Proceedings of the Geologists’ Association 121 (2010) 180–194

Fluvial style changes during the last glacial–interglacial transition in themiddle Tisza valley (Hungary)

C. Kasse a,*, S.J.P. Bohncke a, J. Vandenberghe a, G. Gabris b

a Department of Climate Change and Landscape Dynamics, Faculty of Earth and Life Sciences, Vrije Universiteit, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlandsb Department of Physical Geography, Eotvos Lorand University, Pazmany Peter Setany 1/c, H-1117 Budapest, Hungary

A R T I C L E I N F O

Article history:

Received 2 June 2009

Received in revised form 2 October 2009

Accepted 11 February 2010

Available online 17 March 2010

Keywords:

Fluvial morphology

Tisza valley

River pattern change

Avulsion

Vegetation development

Glacial–interglacial transition

A B S T R A C T

The Weichselian Late Pleniglacial, Lateglacial and Holocene fluvial history of the middle Tisza valley in

Hungary has been compared with other river systems in West and Central Europe, enabling us to define

local and regional forcing factors in fluvial system change. Four Weichselian to Holocene floodplain

generations, differing in palaeochannel characteristics and elevation, were defined by geomorphological

analysis. Coring transects enabled the construction of the channel geometry and fluvial architecture.

Pollen analysis of the fine-grained deposits has determined the vegetation development over time and,

for the first time, a bio(chrono)stratigraphic framework for the changes in the fluvial system.

Radiocarbon dating has provided an absolute chronology; however, the results are problematic due to

the partly reworked character of the organic material in the loamy sediments. During the Late

Pleniglacial, aggradation by a braided precursor system of the Tisza and local deflation and dune

formation took place in a steppe or open coniferous forest landscape. A channel pattern change from

braided to large-scale meandering and gradual incision occurred during the Late Pleniglacial or start of

the Lateglacial, due to climate warming and climate-related boreal forest development, leading to lower

stream power and lower sediment supply, although bank-full discharges were still high. Alternatively,

this fluvial change might reflect the tectonically induced avulsion of the River Tisza into the area. The

climatic deterioration of the Younger Dryas Stadial, frequently registered by fluvial system changes

along the North Atlantic margin, is not reflected in the middle Tisza valley and meandering persisted. The

Lateglacial to Holocene climatic warming resulted in the growth of deciduous forest and channel incision

and a prominent terrace scarp developed. The Holocene floodplain was formed by laterally migrating

smaller meandering channels reflecting lower bank-full discharges. Intra-Holocene river changes have

not been observed.

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1. Introduction

Changes in fluvial styles and phases of fluvial instability, leadingto incision or aggradation, are related to changes in the ratiobetween sediment supply and transport capacity. Tectonic activity,base-level and climate changes are the main factors forcing this ratio(see Blum and Tornqvist, 2000, for a review and further references).During glacial to interglacial transitions, and vice versa, climate andclimate-related discharge and vegetation changes have been shownto have been important factors determining river behaviour(Vandenberghe, 2002). In NW and northern Central Europe,climate-related fluvial changes of the last glacial–interglacial cyclehave been studied in great detail in recent decades (e.g. Pons, 1957;Kozarski, 1983; Vandenberghe et al., 1994; Bohncke et al., 1995;Antoine, 1997; Schirmer, 1995; Van Huissteden and Kasse, 2001;

* Corresponding author.

E-mail address: kees.kasse@falw.vu.nl (C. Kasse).

0016-7878/$ – see front matter � 2010 The Geologists’ Association. Published by Else

doi:10.1016/j.pgeola.2010.02.005

Houben, 2003; Pastre et al., 2003; Kasse et al., 2005; Busschers et al.,2007). Clear fluvial changes have been established associated withmajor climatic events at the start of the Lateglacial interstadial at c.14.5 ka cal BP, the Younger Dryas Stadial at 13 ka BP (Kasse, 1995)and the start of the Holocene at 11.7 ka BP (Starkel, 2002). A delayedresponse of river systems to climate warming at the start of theLateglacial could have been related to the gradual immigration ofvegetation into the previously barren Pleniglacial landscape (Hoek,1997). In the Maas and Niers valleys in the Netherlands andGermany, there was a fluvial transition from braided to meanderingsystems during c. 500 radiocarbon years, coinciding with a gradualincrease in vegetation cover and decrease in sediment supply (Kasseet al., 1995, 2005). Many of the studied late Weichselian riversystems in NW and northern Central Europe are situated in thesandy periglacial zone south of the Weichselian ice sheet margin.Despite different tectonic settings, the responses of the river systemsappear to have been synchronous over large areas, thereforeindicating that climate change was the dominant factor forcingfluvial system change.

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C. Kasse et al. / Proceedings of the Geologists’ Association 121 (2010) 180–194 181

The late Weichselian environmental setting (climate andvegetation) in Hungary differed from that of NW and northernCentral Europe. Vegetation refugia were probably present nearby.Willis et al. (2000) described the presence of trees, probably inmicroenvironmentally favourable sites, during the full glacialbetween c. 32,500 and 16,500 BP in Hungary. The early presence of(forest) vegetation is an important factor in river response, since itdetermines sediment supply and water discharge. Wind-blownloess, reworked by water flow, was the dominant sediment in riversystems. Climate was probably different also, as present-dayclimate is more continental with high summer temperatures(Tjuly 20 to 25 8C), low winter temperatures (Tjanuary 0 to�5 8C)and low precipitation values (400–600 mm/y). Climate models ofthe full glacial (GS-2a) suggest winter temperatures of �10 to�15 8C and summer temperatures of 20 8C (Renssen and Isarin,2001). Palaeoclimate reconstructions have established similarwinter temperatures of �11 to �15 8C but lower summertemperatures of 11 to 15 8C (Borsy, 1991). Hungary was probablyat the southern limit of the permafrost zone (Renssen andVandenberghe, 2003) and continuous permafrost may have beenpresent (Kovacs et al., 2007). These differences in vegetation(early woodland development), sediments (loess) and climate(continental) in Hungary in comparison to NW and north CentralEurope provide the rationale for studying the late Weichselianresponse of the Tisza to climate change.

The long-term development of the Tisza and its changingpositions in the Hungarian Plain during the Pleistocene werestrongly controlled by changes in tectonic subsidence rates indifferent parts of the plain (Gabris and Nador, 2007). Furthermore,the present-day channel sinuosity of the Tisza has been related toactive deformation and differential subsidence (Timar, 2003).Previous studies dealing with the last glacial–interglacial cyclewere mostly based on morphological and tectonic analysis of

Fig. 1. Location map of the study area in Hungary. The Great Hungarian Plain, generally

Mures rivers. NL = Northern Latitude; EL = Eastern Longitude.

catchments (Mike, 1975; Borsy, 1995; Timar et al., 2005). Recentsedimentological and sediment provenance studies, in combina-tion with absolute dating, have resulted in a better understandingof climate-related fluvial changes and tectonically driven riveravulsions in the Great Hungarian Plain (Nador et al., 2007; Thamo-Bozso et al., 2007). Sea-level changes are not likely to have affectedthe river system in the study area because of the intermontanetectonic setting and the long distance to the Black Sea coast(1250 km). However, the fluvial sedimentology and timing of thefluvial system changes in the middle Tisza valley, and their relationto climate and vegetation changes, have remained unclear. Thispaper aims to reconstruct the late Weichselian to Holocenemorphology and alluvial architecture of the Middle Tisza valley, toexplore the climate-driven vegetation development and itssignificance for fluvial system changes, to discuss the role oftectonically driven fluvial changes in the Great Hungarian Plainand to compare the fluvial response of the Tisza with those of otherCentral and NW European rivers.

