivided into three sections (Panin, 1976): (1) the Dobrogean section of limited meandering (between km 104 and km 90); (2) the free meandering segment of the St. George arm (between km 90, where the Dobrogean unit ends, and km 22) with a succession of 6 meander loops; and (3) the downstream section of limited meandering (between km 22 and km 0). The St. George meander loops were rectified in 1981-1992; these cut-offs led to a shortening of the distributary by about 31 km and, consequently, increased the free water surface slope and water flow velocity determining higher water and sediment discharges.

14At its mouth, the St. George distributary forms a small secondary delta with two secondary branches, which fork at km 5: the prolongation of the main St. George channel (also called Kedrilles in historical documents) and the Olinca branch on the right. This latter branch bifurcates to two small branches: the Seredne on the left, about 3.5 km long (already silted to-day), and the Turetzkii (or Gârla Turcului) on the right, some 4.5 km in length.

1.4 - The Danube Delta’s Geomorphological and Depositional Units

15The Danube Delta can be divided into three major depositional systems (PANIN, 1989) (Fig. 2): (1) the delta plain with a total area of about 5800 km2, of which the marine delta plain area comprises 1800 km2; (2) the delta-front, with an area of ca 1,300 km2, which is divided into the delta-front platform (800 km2) and the delta-front slope (ca. 500 km2) and extends offshore to a water depth of 30‑40 m; (3) the prodelta lies offshore, at the base of the delta-front slope, down to a depth of 50‑60 m, and covers an area of over 6,000 km2. To the depositional system should be added the Danube deep-sea fan system that occurs off Romania, Bulgaria and Ukraine in the northwestern Black Sea and extends from a depth of several hundred meters down to the abyssal plain (over 2,200 m).

Fig. 2 ‑ The Danube Delta major morphological and depositional units

Fig. 2 ‑ The Danube Delta major morphological and depositional units

Legend 1: delta plain with: (1a) the fluvial delta plain, (1b) the marine delta plain, (1c) the fossil and modern beach-ridges and littoral accumulative formations built up by the juxtaposition of beach-ridges; legend 2: the delta front with: (2a) the delta front platform, (2b) the relics of the “Sulina Delta” and its delta front, (2c) the delta front slope; 3: the Danube prodelta; 4: depth contour lines in meters.

After Panin, 1989

16The delta plain starts from the delta apex, at the first bifurcation of the Danube Ceatal Izmail. The delta plain is roughly triangular in shape, with a strong southward asymmetry. The average altitude of the Delta relief is about 0.52 m with a mean inclination of ca. 0.0428 %. The delta plain contains different elements of positive and negative relief: the positive relief components include continental relics (e.g. Stipoc remnants) and promontories (e.g. Kilia) entering the delta territory, fluvial subaerial levees, old and present-day marine beach ridges and littoral accumulative formations formed by the juxtaposition of numerous ridges (among which the most important are: Jibrieni in Ukraine, and Letea, Caraorman, Sãrãturile, Perisor, Chituc etc. in Romania), lacustrine spits (e.g. Stipoc spit) (Fig. 3).

Fig. 3 ‑ The Danube Delta’s geomorphological-sedimentological structure

Fig. 3 ‑ The Danube Delta’s geomorphological-sedimentological structure

The map outlines the main sets of beach ridges and the development phases of the delta during the Holocene. 1: marine beach ridges; 2: lacustrine spit; 3: directions of main beach ridges and beach ridge sets; 4: river meandering zone

After Panin, 1989.

17Negative relief forms are represented by areas covered by water constituting the Delta’s hydrographic network. About 54.5 % of the Danube delta plain consists of areas having altitudes between 0 and 1 m above the Black Sea - Sulina reference system, and 18 % with altitudes between 1 and 2 m.

18The delta front area is about 1,300 km2 and can be divided into the delta front platform (ca. 800 km2) and the delta front slope (ca. 500 km2). The main distributaries debouche into zones with different bathymetry: in the North, the Kilia branch flows into a continental shelf area of 20‑25 m depth, while in the South, at the mouth of the St. George distributary, the water depth is considerably higher (30‑40 m). This differentiates the morphology of the delta front in the two mentioned areas.

