Surprisingly small increase of the sedimentation rate in the floodplain of Morava River in the Strážnice area, Czech Republic, in the last 1300 years

Catena 86 (2011) 192–207

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Surprisingly small increase of the sedimentation rate in the floodplain of Morava River in the Strážnice area, Czech Republic, in the last 1300 years T. Matys Grygar a,⁎, T. Nováková a,b,c, M. Mihaljevič b, L. Strnad b, I. Světlík c, L. Koptíková d, L. Lisá d, R. Brázdil e, Z. Máčka e, Z. Stachoň e, H. Svitavská-Svobodová f, D.S. Wray g a

Institute of Inorganic Chemistry ASCR, v.v.i., 250 68 Řež, Czech Republic Charles University, Albertov 6, 128 43 Prague, Czech Republic Nuclear Physics Institute ASCR, v.v.i, 250 68 Řež, Czech Republic d Institute of Geology ASCR, v.v.i., Rozvojová 269, 165 00 Prague, Czech Republic e Masaryk University, Institute of Geography, Kotlářská 2, 611 37 Brno, Czech Republic f Institute of Botany AS CR, v.v.i., Zámek 1, 252 43 Průhonice, Czech Republic g University of Greenwich at Medway, Chatham Maritime, Kent, ME4 4TB, UK b c

a r t i c l e

i n f o

Article history: Received 20 October 2010 Received in revised form 10 March 2011 Accepted 21 April 2011 Keywords: Fluvial archives Environmental change Proxy analyses Anthropogenic impact Floodplain fines Chemostratigraphy

a b s t r a c t Sediment profiles from the floodplain of Morava River in the Czech Republic have been collected from exposed river banks (4–6 m long sections) and cores (2–4 m deep) and investigated using a set of geochemical proxies validated by granulometry and conventional geochemical analysis, outlined in our previous paper. The work was conducted to evaluate the increase in sedimentation rate during Medieval and modern time periods. Correlation of sediments along the current channel belt allows identification of two most important synchronous changes in the channel structure over the past 1300 years: in the 13th century and at the end of the 16th century. These changes could be related to central European climatic extremes rather than to land cover/land use practises. Analysis of the pollen record in peaty deposits at the floodplain edge allows excluded dramatic deforestation in Medieval times. Maps of the area from the last five centuries revealed direct and indirect signs of past avulsions and clearly show how the original multichannel system was transformed into a single meandering channel in the early 20th century. The extrapolated aggradation rate (net vertical accretion of floodplain fines except for levee sediments) increased from 0.2–0.3 cm/year in 700 AD to 0.3–0.4 cm/year in 2000 AD depending on the grain size of the sediment. This is the smallest yet reported enhancement of siliclastic deposition, although Morava River watershed has been intensively used for agriculture and its land cover has changed in a manner similar to west and central European rivers. © 2011 Elsevier B.V. All rights reserved.

1. Introduction In previous paper (Grygar et al., 2010) geochemical tools have been developed for the determination of the sedimentation rate of floodplain fines by Morava River in Strážnice area. We have shown that the sedimentation rate has depended on the actual facies deposited, the study of deposits from erosion banks must be extended by coring the floodplain more distant to the current channel, and that some of the previous interpretations of the sediment profiles by Kadlec et al. (2009) should be reconsidered. The current paper addresses these questions, hand-drilled cores were analysed, historical maps from 17th and 18th centuries were evaluated, one profile of floodplain sediments with pollen record was analysed, and the output of the previous studies is critically discussed in the light of current knowledge on fluvial response to the changing environment.

⁎ Corresponding author. Tel.: + 420 2 66173113; fax: + 420 2 20941502. E-mail address: [email protected] (T. Matys Grygar). 0341-8162/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.catena.2011.04.003

Research on fluvial systems to elucidate human impacts on soil erosion and sedimentation dynamics in European river systems has flourished in the last decade; the works have recently been reviewed by several authors (Dotterweich, 2008; Lewin, 2010; Macklin et al., 2010; Notebaert and Verstraeten, 2010). Investigations of aggrading rivers hoped to trace anthropogenic environmental changes due to early occupation of lowlands and especially river valleys but, unfortunately, the sedimentation dynamics of floodplains significantly obscures that sediment archive whenever lateral erosion is prevalent (Lewin and Macklin, 2003). Dotterweich (2008) reviewed the sediment record of soil erosion and considered fluvial sediments insufficiently straightforward because of their delayed response to catchment changes. The actual transfer mechanism of eroded soil to river floodplains continues to be investigated and modelled. Recent studies have demonstrated that floodplain sedimentation is delayed after land use change due to temporary sinks of eroded soil, such as colluvia and higher order valleys (Dotterweich, 2008; Kalicki et al., 2008; Lang et al., 2003; Notebaert and Verstraeten, 2010; Rommens et al., 2006) whilst further transport from the temporary sinks needs activation by climatic extremes (Lang et al.,