2. Geomorphological setting

The southward-flowing Tisza, almost 1000 km long, is one ofthe main tributaries of the Danube (Fig. 1). Its source is located inthe >2000 m high Carpathian mountains in Ukraine. The Tiszacatchment is >57,200 km2 and discharges water from Romania,Ukraine, Slovakia and Hungary. The mean annual discharge (1931–1970) at the confluence with the Danube is 766 m3/s, ranging from371 m3/s to a maximum of 1644 m3/s (http://www.rec.org/DanubePCU/outputs/icdrbm.html). The present-day Tisza is c.170 m wide and c. 10 m deep and the Holocene floodplain has agradient of c. 15 cm/km. The river was canalized and embanked inthe middle of the 19th century. Its present-day sediments aremainly fine-grained sands and silts. The middle and lower reaches

less than 100 m above sea level, is a vast alluvial plain of the Tisza, Sajo, Koros and

C. Kasse et al. / Proceedings of the Geologists’ Association 121 (2010) 180–194182

of the Tisza are located in the Pannonian Basin, which is a largeintermontane basin, surrounded by the Carpathians, Dinarides andAlps. Late Tertiary and Quaternary tectonic compression hasresulted in uplift of the marginal mountain areas and subsidence ofthe central part of the basin (Ruszkiezay-Rudinger, 2007). TheQuaternary fluvial deposits consist mostly of fine-grained clays,silts and sands. The thickness, based on seismic studies andboreholes, is up to 700 m in the southern part of the basin and morethan 200 m just north of the study area (Nador et al., 2003, 2007).The courses of the Tisza and Danube are strongly determined byQuaternary differential tectonic movements (Gabris, 1994; Gabrisand Nador, 2007). During the Early and Middle Pleistocene the

Fig. 2. Late Weichselian and Holocene floodplain levels

strongly subsiding central part of the Pannonian Basin was filledwith sediment delivered by the Dunube and Tisza rivers, flowing tothe southeast and southwest, respectively. The Danube shiftedwestwards to its present course, probably during the LatePleistocene. The present-day course of the Tisza along the northernmargin of the Pannonian Basin was initiated during the LatePleistocene, due to increased subsidence of the Bodrogkoz basinand the foreland of the North Hungarian mountains (Borsy, 1995;Timar et al., 2005). The Great Hungarian Plain, in the central part ofthe basin, is one of the largest alluvial plains in Europe, with amorphology that (in general) is exceptionally subdued. However,the Late Weichselian and Holocene fluvial and aeolian morphology

and fluvial morphology in the middle Tisza valley.

C. Kasse et al. / Proceedings of the Geologists’ Association 121 (2010) 180–194 183

of the middle Tisza valley is well preserved, as is shown by digitalelevation models (see Timar et al., 2005). The elevation of thelandscape in the study area, near Tiszafured, is around 90 m abovesea level (Fig. 2). Local altitudinal differences on the flood plain areup to 5 m, related to point-bar and swale morphology and terracescarps. Local relict dunes are up to 10 m high.

3. Methods

In the middle Tisza valley, near Tiszafured, channel-planformmorphologies and several generations of river systems have beendistinguished, based on satellite images and topographical maps(1:100.000 and 1:25.000) (Fig. 2). Coring transects were made inan east–west direction across the area, in order to study thelithology and alluvial architecture of the different palaeofloodplainsurfaces (Figs. 3 and 4). Detailed coring across abandoned channelswas undertaken (Fig. 5) to study the geometry (width and depth)of the channel fills and to find the deepest locations for samplingfor pollen analysis and radiocarbon dating, in order to establish themoment of channel abandonment or avulsion. The so-calledEdelman auger was used for unsaturated sediments above theground-water table. A semi-closed gouge and a suction-tube corerwere used in water-saturated loamy sediments and saturatedsands, respectively.

Pollen samples, predominantly from clay and silt beds, wereprepared according to the method described by Faegri and Iversen(1975). The material was sieved through a 7–8 mm nylon mesh andclastic material was removed using a sodium polytungstate heavy

Fig. 3. Lithostratigraphic cross-section at Csero, especially illustrating the alluvial archit

Fig. 4. Lithostratigraphic cross-section at Kiraly, especially illustrating the alluvial archite

legend see Fig. 3.

liquid separation. Samples were embedded in glycerine jelly andsealed with paraffin wax. Pollen sum was generally between 100and 200 grains. Pollen counts are presented as regional diagramsand diagrams of selected taxa. The interpretation of the pollendiagrams and vegetation development is based, in particular, onWillis et al. (1995, 2000).

Plant remains for radiocarbon dating were retrieved from theclastic fine-grained sediments. Large silt samples were sieved over a150 mm mesh to retrieve the organic detritus from the sediment.This consisted mostly of indeterminate plant fragments, which havebeen dated by AMS because of the small volume. From two samples,rich in reworked organic detritus, it was possible to select enoughterrestrial macrofossil remains for AMS dating, while the remainderof the organic material was dated by conventional radiocarbondating (Tornqvist et al., 1992). Two dates were obtained fromfreshwater shells. The dating results are presented as uncalibratedradiocarbon years BP in Table 1 and in the cross-sections.

4. Results

4.1. Morphological analysis

The subsiding Great Hungarian Plain has an exceptionallysubdued morphology and can be regarded as a large alluvial plain,occupied by the Rivers Tisza, Sajo, Koros and Mures (Fig. 1).Terrace staircases related to tectonic uplift and climate-relatedchanges in sediment and water supply are therefore missing.However, (palaeo)floodplain levels can be distinguished based on

ecture of floodplain levels C and D. Numbers indicate units. For location see Fig. 2.

cture of floodplain levels B and C. Numbers indicate units. For location see Fig. 2. For

Fig. 5. Cross-sections through the Berekfurdo palaeomeander (up to 600 m wide and 14 m deep). Note the discrepancy in the radiocarbon ages of the meander fill (c. 29 ka BP)

and cut-bank sediments (c. 22–25 ka), indicating reworking of old organic material. Numbers indicate units. For location see Fig. 2. For legend see Fig. 3.

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morphological characteristics (Fig. 2: levels A–D). Levels A, B andC are not river terraces in the sense of fossil floodplains separatedby terrace scarps and related to incision events. Before riverchannel shortening and embankment in the 19th century, major

parts of the area were inundated during high floods. Furthermore,the levels A, B and C have approximately the same elevation andterrace scarps are missing. Only level D is clearly lower andseparated from the older levels by a distinct scarp (Fig. 2).

Table 1Uncalibrated radiocarbon dating of the Late Pleniglacial to Holocene lithostratigraphic units; for location of the dates see Figs. 3–5.

Unit Field code, depth in m Dated material GrN/A-number 14C-date BP Location in figure

5a Kiraly 16: 11.50–11.75 Bulk fine detritus A-16089 5305�40 Fig. 4

5a Csero 6: 5.90 Selected macro remains A-16938 4760�50 Fig. 3

5a Csero 6: 5.90 Bulk coarse woody detritus A-16091 9190�50 Fig. 3

4c Tiszaors B6: 8.40 Selected macro remains A-16939 19,690�150 Fig. 3

4c Tiszaors B6: 8.40 Bulk coarse woody detritus N-25408 19,900�200 Fig. 3

4c Kunmadaras C4: 10.75–10.90 Bulk fine detritus A-16099 16,250�80 Fig. 4

4c Kunmadaras C2: 11.70–12.15 Bulk coarse woody detritus A-16098 19,260�100 Fig. 4