19The prodelta lies off-shore, at the base of the delta front down to a depth of 50‑60 m, and covers more than 6,000 km2. The eastern limit of the prodelta can be accurately identified in Fig. 2. Elongated depressions, such as small valleys or submarine channels (4 – 10 m deep) bordered by lateral levees or ridges, have been identified in the prodelta area. These channels seem to represent discharge courses of turbid flow yield by the river distributaries, especially at high flood.

2 - The Holocene evolution of the Danube Delta

20The Danube Delta was mainly formed during the Upper Pleistocene highstands (Karangatian, Surozhian) and the Holocene. Its present-day geomorphology expresses the interaction of the river (sediment and water discharge, flow energy, etc.) and the sea (wave and littoral currents regime, sea-level changes, etc.) over the past about 12,000 yr.

21The ages of the delta evolution phases are presently under discussion. The first data set of 14C age determinations is quite old (1972‑1978). It was obtained in the framework of a collaborative project between the Romanian Institute of Geology and Geophysics and the Centre for Applied Isotope Studies in the USA. The results were published in 1983 (Panin et al., 1983; Noakes and Herz, 1983) and represented the base of the following succession and timing of evolution phases: (1) the “Blocked Danube Delta” and formation of the Letea-Caraorman initial spit, 11,700-7,500 yr. BP; (2) the St. George I Delta, 9,000‑7,200 yr. BP; (3) the Sulina Delta, 7,200-2,000 yr. BP; (4) the St. George II and Kilia Deltas, 2,800 yr. BP to present; (5) the Cosna-Sinoie Delta, 3,500‑1,500 yr. BP (not calibrated ages).

22L. Giosan et al. (2005 and 2006) suggested younger ages for the initial stages of delta development (for example, in their view, the St. George I Phase could not be much older than ~5,500-6,000 yr. BP). This hypothesis seems to be more consistent with our present-day understanding of water‑level changes in the Black Sea during the Pleistocene – Holocene time. Nevertheless a number of questions are still under discussion.

23A new campaign of age determinations using, this time, two methods – radiocarbon by AMS and thermo-luminescence – is now in progress. The first results obtained gave us comparable ages using the two methods and these results are very similar with the previous data set (for example for the Initial spit: 14C age at 80 cm sampling depth – 9,973 yr. BP while TLD at 150 cm sampling depth – 9,200 yr. BP). The 14C age was obtained on a marine shell and the age is quite difficult to correlate with present understanding of the Black Sea evolution. Consequently, we consider that the timing of the Danube Delta phases is still open and new studies and age determinations are needed. For the moment, it is too early to take a definitive decision. Probably the new data will introduce some corrections to the old scheme and will allow a closer fit with the Black Sea development.

24Bearing in mind that the succession of the main phases of the Danube Delta development is well established and not contested, in the present paper we shall mainly describe the physiographic changes of the delta during these phases. Conventionally we shall use the chronology of the “old” 14C ages data set mentioned above.

2.1 - The “Blocked Danube Delta” and the initial spit of Letea-Caraorman

25At the beginning of the Holocene, the present area of the Danube Delta was transformed into a large marine bay - the Danube Gulf (Fig. 4).

Fig. 4 ‑ The Danube Delta coastline position during the “blocked Danube Delta”

Fig. 4 ‑ The Danube Delta coastline position during the “blocked Danube Delta”

26All the tributary valleys from the north, from the Bugeac Plateau (Kitai, Catlabug, Ialpug, Kahul) had been partially invaded by the sea and then transformed into lakes (lagoons, locally referred to as limans) following the formation of spits at their mouths. At the mouth of the Danube Gulf, between the Jebriani promontory to the north and Murighiol-Dunavăț promontory of Dobrogea to the south, a spit was formed by the littoral sediment drift fed by Ukrainian rivers (Dniester, Dnieper and Southern Bug). The spit was named, in accordance with other predeces­sors, the “Jebriani‑Letea‑Caraorman Initial Spit”. It almost entirely closed access to the Danube Gulf and repre­sented the coastline for this period of time.