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2003). Floodplains of aggrading rivers are generally more prone to hiatuses than other sediment records, and this can produce an apparent increase in the mean sedimentation rate towards the present day (Sadler, 1981; Willenbring et al., 2010). Despite this complication, river floodplains are such an alluring part of the cultural landscape of most European countries that their study cannot be ignored. The Morava River in the eastern part of the Czech Republic is one such river system which invites study. Most fortified settlements of the dynastic Slavonic state, the Great Moravian Empire (833–906/907 AD), were established in its floodplain before the end of the first millennium AD. Morava was formerly an anabranching river (Brázdil et al., in press), and its watercourse was integrated into the defences of fortified structures. Water castles were built along river banks through the entire Medieval period, e.g., in Veselí, about 10 km NE of Strážnice. Hence people have lived in an intimate contact with the river and have influenced the river dynamics for at least the last 1300 years. Several researchers have hypothesised that anthropogenic activities have dramatically enhanced sedimentation in the floodplain (Kadlec et al., 2009; Opravil, 1983; Prudič, 1978; Sterba et al., 1997; Vrbová-Dvorská et al., 2005), but this hypothesis needs further testing (e.g., Grygar et al., 2010). Studies of western European river floodplains have shown that each floodplain must be examined individually and that there are no general rules or simple schemes which can be mechanistically applied to understand the human impacts on fluvial sedimentation. The study of a floodplains sedimentary fill is complicated by its spatial and temporal complexity (Erkens et al., 2009, 2010; Hoffmann et al., 2009; Schirmer et al., 2005), which in turn requires large sample sets to be analysed and compared. Currently this is mostly undertaken by field examination and less frequently by laboratory analyses of extensive sediment cores (de Moor et al., 2008; Erkens et al., 2009, 2010; Houben, 2007; Rommens et al., 2006; Veerts and Bierkens, 1993). Miall (2006) and Houben (2007) identified a lack of simple sedimentological tools which could be used for the classification of over-bank fine sediments, consisting predominantly of silt and clay and commonly lacking sedimentary structures. We believe that efficient laboratory methods could assist with this classification and subsequent interpretation. The uses of granulometry (Kalicki, 2000), geochemical fingerprinting (Owens et al., 1999) and proxy analyses of lithology (Grygar et al., 2010) are the possible ways to improve further classification of the floodplain fines and extract more information from them. The dating of sediments from the last several centuries is not possible using 14C because there were considerable fluctuations of 14 C activity in the environment in 1620–1955 period, and hence the alternative dating methods have to be investigated. The identification of anthropogenic contamination especially derived from lead and other heavy metals, using both their total concentrations (Ciszewski and Turner, 2009; Kadlec et al., 2009; Middelkoop, 2000; Stam, 1999; Zober and Magnuszewski, 1998) and the ratios of Pb stable isotopes (Bird et al., 2010; Kober et al., 1999; Komárek et al., 2008; Novák et al., 2003; Weiss et al., 1999) has been used in floodplain sediments of European countries to distinguish industrially contaminated sediments and indirectly date them (Ciszewski and Turner, 2009; Hobo et al., 2010; Stam, 1999). This approach is also applicable in the Strážnice area (Grygar et al., 2010; Nováková, 2009) and is also used in this work. The aim of this work is to analyse the sediment deposits found in the Morava River floodplain which have accumulated over the last 1300 years and to evaluate whether the aggradation followed the normal dynamics of deposition and reworking of fluvial sediments or whether there were also some dramatic changes related to the historical development of agriculture, climate extremes, and/or other anthropogenic inherences, or a complex interplay of these factors (Erkens et al., 2009; Foster et al., 2009). Our previous papers (Grygar et al., 2010; Kadlec et al., 2009) were mostly focused on the methodology of the research on the floodplain sediments and we proposed in them some evaluation hypotheses, which should now be more critically recon-

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sidered. The main goal of this paper is to quantify the changes of the mean sedimentation rate of fine siliciclastics in the study area. Whilst several authors (Hoffmann et al., 2009; Notebaert and Verstraeten, 2010; Verstraeten et al., 2009) have recently reported that the siliciclastic deposition in European lowland rivers has substantially increased during the last 1–2 millennia, Morava River floodplain has apparently behaved in a very different manner. 2. Study area Morava is the largest river of the eastern part of the Czech Republic. From its source to its confluence with the Danube, the Morava has a length of 353.1 km with a catchment area of 26,578 km2. The long-term mean annual discharge of Morava River at the Strážnice hydrological station (the upstream catchment of Morava River above the Strážnice station has an area of 9146 km2) is Qa = 59.6 m3/s (calculated from the period 1931–1980). Calculated values of peak discharges with return period of N-year (N= 1, 2, 5, 10, 20, 50, 100 years) are the following: Q1 = 375, Q2 = 440, Q5 = 525, Q10 = 588, Q20 = 649, Q50 = 730, and Q100 = 790 m3/s. The highest peak discharge so far recorded at Strážnices is 901 m3/s1, which occurred on the 10th of July 1997, during the “flood of the 20th century” on the River Morava (Dostál et al., 2002; Matějíček, 1998) and significantly exceeded the value of Q100 (Brázdil et al., in press). The Morava is a lowland river, currently with a single channel and parallel, mostly artificial navigation channels and drainage ditches mainly from the 20th century. Originally it had an anabranching pattern (Grygar et al., 2010). The late Holocene sediments lay on the remnants of coarser pre-Holocene sediments (Havlíček, 1991; Havlíček and Smolíková, 1994). The detailed architecture of the floodplain from the last millennia is not yet known. Floodplain fines deposited in the last several millennia cover top of the floodplain to the depth up to about 5 m with respect to the floodplain surface, which indicates that net accumulation of sediments (aggradation) is a very important feature. In the Strážnice area, the width of the floodplain inundated during seasonal floods, has been reduced to about one quarter of the original width by flood defences erected in the 1930s (Grygar et al., 2010; depicted in Fig. 1). 3. Methods 3.1. Sediment sampling The vertical surface of exposed banks was cleaned and sediments were then sampled at vertical resolution of between 3 and 20 cm. The sampling of erosional banks was not continuous to avoid too many samples of monotonous lithology, however all visible heterogeneities were sampled. Individual samples extended over approximately 2 cm of stratigraphic height. The erosion bank profiles M215, M127, M219, and M107 have been included in our previous study (Grygar et al., 2010), other profiles (M312, M139, M331, M332, and M328) have not yet been reported. Cores M328, M333, M334, M403, M336, M404, M501, M502 and M503 lie all in the altitude 168 m a.s.l., belonging apparently to a single river terrace. However, elevation drops gently and smoothly towards southwest, where cores M405 and M407 are situated in 167.5 m a.s.l. Floodplain sediments were sampled using Eijkelkamp soil corers with a diameter of 2 or 3 cm. In this case the cores were sampled continuously. The positions of localities and sampling sites are shown in Fig. 1. M127 core have been included in our previous study (Grygar et al., 2010), all other cores have not been reported yet. 3.2. Dating 14 C dating of macroscopic wood remnants was performed in the laboratory of Nuclear Physics Institute AS CR as described previously

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Fig. 1. Map of the study area with localities of fieldworks and location of core PW1 and position of channel belt segments and sites of coring.