4c Berekfurdo C4: 8.70–8.90 Bulk fine detritus A-16094 28,990�250 Fig. 5

4c Berekfurdo C4: 13.20–13.80 Bulk fine detritus A-16095 28,640�240 Fig. 5

4a Tiszaors B2: 2.75–2.80 Selected macro remains A-16941 12,490�70 Fig. 3

4a Tiszaors B2: 2.75–2.80 Bulk woody detritus N-25407 13,750�200 Fig. 3

4a Tiszaors 15: 6.35–6.50 Bulk coarse woody detritus N-25406 17,870�220 Fig. 3

2a Kiraly 13: 2.65 Snails A-16117 13,560�60 Fig. 4

2 Kiraly 52: 7.50–8.50 Bulk fine detritus A-16090 15,260�70 Fig. 4

2a Csero 20: 6.10–6.18 Bulk fine detritus A-16093 18,010�90 Fig. 3

2 Berek B2: 3.70–4.20 Snails/shells A-15939 21,970�100 Fig. 5

1 Kiraly 5: 10.61–10.75 Bulk fine detritus A-16088 22,200�120 Fig. 4

1 Tiszaors 5: 7.30–7.43 Humic clay, alkali extract A-15875 19,450�120 Fig. 3

1 Tiszaors 5: 7.30–7.43 Humic clay, residue A-15874 24,070�180 Fig. 3

1 Berekfurdo B1: 8.35 Humic clay, alkali extract A-15872 25,830�200 Fig. 5

1 Berekfurdo B1: 8.35 Humic clay, residue A-15871 24,710�170 Fig. 5

C. Kasse et al. / Proceedings of the Geologists’ Association 121 (2010) 180–194 185

The oldest level, A, is located in the east of the area. Itselevation is between 90 m above sea-level (a.s.l.) in the northand 87 m in the south; surface morphology is weakly expressed,probably due to long-lasting vertical accretion that obscures theoriginal fluvial morphology. Remnants of sinuous channel scarsand point-bars with differing orientations suggest formation bya meandering river.

Level B has a well-expressed morphology of c. 5 m, with almoststraight to slightly sinuous fine sandy bars and loamy swales. Theheight is c. 92 m a.s.l. in the north to 89 m in the south, whichpoints to aggradation with respect to the previous system,represented by level A. The morphology suggests a straight tolow-sinuosity river system, as can be found in low-sinuositymeandering or braided river environments. In the next paragraphthe sedimentary environments are interpreted in more detail,based on sediment characteristics. Locally, the morphology of levelB is characterized by aeolian forms (Fig. 2). There are small dunefields, particularly south of Tiszafured and southeast of Tiszaderzs.The dune relief, occasionally up to 10 m high, is undulating withhummocks and depressions without clear orientation. Southeast ofTiszaderzs, some parabolic forms are present, based on thegeomorphic study of 1:25.000 topographic maps.

Level C is typified by large meander scars that have been formedby three successive meander neck cut-offs (Fig. 2). The oldestpalaeochannel is up to 600 m wide and locally more than 14 m deep,and is filled with fine sandy to clayey silts. The younger ones are 500–400 m wide and shallower. The morphology is characterized bywell-developed point-bars with ridge and swale topography, andthe sedimentary succession is clearly fining upwards. The oldestpoint-bars, at Berekfurdo, are 90 m a.s.l., whereas the successivelyyounger ones at Kunmadaras and Tiszaors are 89 and 88 m, pointingto gradual incision during meander formation. The cross-cuttingrelationship of the level C channels indicates that floodplain level C isyounger than B. However, it cannot be excluded that the oldestmeander near Berekfurdo is as old as adjacent parts of level B, since itis unlikely that point-bars were formed only in the Berekfurdomeander without any continuation in the upstream and down-stream direction. The upper vertical-accretion deposits of floodplainlevels A, B and C are possibly the same age because the levels couldhave been flooded at the same time.

The youngest level, D, is separated from levels B and C by aterrace scarp of up to 4 m, pointing to a phase of incisionbetween levels C and D (Fig. 2). Most of the local villages are

located on this scarp so as to be protected from flooding. Level Drepresents the flood plain of the Tisza prior to embankment (88–86 m a.s.l.) and its morphology shows small-scale abandonedmeanders and point-bars. The smaller channel parametersreflect the lower bank-full discharges in comparison with levelC. The sedimentary succession is around 10 m thick and finesupward from fine sands to clayey silts.

4.2. Lithology and sedimentary environments

Two large-scale east–west cross-sections (Csero and Kiraly)have been made, more or less perpendicular to the morphologicalunits, in order to study the lithology and alluvial architecture ofthe different palaeofloodplain levels (Figs. 2–4). Interpretationsof the sedimentary environment have been made by integration ofthe sedimentary data and the planform morphology, as describedin the previous paragraph.

The profiles cross level B with straight ridges in the east andsouth (Fig. 2). In the central part they cross the well-developedridges and swales of the large palaeomeanders of Tiszaors andKunmadaras (level C). In the west the cross-sections are locatedin the subrecent floodplain (level D) with smaller-scale ridgeand swale morphology. The abandoned meander scar in thewesternmost part of the Csero cross-section is the result of man-induced meander cut-off in the middle of the 19th century.Detailed transects have been reconstructed across the aban-doned meander channel near Berekfurdo, in order to study itsinfill, geometry and age (Fig. 5). The results from detailedtransects near Kunmadaras and Tiszaors have been incorporatedin the Csero and Kiraly cross-sections (Figs. 3 and 4).

At the base of the cross-sections, at a depth of c. 8 m below thesurface, a clay layer has been found (unit 1). The clay is massive,crumbly and locally humic and non-calcareous. It probablyrepresents the top of a fluvial unit, present in the subsurface ofthe area. The fluvial style of the corresponding river system isuncertain but the fine-grained texture and soil characteristicspoint to surface exposure associated with soil formation on afloodplain or in a floodbasin environment.

Unit 2 in floodplain level B has a thickness of circa 8 m. Thesediment consists of fine-grained, well-sorted sand (unit 2a)changing upward to silt and clay (unit 2b). The ridges are in generalmore sandy and have a thinner silt layer at the top, while theswales often have a thick upper unit of clay and silt. The generally

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fining-upward trend can be interpreted as a result of lateralchannel migration and therefore indicates a fluvial origin. Similarstraight and parallel ridges, northeast of the study area, have beeninterpreted previously as aeolian linear dunes (Borsy, 1991).However, the morphology of area B reveals that the ridges havedifferent orientations, while linear aeolian dunes normally haveparallel orientations and aeolian dune sediments mostly consist ofhomogeneous sand. Near Tiszaderzs the ridges have a northeast–southwest orientation, while in the northeastern part of the studyarea the ridges are northwest–southeast oriented (Fig. 2). Inaddition, the ridges show cross-cutting relationships associatedwith channel migration and bar formation. The fining-upwardsequence in the upper part indicates lateral channel migration,which is characteristic of both meandering and braided systems(Bridge, 1985). The fining-upward trend in unit 2 is locallyinterrupted by grain-size changes or smaller fining-upward cycles.The absence in area B of deep channel scars, formed by chute or neckcut-off, and the straight to low-sinuous morphology of the ridgesand swales suggest that unit 2 was formed by a braided river system.

The transition from unit 1 to unit 2 is locally gradational andshows a coarsening-upward tendency, possibly related to pro-gradation of a fluvial system into lower lying areas as a result ofchannel-belt avulsion (Bristow et al., 1999). According to Borsy(1990) and Gabris and Nador (2007), this fluvial system of level Brepresents the southernmost extension of alluvial fan complexes(the Sajo fan) originating from the mountain ranges to the north.However, according to Blair and McPherson (1994) alluvial fanshave a high gradient and are formed by unconfined flow of sheetfloods and debris flows, whereas unit 2 (level B) was formed bylower-gradient fluvial channel and floodplain systems.