27During the existence of the Danube Gulf, almost the en­tire solid discharge of the Danube River was deposited inside the gulf sheltered by the initial spit, and formed a deltaic body called the “Blocked Danube Delta” phase F (~11,000 – 9,000 yr. BP) (Fig. 4).

2.2 - The “Saint George I Delta”

28Between the southern end of the initial spit and the Murighiol–Dunavăţ Promotory, there was a passage through which the first Danube distributary, Palaeo-St. George, flowed into the sea (Fig. 5). It is here that the first delta of the Danube was formed - the “St. George I Delta” and its development lasted for about 2,000 years (~ 9,000 – 7,000 yr. BP).

Fig. 5 ‑ The position of the Danube Delta coastline during the maximum progradation of the St. George I Delta phase

Fig. 5 ‑ The position of the Danube Delta coastline during the maximum progradation of the St. George I Delta phase

29At present, only the northern wing of this delta can be recognised. This flank is represented by the littoral accumulative formation Caraorman, built up by the juxtaposition of an impressive number of palaeo-beach ridges. These ridges were formed exclusively of sandy sediments from Ukrainian rivers transported along the seashore by littoral drift.

30The progradation of the St. George I Delta coastline totalled around 10 km in about two thousand years. At the end of this phase the Palaeo-St. George distribu­tary was partially clogged and a new distributary named Sulina was formed.

2.3 - The Sulina Delta

31The newly formed distributary had broken the Initial Spit at some 20 km north of the Palaeo-St. George mouth zone and started to build its own delta – the Sulina Delta (Fig. 6).

Fig. 6 ‑ The position of the Danube Delta coastline during the maximum progradation of the Sulina Delta phase

Fig. 6 ‑ The position of the Danube Delta coastline during the maximum progradation of the Sulina Delta phase

32The develop­ment of the Sulina Delta lasted about 5,000 years (after N. PANIN et al., (1983), from 7,200 to 2,000 yr. BP). Initially, its evolution was slow and its shape was controlled by waves and littoral drift. The progressive increase of sediment discharge from the Sulina distributary caused a significant progradation of the Sulina Delta front, which became lobate over time with three and then five distributaries.

33The maximum progradation of the Sulina Delta into the sea (Fig. 6) (the Sulina Delta front was 10‑15 km offshore from the present shoreline), coincides with the Phanagorian regression when sea level was at -2 to -4 m elevation. The total progradation of the Sulina Delta front during these 5,000 yr was around 30 km.

34Detailed descriptions of the structure of the two wings of the Sulina Delta are given in previous papers (Panin, 1989, 1996 and 1997).

35At the end of the Sulina Delta Phase, at about 3,000 yr. BP the Sulina distributary was partially clogged and lost its importance in the delta system, culminating in the gradual erosion and retreat of the Sulina delta front. At that time, the St. George distributary was reactivated in the south and a new distributary, the Kilia, was formed in the north. Both these distributaries started to build their deltas.

2.4 - The Saint George II Delta

36The formation and the development of the St. George II Delta took place during the past ~3000 yr (Panin, 1983, 1989 and 1996). The St. George II Delta’s northern part is represented by the Sărăturile littoral accumulative formation, while its southern flank comprises an impressive number of fossil beach ridges and beach ridge sets that record the progradation of the delta shoreline (Fig. 7).

Fig. 7 ‑ The position of the Danube Delta coastline during the present-day progra­dation of the Kilia and St. George II Deltas

Fig. 7 ‑ The position of the Danube Delta coastline during the present-day progra­dation of the Kilia and St. George II Deltas

37The Sărăturile Formation has a divergent structure, com­posed of numerous beach-ridge sets. The divergent structure is due to the coastline regression in the north as the Sulina Delta was subject to continuous erosion, while in the south the coast was prograding with the development of the St. George II Delta. The southern wing of the St. George II Delta is formed of multiple fossil beach ridges and beach ridge sets, recording successive steps of delta development and progradation dur­ing the last ca. 3,000 yr. BP. The latest ridge is the arcuate lateral mouth bar Sakhalin (Island Sakhalin) that appeared in the 19th century and is developing rapidly nowadays. The overall progradation over 3,000 yr. was of about 16‑20 km, the average rate of progradation being of 8‑9 m.yr-1.