(Grygar et al., 2010; Kadlec et al., 2009). Charcoal fragments were analysed under a contractual arrangement by the AMS facility in the Poznań Radiocarbon Laboratory (Poland). Tree trunks and large branches found in outcrops in erosional river banks were subjected to 14 C dating; infrequently occurring tree roots were not analysed. Obviously recycled macroscopic wood remnants (which would have produced reverse dating orders) were not found. In two cases, markedly older charcoals were found, which may indicate the reworking of older floodplain sediments. 14C datable macro-remnants were rare in the overbank fines and the results described in this paper

represent the outcome of seven field visits over three successive years to search for wood debris in freshly exposed faces derived from lateral erosion of banks. Tree remnants were usually found in the deeper parts of the profiles and mostly on lithological boundaries, such as the base of distal floodplain deposits, on the coarser sediments of abandoned channels and on the boundaries between more silty and more clayey sediments. Datable material was only obtained from one core in the floodplain; several charcoal fragments were separated from a thicker core through backswamp deposits in alder carr PW-1 where predominantly reducing conditions have prevailed in the sediment column.

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3.3. Sediment analyses Cation exchange capacity (ΔCu) of the sediments was determined using the methodology of Meier and Kahr (1999), as evaluated by Czímerová et al. (2006) and optimised for soft geomaterials by Grygar et al. (2009). Samples were air dried and ground to analytical fineness; 100–500 mg of powders was accurately weighted into glass beakers, wetted by 5 ml water and then stirred for 5 min with 5 ml 0.01 M [Cu(trien)]SO4 solution, where trien is triethylenetetramine (1,4,7,10-tetraazadekane). Each suspension was then filtrated into 50 ml volumetric flasks, the solid residue washed with distilled water, and the concentrations of Ca, Cu and Mg were obtained by atomic absorption or emission spectroscopy (AAS3, Zeiss, Jena, Germany). The Cu consumed was expressed in mmol [Cu(trien)]2+ per gramme of sample and denoted as ΔCu. ΔCu in the studied sediments is directly proportional to the clay fraction content (Grygar et al., 2010) and can therefore be used as a lithological proxy. Particle size analysis was performed with selected samples to calibrate the lithological proxy method. Air dried samples were gently disaggregated and dispersed in de-ionised water. Organic matter was removed by addition of hydrogen peroxide and gentle warming to 40 °C until effervescence stopped. The supernatant was removed after centrifugation and the sample rinsed with two changes of deionised water. Samples were then re-homogenised in a minimum of water using a whirly-mixer and then sub-sampled for particle size analysis. Particle size distribution was established using a Malvern Mastersizer 2000. Elemental analysis was performed as described previously (Grygar et al., 2010). The sediments were ashed in a Linn (Germany) programmable furnace at a final temperature of 450 °C. The residues were subsequently dampened with deionised water and dissolved in mixture of HNO3 and HF and the solutions made to fixed volume. The metal contents were determined using inductively coupled plasma mass spectrometry (ICP/MS, X Series 2, ThermoScientific). The Pb isotopic ratio 206Pb/207Pb was measured by diluting the solutions to a concentration of b20 μg/L, and a correction for mass bias was performed using SRM 981 (Common Lead, NIST, USA). X-ray fluorescence analysis (EDXRF) of the content of Pb, Rb, Zn, and Zr was performed with a PANalytical MiniPal4.0 spectrometer with Peltier-cooled silicon drift energy dispersive detector. The ground samples were analysed after pouring into measuring cells with a Mylar foil bottom. The signals of Pb, Rb, and Zn were calibrated using the results derived from the analysis of a representative portion of sediment samples by ICP/MS. Magnetic susceptibility was measured as reported previously (Grygar et al., 2010; Kadlec et al., 2009; Nováková, 2009). Sediment samples were left to dry in air and gently ground. The magnetic susceptibility was measured with Kappabridge KLY-2 (Agico Brno) operating with a magnetic field intensity of 300 A/m and a frequency of 920 Hz and the results were recalculated to the mass of samples to avoid problems with poorly defined volume of materials with different porosity. 3.4. Facial assignment of sediments Facies assignment of the Morava River floodplain in the Strážnice area based on ΔCu proxy analysis was described by Grygar et al. (2010). Rb and Zr analyses are now added as two further proxies of the sediment granulometry; they were newly applied also to the profiles included in a previous work (Grygar et al., 2010). Following the recommendation of Houben (2007), the list of facies used in this work has been "tailored" for floodplain sediments in the study area on the basis of a combination of field observations, proxy analyses, and evaluation of spatial relationships between lithological units from visual examination in the field and correlation of cores. This approach allows the use of lithofacial descriptions combined with the visual