Locally, small dune fields occur on floodplain level B andobscure the underlying fluvial morphology (Fig. 2). The aeoliansediments (unit 3) have been distinguished from the underlyingfluvial sediments by their homogeneous fine-sand texture, lackinggrain-size trends, and their lower silt and mica content (Figs. 3 and4). The aeolian dunes only occur in area B and they have not beenfound on palaeomeander level C, which indicates that they areolder than level C. The dunes overlie the fine-grained top of unit 2and deep deflation hollows are missing. This indicates that thedunes probably formed more or less concomitantly with theformation of braided level B. Deflation occurred from the active,barren braid plain, and aeolian sand accumulated on inactive partsof this plain. Local parabolic dune forms on the dune fields (notvisible in Fig. 2) indicate winds from the WNW during theirformation. This is in accordance with dominant northwesterly tonortherly wind directions during dune formation in the Nyirsegregion in northeastern Hungary during the Late Pleniglacial and onthe Danube–Tisza interfluve during the Lateglacial (Borsy, 1991;Ujhazy et al., 2003).

Unit 4 has been found in floodplain level C, dominated by thelarge meander scars and ridge and swale morphology that occursnear Berekfurdo, Kunmadaras and Tiszaors. Unit 4 has beendivided in three subunits (Figs. 3–5). Unit 4a generally consists offine to medium well-sorted sands with a clear fining-upwardsequence and is interpreted as sandy point-bar sediments (Allen,1965). The thickness is mostly unknown, but locally, nearTiszaors, the base of unit 4a was encountered at c. 8–10 m belowthe surface (Fig. 3). In this case unit 4 overlies unit 1 with anerosive contact, at which coarse sand and fine gravel has beenfound, interpreted as channel bed or lag deposits. The fining-upward sequence reflects lateral migration of the meanderingchannel and its thickness is an indication of a bank-full channeldepth of probably more than 10 m.

Fine-grained unit 4b overlies unit 4a with a gradationalboundary, which implies a genetic relationship between the twosubunits. It consists mostly of massive sandy to clayey silts, with

brown and grey (gleying) colours related to its position above theground-water table. It forms a continuous cover of c. 2 m overlyingboth point-bars and swales (Figs. 3–5). Unit 4b is interpreted as thefine-grained upper part of the meander point-bar sequence thatformed by lateral channel migration. In addition, the homogeneouscharacter implies that unit 4b was formed by vertical accretionduring floodplain inundation.

Unit 4c has been found in the abandoned meanders of level C. Itmostly consists of silt and clay, but locally fine-sand beds occur.The upper part is massive and oxidized and resembles unit 4b.Below the ground-water table, unit 4c has a distinct horizontallamination of silt or fine sand and clay, a soft consistency and a greycolour. Organic material is occasionally present as dispersed,probably reworked organic detritus. Peat and organic lake deposits(gyttja) have not been found. A maximum thickness of c. 14 m hasbeen found at Kunmadaras and Berekfurdo (Figs. 4 and 5). Theinfill of the younger meander scar at Tiszaors is 6–10 m thick.These values indicate a decrease in channel depth for thesuccessive meanders of Berekfurdo, Kunmadaras and Tiszaors. Incombination with a decrease in channel width of the scars (Fig. 2),this points to a decrease in bank-full discharge levels during theperiod of large meander formation.

Unit 4c represents the fine-grained fill of the abandonedmeanders. Lateral migration of the meanders resulted in meanderneck cut-off (Fig. 2) (Allen, 1965). The clastic deposition and well-developed lamination indicate that the abandoned channels werefrequently inundated by floods rich in suspended sediment. Thefine-sand intervals represent zones of slightly higher currentvelocity during inundation of the abandoned meanders. In thepresent-day morphology, small-scale secondary meanderingchannels are present within the large abandoned meanders. Theclastic sediments, sedimentary lamination and absence of soils ororganic material in the meander fills points to a high sedimenta-tion rate in the abandoned channels. Therefore, it is likely that themeander fills represent only a short period (see pollen recordbelow).

Unit 5 is present in floodplain level D. It is subdivided into asand, unit 5a, separated by a gradual boundary from fine-grainedunit 5b (Figs. 3 and 4). Unit 5a consists of fine-grained, well-sortedsands, generally revealing a fining-upward sequence, which areinterpreted as point-bar deposits formed by laterally migratingmeandering channels. Near Abadszalok, at c. 14 m depth, the baseof unit 5a contains medium- to coarse-grained sand, representingthe lower point-bar or channel-base deposits (Fig. 4). Organicdetritus is common, but mostly reworked. The thickness of units 5aand 5b, generally exceeding 8 m, is a minimum estimate of channeldepth during bank-full discharge.

Unit 5b is generally fine-grained sandy silt to silty clay. It formsa continuous c. 5 m thick cover. It is interpreted as vertical-accretion deposits formed by inundations with suspension-richflood water. The large thickness may reflect a long period ofvertical accretion, or alternatively high sedimentation rates in thelate Holocene, due to land clearance and agriculture leading to anincreased sediment supply.

5. Chronology

The chronology of the lithostratigraphic units has beenestablished by radiocarbon dating (Table 1). In situ organicmaterial (peat and gyttja) is generally absent in the units. Residualchannels have been filled with clastic material. Only the humicnon-calcareous clay at the top of unit 1, interpreted as a soil,contains autochthonous organic matter. The dominance of clasticdeposition in this area can be explained by the small elevationdifferences between the different floodplain levels A, B and C.During high floods, abandoned floodplain levels were inundated

C. Kasse et al. / Proceedings of the Geologists’ Association 121 (2010) 180–194 187

with suspension-rich flood waters. In addition, the Tisza carrieshigh amounts of suspended material, probably derived from loessdeposits that cover major parts of the catchment (Frechen et al.,1997). The organic material within the clastic sediments is mostlyreworked and the radiocarbon dates should be regarded asmaximum ages.

The humic clay with soil characteristics at the base of the cross-sections (Figs. 3 and 5: unit 1) revealed dates of 24,070 and 24,710BP for the residue and 19,450 and 25,830 BP for the alkali extract.These dates are probably reliable because the organic material is ofautochthonous origin, resulting from soil formation. The date of22,200 BP in the Kiraly cross-section (Fig. 4) is in goodcorrespondence with the other dates from unit 1. The agesindicate that subsurface unit 1 was formed during the WeichselianLate Pleniglacial and that the overlying lithostratigraphic units 2–5and floodplain levels A–D must therefore be younger.

Palaeofloodplain level A was dated south of Berekfurdo (Fig. 5)at 21,970 BP, indicating a Late Pleniglacial age. The dates from unit2 (morphological level B) range from 18,010 BP (Fig. 3) to 15,260–13,560 BP (Fig. 4), placing unit 2 in the Late Pleniglacial as well. Thelatter date was based on well-preserved freshwater snails, possiblysuffering from secondary calcium carbonate precipitation.

The dates derived from the sandy point-bar deposits of unit 4a(large palaeomeander level C) range between 17,870 and 13,750/12,490 BP (paired ages consisting of bulk dates and AMS dates fromselected macro remains, respectively, are shown in that order,separated as here) (Fig. 3) indicating a Late Pleniglacial or early LateGlacial age. Considering the comparable ages for unit 2 and thereworked character of the organic material, the dates from unit 4aare regarded as possibly too old. The dates from the fine-grainedmeander fills (unit 4c) range between 28,640 and 28,990 BP forthe Berekfurdo meander fill (Fig. 5c), 19,260 and 16,250 BP for theKunmadaras meander scar (Fig. 4) and 19,900/19,690 BP for theTiszaors meander (Fig. 3). However, these dates are probably too oldbecause the meander fill dates are older (instead of younger) thanthe related point-bar deposits and cut banks. The Berekfurdo agesare approximately 7000 years too old (compare in Fig. 5: 28,640 BPat the base of the meander fill versus 21,970 BP of the cut bank). TheKunmadaras dates seem to be several thousands years too old(compare Fig. 4: 19,260 and 16,250 BP for the meander fill versus15,260 and 13,560 BP for adjacent sediments). The Tiszaors ages arealso 7000 years too old (compare Fig. 3: 19,900/19,690 BP ka for themeander fill versus 13,750/12,490 for the related point-bardeposits). Comparable dates of 20–25 ka have been obtained fromlarge palaeomeander fills near Polgar, c. 50 km north of the studyarea (Timar et al., 2005). The probable cause for this ageoverestimation is that the organic material has been reworkedfrom the subsoil. The palaeomeanders have deeply scoured intosubsurface units, e.g. unit 1 with an age of c. 25 ka. Theinconsistencies in the radiocarbon dates from unit 4c hamper aprecise chronologic interpretation. The youngest dates from unit 4a,stratigraphically preceding unit 4c, suggest a final Late Pleniglacialor early Lateglacial age for the basal part of the meander fills.