2.5 - The Kilia delta

38When the Kilia distributary reached and broke the Initial Spit (about 2,500 yr. ago), a new depocentre, the Kilia Delta, began to develop. At the beginning, the progradation was very slow as the Kilia’s sediment supply was low. During this period the Kilia Delta was of a wave-dominated type. Around 1,000 yr ago, the Kilia distributary reached a predominant importance in the delta system, its sediment supply gradually increased and the Kilia Delta became lobated (Fig. 7). The most rapid progradation occurred in the last five centuries and will be discussed further. The sandy sediments supplied by the littoral drift system from the Ukrainian rivers stops north of the Kilia Delta forming the Jebriani Formation ridges. During the past in 2500 yr, the Kilia Delta has prograded by about 18 km.

2.6 - Sinoie Delta

39In the southern Danube Delta territory, during the period from 3,500 to 1,500 yr BP, there was a secondary delta, the Si­noie Delta (Fig. 8).

Fig. 8 ‑ The position of the Danube Delta coastline during the Cosna-Sinoie Delta phase. The coastline at ~ 100 yr AD is also shown

Fig. 8 ‑ The position of the Danube Delta coastline during the Cosna-Sinoie Delta phase. The coastline at ~ 100 yr AD is also shown

40Two successive development stages have been elucidated: Cosna and Sinoie Deltas. These two deltas are, in fact, formed by a secondary distributary, the Dunavăț. Dur­ing the first centuries AD, the Sinoie Delta was eroded and its material redeposited as beach ridges forming the Lupilor, Istria and Chituc accumulative formations. The Lupilor sets of beach ridges were synchronous with the existence of the Greek and Roman cities Istros/His­tria and Orgame/Argamum (~7th century BC to ~7th century AD) on the western coast of the Black Sea. The sediments eroded from the Sinoie Delta drifted southwards and caused the decline and, in the end, the collapse of the town of Istria at about 700 AD.

41Giosan et al. (2006), do not support the hypothesis of the edification of the delta Cosna-Sinoie by the Dunavăț distributary. In their opinion, the sedimentary transport by the littoral drift is sufficient to explain the formation of the Lupilor, Istria and Chituc accumulative formations. Giosan’s opinion on the Cosna lobe appears to be a singular one – Vespremeneanu-Stroe et al. (2013) support the existence of this delta, while for him the lobe was built later and faster than in our interpretation.

42In the littoral drift hypothesis as the only source of sediments for the Lupilor, Sinoie and Chituc Formations, the closest source of sediments is the St. George distributary that is located at about 30-40 km NE of the supposed Dunavăț mouth zone. The St. George – Dunavăț section of the delta coast is characterised by the largest sediment deficit and the weakest littoral drift of the entire delta front, with zones where the drift orientation is opposite to the southward general sediment transfer along the coast. It is not possible that the very large amount of sediments that composes the Lupilor, Sinoie and Chituc Formations had drifted southward from the St. George mouth and passed through the Sf. St. George – Dunavăț section of the weakest littoral drift.

43Additionally, (1) there is some geophysical evidence for an old incised channel and some remains from a frontal beach ridge or presently-submerged mouth zone; (2) the mineralogical composition of the heavy fraction of the littoral sands sustains the area close to the river mouth (close to the mouth zone there are some less stable minerals than those that can support important longshore drift); (3) there is a large residual of heavy minerals (mainly titanite, zircon, rutile, garnet, ilmenite) in the northern part of the Chituc formation that was formed by a strong palaeo-erosion of a large volume of littoral sands from an important sedimentary body (the Cosna lobe).

44Concerning the age of the Cosna-Sinoie Delta: the ages proposed by us are based on 14C and TLD determinations and the differences are compared to Vespreanu-Stroe’s et al. (2013) results. We sampled the first set of beach-ridges marking the onset of the Cosna-Sinoie Delta. The ages we obtained from the entire range of Cosna‑Sinoie beach-ridges sets span the period from about 3,550 yr BP to 1,500 yr BP, while the ages proposed by Vespremeanu-Stroe et al. (2013) are in the interval 2000‑1300 yr BP. In principle, we agree with Vespremeneanu-Stroe’s hypothesis that the maximum development of the Dunavăț and Cosna Delta occurred around 1400-1300 yr BP.