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examination of the floodplain architecture to estimate which architectural elements sensu Miall (2006) are built from these facies, i.e. to assign the facies to architectural elements. 4. Results 4.1. Sediment dating Due to the scarcity of datable material in the uppermost parts of the profile and the equivocal 14C calibration curve from the last three centuries, the base of the industrially contaminated topmost layer of overbank fines was identified by increased magnetic susceptibility, increased concentration of Pb and Zn, and the changed ratios of Pb stable isotopes in the same manner as discussed in previous papers (Grygar et al., 2010; Kadlec and Diehl, 2005; Kadlec et al., 2009). Typical depth profiles of these proxies of industrial contamination are shown in Fig. 2. The lithogenic (background values) of heavy metal contents, 206 Pb/207Pb isotope ratio, and magnetic susceptibility (MS) are summarised in Table 1. The Pb and Zn concentrations were normalised to Rb content, because all these three elements in pre-industrial sediments are positively correlated with the size fraction b 0.01 mm. The EDXRF signal of all these three elements was calibrated by ICP/MS analysis with selected samples (Table 2). The normalisation of Pb and Zn concentrations (Pb/Rb and Zn/Rb ratios) and MS profiles are applicable in sediments with sand content b 30%. Some magnetic particles and Fe(III) oxides, which immobilise a portion of the Pb and Zn, are depleted by gleying processes in deeper part of the sediment profiles, particularly in porous sandy sediments or in very dense clayey sediments with signs of Fe(III) oxides transformations by redox changes, where these proxies commonly depart from the lithogenic background values. Fe(III) oxides transformation is visible by naked eye in field, especially in outcrops in erosion banks, or by spiky variations in the depth profiles of EDXRF signal of Fe. The deposition of the Pb and Zn-contaminated layer was found not only in the current erosion banks and between the current channel and the flood defences, but also in the floodplain behind the flood defences, i.e. outside the area currently inundated during floods (Fig. 3). 4.2. Facial assignment of overbank fines In this report the idea of proxy-based facial assignment of the sediments proposed by Grygar et al. (2010) is further substantially developed. The geochemical proxies of sediment lithology were calibrated by conventional granulometric analyses (Table 2). The resulting list of facies with their "fingerprinting" by proxy values is listed in Table 3. Typical granulometric curves of the facies identified by the proxy analyses are shown in Fig. 4. The grain size boundaries distinguished in this work (0.01 and 0.1 mm) do not coincide with the conventional boundaries of clay, silt, and sand, but are more or less consistent with the minima in the actual particle size distribution of the floodplain sediments from the study area, as shown in Fig. 4. The proxy data related to lithology also shows gradation within profiles of floodplain fines, whereas it is either gradual in the case of slow lateral movement of the nearby river channels or sharp in the case avulsions. Distal floodplain deposits have the highest percentage of clay, whilst in proximal floodplain deposits the silt fraction prevails; both sediment types usually contained 10–15% sand. The proximal and distal floodplain sediments form an apparently continuous series, with either the ΔCu or Rb signal giving the easiest numeric expression for their qualitative assignment. Occasionally, these facies are separated by a centimetre to decimetre thick sandy layer interpreted as a crevasse splay. The bottom part of the majority of sections studied consisted of sandy fillings of abandoned channels or former point bars, sometimes with laminated silty intercalations. In several sections, an unsorted, compacted mixture of sand, silt, and clay (conventionally called loam in

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Fig. 2. Depth profiles of industrial contamination in core M404, from left: magnetic susceptibility, the EDXRF signals of Zn and Pb normalised to Rb, and the isotopic ratio of Pb obtained by ICP/MS. The lines are 3-point adjacent averages (MS and element analyses) and 2-point adjacent averages (Pb isotopes). The dark grey area defines the industrially contaminated sediment. Light grey areas: 3σ interval of the pre-industrial lithological background values.

Table 3) was present at depths below 2 m. The unsorted mixture of grain sizes suggests deposition in proximity of shallow channels, maybe in their dense system resembling braided pattern locally, allowing low energy deposition from suspensions during moderate floods regularly alternated by overbank export of channel sands during more extensive flooding. The loam deposits are spatially restricted to segment B of the studied channel belt, whilst better sorted distal and proximal floodplain deposits are more widespread. 4.3. The sediment record in erosion banks All profiles in erosion banks were dated by 14C of wood macroremnants or charcoal. The oldest Holocene wood debris yet found in the studied area cover the age range from 7580 BC to 1524 BC (Fig. 5; further data are listed in Grygar et al., 2010; Kadlec et al., 2009). These strata, older than 1300 years, have yet not been stratigraphically correlated. Only a single profile, section M130, with three 14C dated charcoal fragments with reasonable age-depth dependence was found. Sections of floodplain sediments in erosional banks containing mostly fine material (not predominantly sands), with datable wood remnants or charcoal, and with reasonable lateral stability of the facies are shown in Fig. 5. Facial assignment and 14C dating allow the correlation of four sections from segment A and four sections from segment B of the studied channel belt. All eight correlateable sections exhibit the same general development in their upper part, where the sediments coarsen stepwise from mainly clayey to mainly silty floodplain sediments during the 15th and 16th centuries. This coarsening could have been due to an increase in transport energy for

overbank sediments in the current channel belt between about 1200 and 1600 AD. In segment A, the finer, distal floodplain sediments, were deposited from at least 700 AD. The thin sandy intercalations are interpreted as crevasse splays according to the criteria in Table 3, they occur mainly at two stratigraphic levels (Fig. 5). Their date can be estimated from linear interpolation of available date points to about 1600 AD and about 1800 AD. Prominent, laterally stable sandy layers deposited at about 1550 AD have also been found in S1 and S5a profiles as reported in Kadlec et al. (2009). In segment B of the studied area, in the three sites shown in Fig. 5, (M107, M331, and M332), sediments deposited before about the 12th to 13th centuries AD meet the definition of loams according to Table 3. The same poorly sorted massive sediment has been found in section M136 (about 500 m upstream from M326). 4.4. The sediment record of the floodplain The sediment record of the floodplain is more difficult to interpret because they are not exposed and can only be studied in cores. No 14C datable material was retrieved in the cores and hence there are no reliable time constraints to directly correlate the older strata with dated outcrops in erosional banks. The topmost, industrially contaminated part was revealed unequivocally by EDXRF and magnetic susceptibility. The coring was performed at two sites with local names Muchárov and Klokov. According to available maps from the end of 18th to 20th centuries, these sites have been covered by forest. The general trend of sediment upward coarsening in erosional banks of the current channel belt (Fig. 5) has probably occurred