Unit 5a and 5b (point-bar and vertical-accretion deposits offloodplain level D) revealed radiocarbon ages of 5305 and 9190/4760 BP. These indicate a Holocene age, which is confirmed by thepresence of deciduous forest vegetation. The large differencebetween 4760 BP for the AMS date (on selected macrofossilremains) and 9190 BP for the bulk date suggests that part of theorganic material has been reworked.

6. Biostratigraphy and vegetation development

Pollen samples have been taken from units 1 (oldest), 2, 4 and5 (youngest). The uppermost sediments, 3–5 m from the surface,are strongly oxidized and have therefore not been sampled for

pollen analysis. All pollen samples are from silt and clay depositsand therefore may contain reworked grains that were trans-ported in the river to the investigated site as suspended material.Pollen preparation followed the method set out by Faegri andIversen (1975). The more continuous sections are presented inpollen diagrams (Figs. 6–8). In addition, individual samples havebeen analysed from unit 1 and 2, not represented in pollendiagrams.

Unit 1 has been investigated in 12 pollen spectra. The pollencomposition is characterized by moderately high Pinus values (20–50%) and low amounts of Betula (<5%) and Picea (<5%). Juniperus isgenerally low or absent. Artemisia (5–20%) and Compositae (2–50%) are important elements in the upland herbs.

Deciduous forest species, which are typical of Holocene andpresent-day vegetation and climate conditions in Hungary (seeWillis et al., 2000, their Fig. 4), are generally missing in unit 1. Thisabsence therefore indicates glacial-period conditions. This is inagreement with the radiocarbon dates of 20–25 ka from the top ofunit 1. It shows that the reconstructed vegetation is part of the LatePleniglacial. The presence of heliophytic elements (Compositae,Chenopodiaceae) points to a steppe-type vegetation. It is uncertainwhether the coniferous pollen types represent long-distancetransport from more southerly refugia or production by in situpopulations. Willis et al. (2000) concluded, on the basis of charcoalinvestigations from soils and archeological sites, that an openconiferous forest was locally present in Hungary, probably atmicroenvironmentally favourable sites, between c. 32,500 and16,500 BP.

Unit 2 has been investigated in 14 pollen spectra. The pollenassemblage of unit 2 resembles that of unit 1, but Pinus values arehigher (c. 50–80%). Betula and Picea values are generally lower than5% and 10%, respectively. Juniper is present in low amounts (<5%).Artemisia values (<10%) and Compositae (�3%) are somewhatlower than in unit 1.

The pollen assemblage of unit 2 indicates cool climaticconditions. This is in accordance with the radiocarbon dates of13.5–18 ka obtained from unit 2, which indicate deposition duringthe Late Pleniglacial. The dominance of Pinus indicates the localpresence of pine forest in the study area. This is in good agreementwith previous palynological studies by Willis et al. (1995, 2000),who concluded that open coniferous forests existed in refugialareas in Hungary in the 16–10 ka interval.

Unit 4c (large meander fills) has been investigated in many pollenspectra. Discontinuous diagrams (32 spectra) have been recoveredfrom the Berekfurdo and Kunmadaras palaeomeanders (Fig. 2). Theyare generally characterized by Pinus (40–60%), Betula (�5%) andPicea (<5%), with a sporadic presence of thermophilous elements inthe upper part of the fills (Corylus, Quercus, Ulmus, Alnus). Artemisia

(5–10%) and Compositae (�3%) are important elements in theupland herbs. Two continuous pollen diagrams have been con-structed from the large palaeomeander 6 km NE of Tiszacsege,25 km northeast of Tiszafured, and near Tiszaors (Fig. 2).

The Tiszacsege diagram (Fig. 6) covers the interval from 400 to1450 cm below the surface of the meander scar. Despite theconsiderable sediment thickness the overall pollen composition isfairly constant, being dominated by Pinus and Betula, whichprobably indicates high sedimentation rates. This is supported bythe lithology of the meander fill being dominated by silt. Thefollowing local pollen assemblage zones (LPAZ) have beendistinguished.

Zone TSZ-1 (1450–1335 cm) is characterized by high Pinus

values (60%), an increase of Betula, Juniperus and Salix and thepresence of Artemisia and Compositae.Zone TSZ-2 (1335–1107 cm) shows a strong increase of Betula

to 25% and a decrease of Pinus to 35%. Juniperus, Salix and

Fig. 6. Pollen diagram (selected taxa) from the first generation of large meanders near Tiszacsege, showing Late Pleniglacial and Lateglacial vegetation development (local

coordinates x = 4505.775; y = 5286.800 topographic map 1:25.000 sheet Ujszentmargita L-34-7-C-c).

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especially Populus show an increase to 10%. The first appearanceof Typha is registered.Zone TSZ-3 (1107–1015 cm) shows high Pinus values (�60%)while Betula (�15%) and Salix are declining. Juniperus andPopulus show continuous curves. Selaginella temporarily dis-appears from the record whereas the thermophilous Typha

becomes more frequent.Zone TSZ-4 (1015–907 cm) shows a decline in Pinus and Salix

and a simultaneous increase in Artemisia and Selaginella.Chenopodiaceae form a continuous curve and the thermo-philous Typha disappears from the record.Zone TSZ-5 (907–677 cm) shows a decrease in Pinus andJuniperus, while Betula and espescially Salix show an increase.Deciduous forest elements (Alnus, Quercus, Ulmus) are spor-adically present.

Zone TSZ-6 (677–400 cm) shows fairly high Pinus (45%) andBetula (10%) values. Juniperus has disappeared and continuouscurves of Quercus and Ulmus occur for the first time.

The pollen zones can be interpreted as a typical Lateglacialvegetation succession. TSZ-1 shows a strong resemblance to thepollen composition of unit 2 and probably reflects the final LatePleniglacial with steppe-type vegetation conditions and localconiferous growth. TSZ-2 represents a Betula and Juniperus phaseand the spread of forest cover, probably related to the climaticamelioration (with increasing temperature and precipitation) atthe start of the Lateglacial (Willis et al., 1995; Hoek, 1997; Renssenand Isarin, 2001). TSZ-3 is interpreted as a Pinus phase, followingthe birch phase of TSZ-2, in which boreal forest continued to spreadin the area. Willis et al. (2000), in their reconstruction of the

Fig. 7. Pollen diagram from Tiszaors (selected taxa), from the younger generation of large meanders on floodplain level C, showing the Lateglacial and early Holocene

vegetation development. For location see Fig. 3.

Fig. 8. Pollen diagram from Kiraly (selected taxa), from floodplain deposits on level D, showing the Middle Holocene deciduous forest vegetation. For location see Fig. 4.

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Table 2Summary of the morphological, sedimentological and ecological characteristics of the subsurface units in the middle Tisza valley.