2.7 - The evolution of the Danube Delta hydrographic network during the Holocene (Fig. 9)

Fig. 9 ‑ Hypothetical courses of the Paleo-St. George, Paleo-Sulina and Paleo-Kilia distributaries

Fig. 9 ‑ Hypothetical courses of the Paleo-St. George, Paleo-Sulina and Paleo-Kilia distributaries

After Panin, 1976 with completions.

45The bifurcation of the Danube River at the first hydro­graphic knot probably occurred during the “Danube Blocked Delta”. The northern distributary – Palaeo‑Kilia, during this period appears to have been less important, with a smaller discharge than the southern one – Palaeo‑Tulcea branch. The evolution of the southern branch was influenced by the land of the northern Dobrogea: there were four impingements against Dobrogea - the first at Tulcea, the second, about 7 km downstream, at Nufăru (Preslav), fol­lowed by Carasuhat and Mahmudia impingements (PANIN, 1976). After the impingement at Preslav, (km 104 upstream from the mouth zone of the St. George arm) the distributary divided into two distributaries – Palaeo-St. George (southern branch) and Palaeo-Sulina (northern branch) as a result of disturbed flow conditions that occurred immediately after the impinge­ment. Consequently, initially the second hydrographical knot (Ceatal St. George) was located a few kilometres downstream from the present-day bifurcation point, immediately after Preslav. The Palaeo-St. George branch became the most important and active distributary for a long period and was responsible for the formation of the first Danube Delta, the St. George I Delta.

46The Kilia distributary flowed northwards, touching the Bugeac Plateau in the Izmail area, where it changed the flow direction towards the east, probably, along the Stipoc lacus­trine spit and then followed the present-day course of the Sontea channel to join the Paleo-Sulina distributary around Mile 25‑Mile 24 (on the so-called Old Danube) (Panin, 1976 and 1997).

47During the phase of maximum progradation of the St. George I Delta, the palaeo-distributary St. George formed me­anders of 10-14 km wavelength (λ), with amplitudes (α) of 5-7 km and curvature indices rm/w exceeding 2.0 (Panin, 1976) (Fig. 9).

48Only at the end of the Phanagorian regression, when the sea level was lowered by several meters (‑2 – ‑4 m) and the relief energy increased, was the St. George distributary drained and a new generation of meander bands characterised by λ = 2.4-5.0 km, α = 1.5-4.0 km and rm/w values of 1.49‑2.1 was formed (Panin, 1976) (Fig. 8). Then the excessive length of the distributary channel and associated low-energy relief led to a partial clogging of the Palaeo-St. George which lost its major role in the hydrographic system of the delta during the following period. The Palaeo-Sulina branch gradually be­came the most important distributary (for a period of almost 5,000 yr) and some 6,000‑7,000 yr. BP broke the initial spit in the Răducu area, (Fig. 9), starting to build the “Sulina Delta”. The Palaeo‑Sulina distributary meander system (composed of Maliuc and “Big M” meander bends), had wavelengths and am­plitudes very similar to those of the Palaeo‑St. George branch (λ = 14-16 km; α = 5-7 km; rm/w = 2.67-2.78) (Panin, 1976).

49The Kilia distributary may have adopted its present course around 3,000-3,500 years BP. The area north of Stipoc spit, the Pardina Depression, was occupied by the Lake Thiagola, mentioned by ancient texts (among them Ptolemy or Claudios Ptolemaios – ~90‑168 AD), that was a lagoon formed in the drowned valleys deriving from the Bugeac Plateau (Panin, 1983). The Kilia arm probably entered the Pardina Depression after exceptionally high water in the Danube River and Gulf. The distributary probably found a way out to the sea through the Initial Spit at almost the same time. Since then, the Kilia distributary began introducing an increasing quantity of sediment to the coastal area forming its own depocentre, the Kilia Delta.