Table 1 Discrimination of the industrially polluted sediments from profiles at the Muchárov and Klokov sites. Threshold values were calculated as a sum of lithogenic (background) concentrations and 3σ of the means from individual profiles; in the case of isotopic composition as mean of lithogenic 206Pb/207Pb minus 3σ of all values. Lithogenic values

a

Pb/Rb (c.p.s./c.p.s.) EDXRF analysis Zn/Rb (c.p.s./c.p.s) EDXRF analysisa 206 Pb/207Pb ICP/MS analysisb MS (10− 9 m3/kg)a a b

0.0824 0.367 1.202 134

Threshold values

0.0895 0.405 1.195 166

Most contaminated sediments in top of profiles (median values) within flood defences

outside flood defences

0.122 0.65 1.191 260

0.096 0.45 1.193 213

18 profiles at Muchárov and Klokov. Five profiles from the entire studied area and nine couples of polluted and unpolluted sediments from Muchárov and Klokov.

T. Matys Grygar et al. / Catena 86 (2011) 192–207 Table 2 Results of calibration of the proxy analyses of sediment lithology using granulometric data and ICP–MS analyses. Proxy analysis

Calibration curve

ΔCu (mmol/g)

% clay fraction b 2 μm = 0:3314⋅ΔCu; R2 = 0.86, Grygar et al., 2010 % fraction b10 μm = 0.949·Rb — 33.8; 97 analyses, R2 = 0.6618 % fraction 10–100 μm = 0.171·Zr + 7.0; 96 analyses, R2 = 0.6316 Rb (ppm) = 1.403·Rb; 45 analyses, R2 = 0.8321 Pb (ppm) = 2.694·Pb; 187 analyses, R2 = 0.6444 Zn (ppm) = 2.443·Zn; 191 analyses, R2 = 0.8714

EDXRF signal of Rb (c.p.s.) EDXRF signal of Zr (c.p.s.) EDXRF signal of Rb (c.p.s.) EDXRF signal of Pb (c.p.s.) EDXRF signal of Zn (c.p.s.)

simultaneously, with sediment fining upward from mostly sand or silt to mostly clay at sites in Muchárov (Fig. 6). Although we do not have 14 C dating in the floodplain cores depicted in Fig. 6, the fining upward from proximal to distal floodplain sediment (correlation area III in Fig. 6) could have proceeded simultaneously with sediment coarsening from distal to proximal floodplain (correlation area II) due to the reorganisation of the channel system. The observed lithological development would be consistent with trunk channel movement from a position south-east from the site M404 to the current channel belt (the site M215). 4.5. Changes in pollen record Pollen grains have not been found in most of the fine overbank sediments, probably because the pollen has been oxidised. A pollen assemblage has successfully been retrieved from the fill of an abandoned channel within core PW-1. Its position is shown in Fig. 1 and an evaluation of the pollen analysis is in Table 4. Only the upper part of the core (top 179 cm) is described in Table 4; the lower part (180–340 cm) consisted of sandy sediments and peats of the early

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Holocene and pre-Holocene age (Bláhová et al., 2008). The past change in land use or land cover has hence been reconstructed from that core, because even at the present time, when the Morava River is incised deeper than before the river regulation of the 20th century, the groundwater level is close to the current floodplain surface. The diatom analysis of the core PW-1 (Bláhová et al., 2008) indicates standing or flowing water conditions over the 1.8–0.25 m core interval and higher nutrient, wet conditions in the uppermost 25 cm of the core profile. Age constraints are given by 14C dating, by the occurrence of buckwheat pollen together with an assemblage of cultural plants over the 0.6–0 m interval, and by the presence of an industrially contaminated layer (0.1–0 m) (see Table 4). The pollen assemblage in the PW-1 core from the last 1300 y consists mostly of alder (Alnus). The percentage of tree pollen grains, including oak, does not exhibit any dramatic changes in the forest structure except for an increase in pine during the 19th century, which is consistent with the afforestation of the sand body behind the NW edge of the studied floodplain area as illustrated on historical maps. Land use has been changed substantially from crop fields before the 13th century to pasture.

4.6. Palaeochannels in the current channel belt Minor old channel has been found in section M221 (Fig. 7) and dated by one branch and one wood fragment in the gravelly sand of the channel bottom to between1012–1266 AD (wood fragment) and 1251–1434 AD (branch. This old channel was incised in the fine clayey-silty unit dated in profile M110 to ~ 700 AD. The proximity of the channel was responsible for the laterally variable stack of sandy and silty sediments in the nearby profiles M15 and M225. Another relatively shallow, narrow old channel was found in profiles M319 to M321, with a channel base containing a large chopped oak beam dated to 967–1258 AD. These past channels, originally incised into distal to proximal floodplain sediments and now occasionally exposed in the erosion banks, occur as lenses about 2 m thick and about 10 m wide, and are now covered by planar strata of floodplain fines.

Fig. 3. Contamination of the sediments from the 20th and 21st centuries by Pb within and outside the flood defences erected in the 1930s. Light grey areas: 3σ interval of the preindustrial lithological background values.

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Table 3 A list of major facies distinguished in the studied floodplain. Facies Channel sediments (CH)

a

Point bar sediments (LA)a

Levee sediments (LV)a

Crevasse splay deposits (CS)a

Proximal floodplain sediments (PF)b

Proxy values

Lithology, appearance, Spatial relations

ΔCu b 0.01 mmol/g Zr 50–90 c.p.s. Rb 50–55 c.p.s. ΔCu 0.03–0.06 mmol/g Zr 150–300 c.p.s. Rb 50–80 c.p.s. ΔCu 0.04–0.07 mmol/g Zr 250–300 c.p.s. Rb 50–60 c.p.s. ΔCu b 0.05 mmol/g Zr 230–290 c.p.s. Rb 60 c.p.s. ΔCu 0.08–0.12 mmol/g Zr 200–300 c.p.s. Rb 60–80 c.p.s.