Palaeofloodplain level Level A Level B Level C Level D

Elevation 87–90 m 89–92 m, dunes up to 100 m 88–90 m 86–88 m

Surface morphology Subdued point-bars

and swales

Straight to low-sinuosity

ridges and swales

Large meander scars,

clear point-bars en swales

Small meander scars,

clear point-bars and swales

Lithology and thickness Unit 1: clay/loam, >4 m Unit 2: fining upward of fine

sand to loam; base coarsening

up; 5–9 m, unit 3:

homogeneous fine sand; 4 m

Unit 4: fining upward of

medium and fine sand

to loam; 10–>14 m

Unit 5: fining upward of

medium and fine sand to

loam; 10–14 m

Radiocarbon age 22–25 ka 18–15 ka Unit 4a: 18–13 ka, unit

4b: 16–29 ka

5–9 ka

Vegetation Steppe Open coniferous forest Coniferous forest, some deciduous

forest elements

Deciduous forest

Sedimentary environment Floodplain/flood basin Braided channels/alluvial fan Large meandering channel Small meandering channel

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vegetational history of Hungary, dated these Betula and Pinus

phases between approximately 12 and 14 ka cal BP. The sporadicpresence of Quercus, Ulmus and Alnus can be interpreted as pocketsof deciduous trees (temperate refugial populations) within theconiferous forest (Willis et al., 1995). For zone TSZ-4 the combinedevidence points to a short-lived cooling and or drying phase, withthe spread of steppe elements. This zone is possibly related to theYounger Dryas. In zone TSZ-5 Pinus and Juniperus are decreasingwhile Salix is spreading again. In combination with the occurrenceof deciduous trees, this is interpreted as the transition from theLateglacial to the Holocene. In TSZ-6 the light-demanding Juniperus

has disappeared and deciduous forest shows a further expansion.According to Willis et al. (1995, 2000), the disappearance ofJuniperus and the shift from coniferous to deciduous woodlandoccurred around the Lateglacial to Holocene boundary, at c. 11 kacal BP.

The Tiszaors diagram (Fig. 7) has been taken from the youngestlarge palaeomeander on floodplain level C (see Figs. 2 and 3 forgeomorphological position and relative age). The diagram coversthe interval from 275 to 1050 cm below the surface of the meanderscar. The pollen composition is fairly constant, which againprobably indicates high sedimentation rates. The diagram shows agradual trend of decreasing arboreal pollen. The following localpollen assemblage zones (LPAZ) have been distinguished.

Zone ORS-1 (1050–875 cm) shows high Pinus (c. 45%) and Salix

values. Juniperus is present in very low amounts.Zone ORS-2 (875–410 cm) reveals declining Pinus and Salix

values. Artemisia is rising and reaches its maximum in the upperpart. Deciduous forest species (Alnus, Fraxinus, Quercus, Ulmus)show discontinuous curves.Zone ORS-3 (410–275 cm) shows Pinus values of 30%. Salix islow and Juniperus is absent. Quercus and Ulmus reach highervalues up to 5%. Artemisia remains relatively high.

The Tiszaors pollen diagram shows a good correspondence tothe upper part of that from Tiszacsege, with gradually decreasingarboreal pollen values. The meander fill at Tiszaors and the timingof channel abandonment there are thus later than in Tiszacsege.Zone ORS-1 shows the final presence of Juniperus and is probablyequivalent to zone TSZ-5 at Tiszacsege, marking the finalLateglacial to early Holocene period (Willis et al., 1995, 2000).The transition from ORS-1 to ORS-2 shows the first establishment(zone ORS-2), followed by further expansion (zone ORS-3) ofdeciduous woodland, indicating the early Holocene. The Artemisia

maximum in zone 2 and 3 has similarly been reported by Willis etal. (1995) from the final Lateglacial.

Unit 5 (floodplain level D) has been investigated in 14 pollenspectra. Five individual spectra have been recovered from unit 5 inthe Csero cross-section (Fig. 2). One continuous pollen diagram

has been recovered from the Kiraly cross-section (Fig. 8). The corewas taken at some distance from the present-day Tisza channel,along the easternmost side of the floodplain, close to the terracescarp, and may therefore represent a relatively old segment of theHolocene floodplain (see Figs. 2 and 4 for location). Although thesection was over 8 m deep, the pollen composition is dominated bydeciduous trees and is remarkably constant, which again probablyindicates the high sedimentation rates of the silt-dominatedfloodplain deposits.

The Kiraly 16 pollen diagram is characterized by low Pinus andBetula values (Fig. 8). Corylus, Alnus, Quercus, Ulmus, Tilia andCarpinus are important elements of the deciduous forest vegeta-tion. Fagus is present at low values. The dramatic change in pollencomposition from predominantly coniferous forest in unit 4(Figs. 6 and 7) to mixed deciduous woodland in unit 5 (Fig. 8)marks the transition from the Lateglacial to the Holocene.According to Willis et al. (1995), this vegetation transition resultedfrom competition driven by climate change. The absence at Kiraly16 of high Tilia values, which are characteristic of the earlyHolocene, and the presence of Carpinus and Fagus indicate that thediagram probably represents the Middle Holocene (Willis et al.,1995, 2000).

The individual pollen spectra from the Csero cross-section havethe same pollen composition. However, two samples from thewesternmost part of the floodplain have higher Alnus values and astrong presence of Fagus (c. 10%). The geomorphic position, close tothe present-day river and to the 19th century course of the Tisza(Fig. 2), suggests a Late Holocene age.

7. Discussion on climatic and tectonic forcing of changes influvial style

The geomorphical, sedimentological and palynological resultshave been summarized in Table 2. The biostratigraphy andradiocarbon dates enable us to reconstruct the changes in thefluvial system over time and to compare valley development in theTisza with other fluvial systems in Europe (Fig. 9). The timing ofthe fluvial system changes and the avulsion of the Tisza to themiddle Tisza valley is still somewhat uncertain, due to problematicradiocarbon dating results.

The general geomorphological evolution of the middle Tiszavalley reveals strong similarities with other river systems in NWand Central Europe (Vandenberghe et al., 1987; Antoine, 1997(Somme basin); Kasse et al., 1995 (Maas), 2005 (Niers-Rhine);Lipps and Caspers, 1990 (Weser); Rose, 1995 (British catchments);Bohncke et al., 1995 (Warta); Van Huissteden and Kasse, 2001;Pastre et al., 2003 (Seine basin)). Pleniglacial braided river systemschanged to large-scale meandering systems during the glacial–lateglacial transition and smaller-scale meandering systems persistedduring the Holocene (Fig. 9).

Fig. 9. Fluvial system changes and vegetation development in the Netherlands (Maas river: Kasse et al., 1995; Hoek, 1997; Huisink, 1997; Van Huissteden and Kasse, 2001),

Poland (Warta river: Vandenberghe et al., 1994; Bohncke et al., 1995) and Hungary (Tisza: this study; Willis et al., 1995, 2000) during the last glacial to interglacial transition.

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7.1. Pleniglacial

In the Tisza valley the cross-sections (Figs. 3 and 4) andradiocarbon dates of c. 20–25 ka BP (Table 1) show thatsubsurface unit 1, with soil formation at its top, was formed inthe early Late Pleniglacial (Table 2). This buried fluvial systemmay have been formed by southward-flowing precursors of thepresent-day Tisza, like the Bodrog or Sajo (Gabris and Nador,2007), since it is argued below that the Tisza river has occupied theMiddle Tisza valley since the end of the Pleniglacial. River stylemay have been meandering, as suggested by the weakly expressedpoint-bar surface morphology east of the study area in theHortobagy region, where unit 1 is not fully covered by unit 2. Thevegetation during the deposition of unit 1 was characterized bysteppe, while open coniferous forest was possibly present locally(see biostratigraphy section), which is in agreement with previousresults by Willis et al. (2000). Fluvial and vegetation patterns inHungary were clearly different from those in NW Europe, wheremost river systems changed from meandering or anastomosing tobraided at the Middle to Late Pleniglacial transition. This change iscommonly attributed to climatic deterioration, a decline orabsence of vegetation cover and increasing peak discharges andsediment supply at the start of the Late Pleniglacial (VanHuissteden and Vandenberghe, 1988; Mol et al., 2000). Accordingto Van den Berg (1995) a transition from meandering to braidingcan be caused by an increase in the stream power (higher gradient,higher discharge) of the river. Apparently, the stream power in theMiddle Tisza region did not increase sufficiently to cross thethreshold towards braiding at the start of the Late Pleniglacial. Itmay be that the climatic and related discharge and vegetationchanges were smaller in the Great Hungarian Plain in comparisonwith NW Europe.