2.8 - Present state of the Danube Delta hydrographic system

50Anthropogenic pressure in the last 150 years has transformed the majority of river systems (PETTS, 1984; CHURCH, 1995; SEAR, 1995; XU, 1996; BRANDT, 2000; SHIELDS et al., 2000; BATALLA, 2003; GAEUMAN et al., 2005; MAGILLIGAN and NISLOW, 2005; PHILLIPS et al., 2005). Disturbances like river-draining operations, such as meander cut-offs initiated for navigation or flood mitigation purposes, often lead to dramatic changes in fluvial profiles (HOOKE, 1986; KESEL, 2003; KISS et al., 2008).

51For the lower Danube River, and especially for the Danube Delta, anthropogenic changes started in the 19th century, mainly after the Crimean war with the establishment of the European Danube Commission in 1856. The Commission’s objective was to improve the navigability of the delta distributaries and to transform the river into an international waterway, stretching from the Black Sea to Central Europe. The anthropogenic interventions in the delta continued in the 20th century and their cumulative impact on the delta system has been very important. The measures and the actions during this period of time are described in part 1.3. of the present paper.

52Furthermore, within the lower Danube, two barrages (Iron Gates I and II, built up in 1970 and 1983, respectively) and the hydrotechnical regulation works along the Danube tributaries have dramatically decreased the sediment discharge at the Danube mouths (by around 25‑40 %) (Panin and Jipa, 2002; Panin, 2003; Panin and Overmars, 2012) (Fig. 10).

Fig. 10 ‑ Changes in water discharge between the Danube Delta’s main distributaries during the period 1840‑2003

Fig. 10 ‑ Changes in water discharge between the Danube Delta’s main distributaries during the period 1840‑2003

After Bondar and Panin, 2000.

53This had an adverse impact on the sedimentary budget of the delta’s coastal zone, which became unbalanced and characterized by strong erosion of the delta front.

54The distribution of Danube River water discharge through the main delta branches has varied in the last two centuries, mainly as a result of human intervention: deviation projects, damming, channel construction. Table 1 and Fig. 10 show the estimated variations in water discharge over the last century (Bondar and Panin, 2000).

Tab. 1 ‑ Danube River water discharge distribution through the delta’s main distributaries

Tab. 1 ‑ Danube River water discharge distribution through the delta’s main distributaries

After Bondar and Panin, 2000.

55At present, the Kilia distributary is still the largest of the delta system, but its water and sediment discharges have decreased significantly and, consequently, progradation of the Kilia Delta has slowed down.

56The hydrotechnical works carried out along the Sulina distributary explain the gradual deepening of the river channel from less than 2.5 m in 1857 to at least 9.5 m in 1959 (Bondar and Papadopol, 1972; Bondar and Panin, 2000) and to 14‑15 m at present (Fig. 11), (Duţu, 2014).

Fig. 11 ‑ Diversion of the Sulina distributary meander belts during the period 1868-1902

Fig. 11 ‑ Diversion of the Sulina distributary meander belts during the period 1868-1902

57Recent studies (Duţu, 2014) show that the Sulina channel is very dynamic, with a high capacity for erosion and sediment transport. Active bed forms (mega-ripples, small and high dunes) have been identified across the entire channel bed. The present-day evolution of the Sulina branch is controlled by the continuous anthropogenic works and interventions (dredging for navigation purposes) with direct implications for the fluvial profile.

58The St. George distributary (Fig. 12) meander cut-offs caused dramatic changes in the local distribution of river-flow velocities, discharge, and sediment fluxes (Ichim and Radoane, 1986; Popa, 1997; Panin, 2003; Tiron, 2010, Tiron Duţu et al., 2014).

Fig. 12 ‑ The St. George distributary – the meander belts diversion project (1981‑1992)

Fig. 12 ‑ The St. George distributary – the meander belts diversion project (1981‑1992)

Crédit: Satellite Landsat image.

59A rupture of the natural bend evolution occurred – strong silting processes are more actively expressed in the aggradation of the channel bed, the narrowing of channels and the development of bars and islands along the natural meander bend sections (Jugaru Tiron et al., 2009; Tiron Duţu et al., 2014).