Sand, poorly sorted, rarely with gravel. Deeper part of the sections. The units have poor lateral stability.

Distal floodplain sediments (DF)b

ΔCu N 0.12 mmol/g Zr 140–200 c.p.s. Rb 80–100 c.p.s.

Mixed floodplain sediments, loam (MF)b

ΔCu ~ 0.1 mmol/g Zr 100–240 c.p.s. Rb 70–80 c.p.s.

a b

Mainly consisting of silt and fine sand, poorly sorted, with silty laminae. Deeper part of the sections. The units have inclined upper boundaries and poor lateral stability. Mainly fine silt and fine sand, unconsolidated, usually with sandy laminae. Present in most of the current erosional banks. Mostly fine sand, cm to dm thick horizontal sheets. Mostly on the interface between DF and overlying PF or within PF. Laterally stable for tens to hundred metres. Silt N clay N sand, sand component very fine. Usually with fine sand intercalations or inclusions, yellowish to greyish. The units have horizontal boundaries. Thickness from about 0.5 to about 2 m. Laterally stable for hundred metres. Clay ≥ silt N sand. Total sand b20%, sand component very fine. Massive, usually homogeneous, brownish colour, usually with clear signs of gleying. The units have horizontal boundaries. Thickness from about 0.5 to about 2 m. Always in contact with PF. Laterally stable for hundred metres unless interrupted by minor palaeochannels. Very poorly sorted sediment, clay ~ sand N silt or clay, sand and silt in comparable percentages. Massive (without lamination), hard, in dm to m thick strata, sand grains visible by naked eye. Less common, usually on the base of sediment profiles.

Facies name equal to Miall's name of architectural elements (Houben, 2007; Miall, 2006). All these facies would be included in Miall's name of architectural element “floodplain fines” (Miall, 2006).

4.7. Changes of channel pattern as documented by historical maps Maps available for the studied area (Fig. 8) cover the period from the second half of the 16th century up to the present day. The scale of maps varies from 1:530,000 (Comenius Map of Moravia, 1624) to 1:10,000 (present day topographical maps). The planimetric accuracy of maps from the 16th to 18th century is not to comparable modern standards but they can be used for evaluation of the overall river pattern and its evolutionary tendencies over hundreds of years. It is evident (Fig. 8) that the Morava River has an anabranching pattern through its middle and lower course as it flows through the Outer Carpathian Depressions and the Vienna Basin. Taking a transect across the floodplain from Strážnice (Strasnitz or Strassnitz in German in maps) to Rohatec (Rohatetz), all the historical maps portray a channel pattern consisting of several parallel branches; from two to five parallel channels are shown depending on the map scale and cartographic generalisation. Historical maps indicate that individual river branches varied in their width, course and sinuosity. Two large channels flowed along the opposite margins of the floodplain and formed the lateral boundaries of the channel network (Figs. 8A, C, D and 9). From the second half of the 18th century these two main channels were intensively meandering and laterally active. The overall pattern of the channel network included numerous smaller meandering or straight river branches that diverted and re-joined the main channels, or in some cases crossed the floodplain and connected the two main channels (Fig. 8, maps A and C). The Morava River thus had an anabranching character with meandering and straight individual channels. The sinuosity of the individual channels reflected the stream power; large branches (trunk channels) with a higher discharge meandered and were laterally more active. Small branches (minor channels) were rather straight in plan and laterally more stable. Some of the minor channels are occasionally exposed in the current channel belt as discussed above. It can be inferred from historical maps that the arrangement of the channel network has not been affected by any dramatic changes at least since the first half of the 17th century. The overall pattern with two

dominant channels can be traced back to the Comenius Map of Moravia which dates from 1624 (Fig. 8A). The most valuable information on the dynamics and development of the channel network is provided by the maps of the First, Second and Third Austrian Military Surveys, which were conducted from the second half of the 18th to the second half of the 19th centuries (Figs. 8D and 9). These maps provide the indirect and direct evidence of channel avulsions as well as the lateral shifts by gradual bank erosion. The relatively dense network of degrading, ephemeral watercourses, either still connected to active channel network or preserved as isolated fragments within the floodplain (Fig. 9), provides the indirect evidence of historical avulsions. Véšky and Medvídka channels are the current remnants of such abandoned channels (Fig. 1). These remnants are interpreted as derived from repeated avulsions that created several generations of superimposed channel belts. The direct evidence of avulsions is depicted on maps from the first half of the 19th century, when the southernmost of the two trunk channels splits into two branches (Fig. 9A and B). Old and new branches coexisted for several decades until the entire discharge eventually diverted into the new channel (Fig. 9C). Cessation of natural avulsions, the abandonment of many smaller channels, and the concentration of discharge into one of two main channels are the main trends in channel network development during the second half of the 19th and the whole of the 20th century. The transformation of the anabranching system into a single channel meandering system was completed during the 1930s by the erection of flood defences that constrained the channel network and also by local channel straightening in the study area. It is, however, possible that the "degradation" of anabranching system has been initiated much earlier, in Medieval times similarly as it was reported by Lewin (2010) for lowland rivers of England and Wales. Fluvial processes have been inhibited in the side channels but, on the other hand, the dominant meandering channel experienced profound morphological changes. Increased discharge has caused channel incision and widening and fast lateral migration occurs. Thus, the Morava River between Strážnice and Rohatec documents the transformation from

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Fig. 4. Examples of granulometric curves of typical representatives of the facies according to Table 3.

anabranching system with both sinuous and straight branches to single channel meandering system with high lateral activity (Brázdil et al., in press).