Subsurface unit 1 has been covered by a braided river unit 2,previously interpreted as an alluvial fan of the Sajo (Gabris andNador, 2007; Table 2). According to the radiocarbon dates, rapidaggradation occurred between c. 18 and 14 ka (Fig. 2: level B).The palaeobotanical analysis of unit 2 indicates the localpresence of coniferous forests, possibly in refugial areas, in or

near the study region (cf. Willis et al., 1995, 2000). The fluvialsystem change from meandering (unit 1) to braided (unit 2) canbe attributed to climatic changes during the Late Pleniglacial.The second part of the Late Pleniglacial was cold and arid, aperiod of intense loess deposition in Hungary (Frechen et al.,1997), and probably resulted in an increased (coarser-grained)sediment supply from the mountain headwaters, promotingbraiding and aggradation (Van den Berg, 1995). Such a change inriver style from meandering to braided, followed by rapidaggradation, has frequently been reported form river systems inNW and Central Europe (Van Huissteden, 1990; Kasse et al.,1998; Huisink, 2000). Late Pleniglacial river systems in theNetherlands and Poland are clearly of the braided type, withstrongly bifurcating channels and braid bars (Bohncke et al.,1995; Fig. 9), and concurrent aeolian activity resulted in theformation of fluvio-aeolian deposits in the upper part of thePleniglacial successions (Kasse et al., 2007).

Although a climatic cause (colder climate with less vegetationand higher bank-full discharges) might have been responsible forthe regime change from meandering to braiding, it cannot beexcluded that the change in fluvial style was related to system-intrinsic processes. The braided system of unit 2 was attributed byGabris and Nador (2007) to the Sajo river flowing from thenorthwest into the Great Hungarian Plain, forming the Sajo alluvialfan. Southeastward progradation of the fan into the study area mayhave resulted in higher gradients and stream power, leading theriver system to adopt braiding patterns (Van den Berg, 1995).

Dunes were formed locally on inactive parts of braid-level B,probably by deflation from active parts of the braid plain (Fig. 2:level B; Figs. 3 and 4: unit 3). Such local dunes are generallydescribed as source-bordering dunes or river dunes. This LatePleniglacial aeolian activity and dune formation on floodplain levelB was earlier than the aeolian deposition in northeastern Hungarythat occurred predominantly in the Lateglacial (and Holocene)periods (Ujhazy et al., 2003). The aeolian deposition suggests atleast local bare soil conditions, which is in accordance withgenerally dry conditions and strong aeolian activity in Europeduring the Upper Pleniglacial (Kasse et al., 2007).

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7.2. Final Late Pleniglacial and Lateglacial

The problematic radiocarbon dating control hampers our abilityto pinpoint the onset of large meander formation (Fig. 2: level C).The dates from the meander fills seem to indicate meander activityprior to 20 ka (Tiszaors) or even 30 ka (Berekfurdo meander)(Table 2). Comparable dates of 20–25 ka have been obtained frommeander fills north of the study area (Timar et al., 2005). However,the radiocarbon dates from related point-bar sediments (unit 4a:�18–13 ka) and the biostratigraphy of the meander fills (mostlyLateglacial) have shown that the large meanders in the study wereabandoned at the end of the Pleniglacial and during the Lateglacial(Fig. 9). Therefore, the fluvial system change from braiding tomeandering can be placed in the final stage of the Late Pleniglacial.This change in channel planform from braided towards large-scalemeandering in the river systems of Central and NW Europe iscommonly related to climatic warming at the onset of theLateglacial (12.5 ka 14C BP/14.8 ka cal. BP) and the associatedincrease in vegetation cover, higher bank stability, decreasedsediment supply and more regular discharges (Kozarski, 1983;Lipps and Caspers, 1990; Bohncke et al., 1995; Antoine, 1997; VanHuissteden and Kasse, 2001; Kasse et al., 2005).

The dates from the study area suggest that the change to a large-scale meandering system occurred earlier in the Tisza valley than inNW and Central Europe, where a meandering system graduallydeveloped during the course of the Bølling-Allerød interstadial(Fig. 9). The different timing of the fluvial response may be related todifferences in vegetation development. In NW Europe the firstexpansion of boreal forests occurred during the Bølling-Allerødinterstadial, resulting from the gradual vegetation migration fromthe south over the previously barren Late Pleniglacial surface (Hoek,1997). In the Maas and Niers-Rhine valleys, it has been establishedthat the change in fluvial regime from braiding to meandering was agradual one, taking c. 500 years as a delayed fluvial response to therapid Lateglacial warming (Kasse et al., 1995, 2005). This gradualfluvial response can be explained by a gradual decrease in streampower, accompanied by increases in evapotranspiration anddecreases in discharge and sediment supply related to thevegetation development (Vandenberghe et al., 1987). The vegetationreconstruction from the Tisza study area has shown that coniferousforests already existed (in refugia) during the Late Pleniglacial andthat deciduous forest was already present in the area during theLateglacial (Willis et al., 1995, 2000; Figs. 6 and 7). Thus the fluvialresponse to the Lateglacial warming may have been more rapid herethan in NW Europe.

The large dimensions of the palaeomeanders of level C (up to600 m wide and over 14 m deep) indicate high flood discharges incomparison with the Holocene system (level D). Successivepalaeomeanders (Berekfurdo, Kunmadaras, Tiszaors) show adecrease in width and depth, indicating a decrease in bank-fulldischarge during the course of the Late Pleniglacial and Lateglacial.It is unknown whether this decrease in discharge had a directclimatic cause (less rainfall or snowmelt) or was related to theindirect effect of boreal (and deciduous) forest development andincreased interception and transpiration by the vegetation. Wardet al. (2008) have argued that changes in the forest cover have hada strong impact on flood frequency and discharge of the river Maasduring the Holocene. Therefore, we assume that the early forestcover development (Figs. 6 and 7) and associated increase inevapotranspiration in the catchment during the course of theLateglacial resulted in lower discharges and hence smallermeander channels.

Concurrent with the meander-channel changes, a gradualtendency towards floodplain incision occurred in the meanderingsystem. Each younger generation of palaeomeander point-bars(Fig. 2: level C, Berekfurdo, Kunmadaras, Tiszaors) has been

formed at a lower level (Table 2). The floodplain-wide incision bythe meandering system was probably caused by a decrease of thesediment-discharge ratio, because sediment supply from thecatchment slopes declined as a result of expanding vegetationcover. Similar early Lateglacial incision has been reconstructed forriver systems in NW and Central Europe (Vandenberghe andBohncke, 1985; Bohncke et al., 1995; Huisink, 1997). As analternative to a climate cause, the change from braiding (level B) tomeandering (level C) can perhaps be explained in terms oftectonically controlled avulsion by the Tisza into the study area(Timar et al., 2005). The palaeo-Tisza flowed in a northeast–southwest direction along the Ermellek depression east ofDebrecen in northeastern Hungary during the Late Pleniglacial(Fig. 1). Increased subsidence of the Bodrogkoz area in northernHungary resulted in dramatic avulsion by the Tisza and theformation of the Zahony bend (Fig. 1). The avulsion event and theformation of the present-day meander belt has been dated at circa14–18 ka (Timar et al., 2005; Nador et al., 2007). This timing of theTisza avulsion is largely in agreement with our radiocarbon andbiostratigraphic dates from level C, which indicate the onset of thelarge meander system at the end of the Late Pleniglacial. Thesimilarity in timing of the Late Pleniglacial–Lateglacial climaticchanges and the Tisza avulsion event makes it impossible todetermine a single cause for the fluvial system change.