60Additionally, different other works have been undertaken on the Danube Delta territory in or­der to open the fresh water supply to different zones of the delta or to facilitate the navigation and the access to these areas. Thus, a network of 300 km of canals was artificially created. Among these canals, the following had a particularly important impact on the environment: the canal “King Charles I” (canalul “Regele Carol I”), dredged in 1906 on 23 km to rectify a small, second­ary, distributary Dunavăț that at that time was almost completely silted; the canal “King Ferdinand”, dug between 1911 to 1914 along 28 km to open a waterway to the Lake Dranov area and finally, to the Razelm lagoon and the canal “Mila 36”, cut in 1983‑1985 to facilitate the access from the Tulcea distributary (from Tulcea city), to the Kilia distributary (to Kilia Veche village).

3 - Discussions and conclusions

3.1 - The chronology of the delta development phases

61As shown in chapter 2, the chronology of delta evolution is presently under discussion.

62There are at least two prevailing hypotheses concerning the timing of delta development phases, differing notably with regards to the ages for the initial stages of delta evolution. The first hypothesis, based on an older data set, considers that the onset of delta formation occurred around 10-9 ka BP and the first delta lobe (“St. George I Delta”) was formed and existed at 9‑7.2 ka BP. The second hypothesis, based on a more recent data set, suggests that the first delta lobe St. George I was formed around 6 ka BP. To solve this controversy a new age determination campaign is currently in progress. We consider that the problem of the timing of the Danube Delta phases needs new studies and age determinations. These new data will also contribute to a better understanding of the Black Sea region’s development.

3.2 - Influence of the sea-level changes on the Danube depocentre

63The development of the Danube Delta is closely linked to the evolution of the Black Sea water level on the one hand and to the development of the Danube River, of its sediment and water discharge on the other.

64In the last 100 ka there have been at least three high-stands of the Black Sea: the Karangatian phase (~125 – ~65 ka BP), the Surozhian phase (~40–25 ka BP) and after the melting of Würmian icecap (after ~16-15 ka BP). A low stand is documented during the Younger Dryas, followed by a quite rapid transgression of the sea up to the present-day water level.

65During the Karangatian, the water level was a few meters higher than present-day and this meant that the Mediterranean water entered the Black Sea and the water covered the lowlands such as the present-day delta territory. The Surozhian high-stand brought the water level to a slightly lower mark similar to today, and consequently the water did not cover the entire delta area, but it seems that at least the eastern part of the delta was submerged. The Würmian icecap melting again brought the water level to a stand that allowed the water to cover the delta territory and restored the connection between the Black and the Mediterranean seas.

66These high stands are the periods when the Danube River sediment load accumulated within the present-day delta territory. By contrast, during low-stands, the river continued to flow down towards the Black Sea depression, to the low-stand coastal zones situated at or even beyond the shelf break. On the present-day delta territory the low-stands are marked by important incisions of the river valley and strong erosion when the older deposits were washed out almost completely. The following transgressions infilled the incisions and covered the area with new blankets of sediments.

67The evolution phases of the Danube delta have also been influenced by small-scale sea-level variations (for instance, the Phanagorian regressive phase), as well as by the possible autocyclic shifting of the main sediment supply between the Danube distributaries and correspondingly of depocentres from one area to another.