5. Discussion 5.1. Correlation of industrially contaminated sediments In this and in the previous works (Grygar et al., 2010; Nováková, 2009) we have used the onset of significant industrial contamination in the top part of floodplain sediment sequences as a chronostratigraphic proxy. The increase in heavy metal concentration in Morava River sediments during the 20th century has already been reported by Bábek et al. (2008). The base of the contaminated layer in Strážnice area is marked by an increase in magnetic susceptibility (MS), Pb and Zn concentrations as well as a change in the isotopic composition of lead in the currently inundated floodplain area (Grygar et al., 2010;

199

Kadlec et al., 2009; Nováková, 2009). A similar, nearly simultaneous change in all these proxies was also found outside the flood defences (Fig. 2), i.e., in areas regularly inundated only before the 1930s. Kadlec and Diehl (2005) and Kadlec et al. (2009) attributed the increase of MS and other magnetic parameters in the top layers of the floodplain sediments to enhanced erosion of soil in the Morava River watershed related to the intensive agricultural practises of the last 50 years, but this hypothesis would require verification. The simultaneous increase in heavy metal content and magnetic minerals is usual in industrially polluted topsoils (Kapička et al., 1999; Matýsek et al., 2008) and river sediments (Knab et al., 2006). In areas without nearby point sources of Pb contamination (such as metal mining and smelting), Pb contamination and simultaneous Pb, Zn and magnetic contaminations have been related to coal combustion and/ or leaded gasoline use (Desenfant et al., 2004; Komárek et al., 2008). The observed shift in stable Pb isotope ratios is in agreement with the change from typical lithogenic values towards the mean value for coals from the region (Komárek et al., 2008; Novák et al., 2003). The base of an almost stepwise increase of coal combustion in the region was dated back to the end of the 19th century (Novák et al., 2003). In Switzerland, the first regional increase of Pb emissions linked to the industrial revolution is dated to between 1880 and 1920 (Weiss et al., 1999), or to the early 20th century (Kober et al., 1999). This timing is in agreement with the hypothesis that the base of the industrially contaminated layer in the Strážnice area can be attributed to the first wave of industrialization in the region at the end of the 19th or beginning of the 20th century (Grygar et al., 2010). No important local sources of Pb and Zn contamination have been identified in the vicinity of the study area or close upstream, as this area of Moravia is predominantly agricultural. The maximum Rb-normalised concentrations of Pb and Zn, as well as the thickness of the contaminated layer, decrease with increasing distance from the main channel, implying that riverine transport rather than emissions (atmospheric fallout) is the dominant source of pollutants (Martin, 2000; Fig. 6, correlation area I). It is noteworthy, that about 50 km north of Strážnice, upstream from the study area, the Baťa's shoe making factory was established at the very end of the 19th century in Zlín (on the bank of Dřevnice River, which is a tributary of Morava River), and the factory later extended to Otrokovice (a city on confluence of the Morava and Dřevnice). The discovery of industrial contamination outside the flood defences erected in the 1930s (Figs. 2 and 3), further confirms the time assignment of its onset to about 1900 AD. We used this date in the calculation of mean aggradation rates. There are several geochemical and lithological limits to the use of the relative concentrations of MS, Pb/Rb and Zn/Rb for indirect dating. The Pb and Zn depth logs can only be used in this way if these heavy metals have not migrated vertically within the sediment profile. That vertical migration could be possible in very coarse sediments, such as it was observed in sandy levee sediments (Ciszewski et al., 2008; Grygar et al., 2010). The Pb and Zn depth profiles would not be stable if the particles sorbing and immobilising these elements in the sediment profile could be degraded. Most Pb in floodplain sediments is sorbed on Fe(III) oxides (Nováková, 2009). The first sign of gleying processes, redoximorphic transformation of Fe(III) oxides, occurs at a depth of more than 1 m, significantly below the depth of the industrially contaminated layer. From this it can be inferred that the Pb/Rb depth profiles are reliable in distal and proximal floodplain sediments through at least the uppermost 1 m of their profiles. In sand and coarse silt levee sediments (Table 1), the depth profiles may be affected by vertical migration or reworking the sediment during floods. At greater depth in the floodplain fines where gleying is present (from the colour patterns of the sediment or by spiky variations in Fe concentration), more complex variations in the Pb/Rb and MS record were observed; they are not shown in this work but will be addressed in a separate study.

Fig. 5. Correlation of floodplain sediments from the current erosion banks with available

C ages of wood debris or charcoals. The base of the industrial contaminated sediment is also shown. Facies abbreviations are given in Table 3.

14

200 T. Matys Grygar et al. / Catena 86 (2011) 192–207

T. Matys Grygar et al. / Catena 86 (2011) 192–207 Fig. 6. Sediment lithology in the cores in Muchárov locality with growing distance from the erosion bank toward the floodplain. Chemostratigraphic correlation area I (light grey): industrially contaminated sediment. Facial correlation areas II–IV (dark grey): II coarsening upward from distal to proximal floodplain sediments, III fining upward from proximal to distal floodplain sediments, IV fining upward from channel or point bar sediments to floodplain fines.

201

Low nutrient fen, dessication and floods, pasture meadows. Anthropogenic indicators of dense settlement Development of low nutrient fen with Filipendula, Great riverine flood plain, alder carr well Poaceae and Cyperaceae, Artemisia, development of saturated by water, riverine forest with pasture meadows. Dessications and floods, open oak under development, thermophilous water sources acid oak–pine forest, surface fires Climate change, formation of termophilous Change of vegetation from Subboreal steppe to oak–pine forest (Quercus, Tilia, Ullmus, Pinus) termophilous oak pine forest, local development of alder (Alnus glutinosa) carr. Low nutrient fen, periodical dessication and floods during summer

Settlement of Middle Ages (12th and 13th centuries AD), setlement with cereal fields and pasture Early Medieval settlement with cereal 106 cm, 677 to 868 AD fields followed by Early Medieval settlement with pasture by water sources Prehistoric pasture by water source 156 cm (nearby PW-2), 1129 to 918 BC

5.2. Mean aggradation rates in the last 1300 years

Older Subatlantic

Subboreal

149-93

179-149

Sandy sediment N179

Younger Subatlantic 93-69

Upward fining of siliciclastic component (overbank fines with more clay)

Riverine forest with oak

60 cm, buckwheat pollen

Upward coarsening of siliciclastic component (overbank fines with more silt) Discontinuous pollen sedimentation (periods with corroded pollen grains). Discontinuous pollen sedimentation (periods with corroded pollen grains about 20–30% of total pollen sum) Upward coarsening of overbank fines. Discontinuous pollen sedimentation. Younger Subatlantic

Stratum confussum 14-0

34-14 69-34

Biochronology Sediment components Depth (cm)

Table 4 Results of pollen analysis from core PW-1.