7.3. Younger Dryas

The effects of the Younger Dryas Stadial have not been detectedin the fluvial evolution of the Tisza valley. Level C, attributed to theLate Pleniglacial and Lateglacial, only reveals large meander formsof decreasing size. Also to the north, in the region of Polgar, noseparate floodplain level with specific channel forms has beenrecognized (Gabris and Nador, 2007). This is comparable toprevious findings in the Warta valley in Poland, where meanderingpersisted during the Younger Dryas (Vandenberghe et al., 1994;Fig. 9). In the Netherlands and England, however, the YoungerDryas is reflected by an abrupt change to braided conditions inseveral river systems (Berendsen et al., 1995; Kasse, 1995; Rose,1995; Huisink, 1997). It is possible that the effects of the YoungerDryas Stadial are better expressed in the fluvial systems of NWEurope. According to climate modelling experiments and recon-structions, the Younger Dryas cooling, especially in winter, wasmore pronounced along the western margin of the Europeancontinent, where the effects of changes in Atlantic Ocean currentsand sea ice cover were more clearly felt, than on the continent(Renssen et al., 2002). Palynological investigations indeed showedthat the vegetation response to the Younger Dryas cooling wasstrong in the Netherlands and Great Britain (e.g. Hoek, 1997).Forest cover, especially pine, was strongly reduced. In Poland, onthe other hand, coniferous forest persisted (Bohncke et al., 1995;Fig. 9). The pollen diagrams from the large meander fills (Figs. 6and 7) also show little vegetation change during the Lateglacial,and the Younger Dryas biozone is hard to decipher. Coniferousforests persisted and only the small increase of Selaginella has beeninterpreted as an indication of the Younger Dryas cooling (Fig. 6:pollenzone TSZ-4). As forest cover is an important factordetermining interception and evapotranspiration, it is argued thata strong reduction in forest cover and vegetation density in NWEurope resulted in decreased evapotranspiration and therefore anincrease in discharge and flood frequency (cf. Ward et al., 2008). Incombination with a return to a nival discharge regime during theYounger Dryas Stadial, this resulted in higher stream power andthe threshold to braiding was crossed in several rivers (Van denBerg, 1995). In the Tisza valley stream power possibly evendeclined during the course of the Late Glacial, as successivemeander channels are smaller.

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In addition to climatic and vegetation causes that can explainthe different fluvial responses to the Younger Dryas cooling, thereare also differences in floodplain gradient. While the Maas is ahigher gradient (25–50 cm/km) sand and gravel river, the Tisza is alow-gradient (15 cm/km), sand and silt-dominated river, withlower stream power, and therefore the threshold towards braidingwas not easily crossed.

7.4. Holocene

A well-developed terrace scarp of up to 4 m was found betweenfloodplain levels B/C and D (Fig. 2; Table 2). Pollen analysis of thefloodplain sediments of level D points to deciduous forestvegetation (Fig. 8). In combination with the radiocarbon dates(Figs. 3 and 4), it confirms the Holocene age of level D. Thedramatic shift of coniferous forest (level C) to deciduous forest(level D) has been associated with the Lateglacial to postglacialtransition (Willis et al., 1995). Therefore, this incision phase andthe start of scarp formation probably occurred at the Weichselian–Holocene boundary (Fig. 9).

Similar short-lived fluvial instability and incision at the start ofthe Holocene has been reported for the Maas, Wartha, Niers-Rhineand Vistula catchments (Vandenberghe et al., 1994; Bohncke et al.,1995; Kasse et al., 1995, 2005; Huisink, 1997; Starkel, 2002). Theincision is probably related to the rapid climate warming at theonset of the Holocene. The increase in vegetation cover and fullexpansion of the deciduous forest strongly reduced the sedimentsupply from the catchment slopes and therefore the rivers erodedtheir channel beds. During the course of the Holocene, lateralmigration of the meandering Tisza led to floodplain widening andthe scarp obtained its present form. The Holocene meanderdimensions, based on the meander scars and oxbow lakes on thefloodplain (Fig. 2), are smaller than during the formation of largemeander level C. The higher temperature and full forest cover ofthe early Holocene resulted in higher evapotranspiration, and flooddischarge and frequency probably decreased. As a result theHolocene meandering channels were smaller, a change that hasalso been reported in Polish catchments (Starkel, 2002; Fig. 9).Intra-Holocene fluvial changes have not been observed, althoughanthropogenic deforestation and development of agricultural landsince the mid-Holocene will probably have had an effect on flooddischarge and sediment supply (Ward et al., 2008).

8. Conclusions

The Late Pleniglacial to Holocene fluvial evolution of the vastTisza alluvial plain in Hungary (Fig. 1) is the result of climatic andtectonic forcing (Fig. 9). Climate and climate-related vegetationand discharge changes resulted in channel-pattern changes andphases of incision.

Four floodplain levels have been identified in the middleTisza reach on the basis of their geomorphologic and sedimen-tary characteristics (Fig. 2; Table 2). Their elevation is between85 and 92 m above sea level. During the Late Pleniglacial,aggradation occurred because of a high sediment supply, first bymeandering (level A, unit 1) and later by braided precursorsystems (level B, unit 2) of the Tisza and Sajo rivers (Figs. 3 and4). Locally, parabolic dunes were formed by northwesterlywinds, concurrent with the formation of level B (unit 3).Vegetation was first dominated by herbs (steppe) in unit 1 andlater by open coniferous forest in unit 2.

In the final phase of the Late Pleniglacial or Late Pleniglacial–Lateglacial transition a fluvial change took place to a largemeandering system (level C, unit 4) (Fig. 9). Meander scars upto 600 m wide and 14 m deep were formed (Fig. 5). Point-barsurface elevation decreased from 90 to 88 m during formation of

level C. Climate warming and resultant decreased snow-meltdischarge, early vegetation development of (coniferous) forests(Figs. 6 and 7) and an increase in evapotranspiration resulted indecreased stream power and a change to a meandering system. Thegradual floodplain incision is the result of a decrease in sedimentsupply related to the vegetational development. The fluvial changeto large-scale meandering in Hungary seems to have occurredearlier than similar changes in NW and Central Europe, probablybecause of the earlier development of forest cover in the former(Fig. 9), but the radiocarban age control remains poor due tofluvial reworking of organic material. Tectonically inducedavulsion of the Tisza river to its present-day meander belt alsooccurred at this time.

Younger Dryas cooling is not reflected clearly in the vegetationrecord (Figs. 6 and 7); there was a persistence of coniferous forestwith some deciduous elements. Meandering continued and thebraiding threshold was not crossed because changes in dischargewere limited due to the continental climatic setting of Hungary(Fig. 9).

The glacial–postglacial transition is reflected by a change fromboreal to deciduous forest cover in the region (Fig. 8). The Tiszaresponded to this vegetation change and the resultant decrease insediment supply by incision of its bed and a scarp up to 4 m highwas formed (Fig. 9). The channel pattern changed from large- tosmaller-scale meanders (level D, unit 5), indicating a decrease inbank-full discharge, possibly related to decreased snow-meltrunoff. Intra-Holocene fluvial changes related to climate variabilityor human impact have not been found.

The similarities between the middle Tisza and NW and CentralEuropean rivers show the importance of climatic forcing on fluvialsystem change (Fig. 9). However, the earlier vegetationaldevelopment (thanks to nearby refugia), local tectonic subsidenceand a low gradient have in some cases resulted in such changesoccurring at somewhat different times in the Tisza.

Acknowledgements

The students of the Climate Change and Landscape DynamicsDepartment are thanked for the collection of the data duringfieldwork; Saskia Gietema and Martin Konert are thanked for thepollen analysis.

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