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List of illustrations

Title Fig.1 – Danube Delta general view
Credits Credit: Landsat satellite image.
URL http://journals.openedition.org/mediterranee/docannexe/image/8186/img-1.jpg
File image/jpeg, 1.0M
Title Fig. 2 ‑ The Danube Delta major morphological and depositional units
Caption Legend 1: delta plain with: (1a) the fluvial delta plain, (1b) the marine delta plain, (1c) the fossil and modern beach-ridges and littoral accumulative formations built up by the juxtaposition of beach-ridges; legend 2: the delta front with: (2a) the delta front platform, (2b) the relics of the “Sulina Delta” and its delta front, (2c) the delta front slope; 3: the Danube prodelta; 4: depth contour lines in meters.
URL http://journals.openedition.org/mediterranee/docannexe/image/8186/img-2.jpg
File image/jpeg, 564k
Title Fig. 3 ‑ The Danube Delta’s geomorphological-sedimentological structure
Caption The map outlines the main sets of beach ridges and the development phases of the delta during the Holocene. 1: marine beach ridges; 2: lacustrine spit; 3: directions of main beach ridges and beach ridge sets; 4: river meandering zone
Credits After Panin, 1989.
URL http://journals.openedition.org/mediterranee/docannexe/image/8186/img-3.jpg
File image/jpeg, 824k
Title Fig. 4 ‑ The Danube Delta coastline position during the “blocked Danube Delta”
URL http://journals.openedition.org/mediterranee/docannexe/image/8186/img-4.jpg
File image/jpeg, 568k
Title Fig. 5 ‑ The position of the Danube Delta coastline during the maximum progradation of the St. George I Delta phase
URL http://journals.openedition.org/mediterranee/docannexe/image/8186/img-5.jpg
File image/jpeg, 588k
Title Fig. 6 ‑ The position of the Danube Delta coastline during the maximum progradation of the Sulina Delta phase
URL http://journals.openedition.org/mediterranee/docannexe/image/8186/img-6.jpg
File image/jpeg, 652k
Title Fig. 7 ‑ The position of the Danube Delta coastline during the present-day progra­dation of the Kilia and St. George II Deltas
URL http://journals.openedition.org/mediterranee/docannexe/image/8186/img-7.jpg
File image/jpeg, 788k
Title Fig. 8 ‑ The position of the Danube Delta coastline during the Cosna-Sinoie Delta phase. The coastline at ~ 100 yr AD is also shown
URL http://journals.openedition.org/mediterranee/docannexe/image/8186/img-8.jpg
File image/jpeg, 732k
Title Fig. 9 ‑ Hypothetical courses of the Paleo-St. George, Paleo-Sulina and Paleo-Kilia distributaries
Credits After Panin, 1976 with completions.
URL http://journals.openedition.org/mediterranee/docannexe/image/8186/img-9.jpg
File image/jpeg, 832k
Title Fig. 10 ‑ Changes in water discharge between the Danube Delta’s main distributaries during the period 1840‑2003
Credits After Bondar and Panin, 2000.
URL http://journals.openedition.org/mediterranee/docannexe/image/8186/img-10.jpg
File image/jpeg, 136k
Title Tab. 1 ‑ Danube River water discharge distribution through the delta’s main distributaries
Credits After Bondar and Panin, 2000.
URL http://journals.openedition.org/mediterranee/docannexe/image/8186/img-11.jpg
File image/jpeg, 56k
Title Fig. 11 ‑ Diversion of the Sulina distributary meander belts during the period 1868-1902
URL http://journals.openedition.org/mediterranee/docannexe/image/8186/img-12.jpg
File image/jpeg, 180k
Title Fig. 12 ‑ The St. George distributary – the meander belts diversion project (1981‑1992)
Credits Crédit: Satellite Landsat image.
URL http://journals.openedition.org/mediterranee/docannexe/image/8186/img-13.jpg
File image/jpeg, 641k
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References

Bibliographical reference

Nicolae Panin, Laura Tiron Duţu and Florin Duţu, « The Danube Delta », Méditerranée, 126 | 2016, 37-54.

Electronic reference

Nicolae Panin, Laura Tiron Duţu and Florin Duţu, « The Danube Delta », Méditerranée [Online], 126 | 2016, Online since 01 January 2018, connection on 20 April 2018. URL : http://journals.openedition.org/mediterranee/8186 ; DOI : 10.4000/mediterranee.8186

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About the authors

Nicolae Panin

National Institute of Research and Development for Marine Geology and Geo-ecology GEOECOMAR, 23–25 Dimitrie Onciul Street, 024053 Bucharest, Romania

Laura Tiron Duţu

National Institute of Research and Development for Marine Geology and Geo-ecology GEOECOMAR, 23–25 Dimitrie Onciul Street, 024053 Bucharest, Romania

Florin Duţu

National Institute of Research and Development for Marine Geology and Geo-ecology GEOECOMAR, 23–25 Dimitrie Onciul Street, 024053 Bucharest, Romania

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Copyright

Tous droits réservés

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