Regional vegetation

Pinus aforestation Riverine forest with oak

Local record

Low nutrient fen, eutrophic alluvial meadows Fallow land. Dense alder (Alnus glutinosa) carr, pasture meadows

Settlement

Modern agriculture with maize fields. Top and late middle ages, 14th - 17th century AD, weak signs of settlement, abandoned fields, vineyards

10 cm, base of industrial contamination

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Age constraints

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A calculation of the mean aggradation rates must take into account several possible biases originating from the fluvial sediment dynamics. Only sediment strata lacking hiatuses can be evaluated, to avoid an apparent increase in sedimentation rate towards the present (Sadler, 1981). This phenomenon is not always considered in the evaluation of the long-term rate of aggradation in floodplains (Willenbring et al., 2010). In a number of recent studies, a very dramatic increase in the aggradation rate has been reported from European lowland rivers and river estuaries, and assigned to soil erosion from historical land use (e.g., Hoffmann et al., 2009). It seems unequivocal in cases where lateral erosion (sediment recycling) is negligible, and/or when clearly stratigraphically correlated older, (mostly peat) deposits have been overlain by stratigraphically correlated younger, (mostly siliciclastic) deposits (de Moor et al., 2008; Lang and Nolte, 1999; Rommens et al., 2006) or when erosion and deposition rates have altered (Erkens et al., 2009; Foster et al., 2009). The lower reach of the Morava River has deposited siliciclastics for several millennia (Havlíček, 1991; Havlíček and Smolíková, 1994) although they are poorly preserved (e.g. section M130, Fig. 5). Historical maps show highly sinuous river channels and a complex multichannel network and currently very active lateral erosion is occurring. Avulsion and lateral migration of past channels are also apparent from maps published since the 17th century. Hence, the reworking of the older floodplain sediments must also be considered. In all 14C-dated sequences in erosion banks, the stratigraphic correlation has only been possible for sediments from the last 1300 years. The levee sediments, i.e., the youngest material from the current erosion banks, have been omitted from the comparisons because these sediments are not stratigraphically reliable and comparable to proximal and distal floodplain sediments. The most recent aggradation rate has been obtained from sediments cored in the floodplain between the current channel belt and the flood defences, where industrial contamination allowed their dating. The mean aggradation rate of floodplain fines is, however, dependent not only on the mean age of sediments, but also on their coarseness, i.e., actual sediment facies (Grygar et al., 2010), which is in turn governed by the distance from the active channel. The thickness and mean grain size of the blanket of floodplain fines decrease with increasing distance from the channel (Bridge, 2003). This dependence is nicely visualised, e.g., by decreasing thickness of the recent, industrially contaminated sediment with the increasing distance from the channel (Martin, 2000). To separate these contributions, all available estimates of mean sedimentation rate between two dating points have been processed by linear regression, using sedimentation rates as the dependent variable and the ΔCu and the mean sediment age in the given section as independent variables. The results of the analysis of variance (ANOVA) of that dependence are summarised in Table 5: the lithological bias is statistically relevant at a probability of 95.4%. The net increase in sedimentation rate with the mean age of the sediment is about 0.066 cm/year per millennium between 1000 and 2000 AD. In other words, for proximal floodplain sediments with ΔCu = 0.1 mmol/g the mean deposition rate increased from 0.23 cm/year in 700 AD to about 0.31 cm/year at the end of the 20th century. For distal floodplain sediments with a mean ΔCu = 0.15 mmol/g the corresponding increase would be from 0.14 to 0.23 cm/year. It should be noted that the 20th century mean aggradation rates were calculated from profiles within the flood defences, when the width of the inundated area was decreased to about one quarter of that prior to the 1930's. In our previous paper (Grygar et al., 2010) we reported the deposition rate in the interval 0.2–0.6 cm/year for the period 1000– 1900, because formerly we also processed faster-deposited proximal floodplain sediments with ΔCu ~ 0.08 mmol/g. The currently obtained deposition rates are at the lower part of the previously published interval.

Fig. 7. An example of laterally unstable profiles around minor palaeochannel (in profile M221), showing facial changes related to its moderate lateral movement and then abandonment. Legend as in Fig. 5, facies abbreviation given in Table 3.

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Fig. 8. A series of maps covering the period from the first half of the 17th to the middle of the 19th century showing the branching pattern of the Morava River. Historical maps show that the basic arrangement of river branches and bifurcations has been stable since at least the beginning of the 17th century. A: J.A. Komenský, 1624, original scale ca 1:530,000 (Map collection of the Department of Geography, Masaryk University in Brno), B: J.M.W. Tyrolský, original scale ca 1:187700 (Map collection of the Department of Geography, Masaryk University in Brno), C: J.C. Müller, 1730, original scale ca 1:168,000 (Map collection of the Department of Geography, Masaryk University in Brno), D: Second Austrian Military Survey, Section No. O-12-V, 1841, original scale 1:28,800 (Austrian State Archive/Military Archive, Vienna; Geolab Ústí nad Labem; Ministry of Environment of the Czech Republic).

The sedimentation rate of the top layer of the current erosional banks is not comparable to the mean deposition rate of floodplain fines. The levee sediments were deposited at a markedly high apparent rate of about 0.8 cm/year (see also Kadlec et al., 2009). This rate may be biased by the reworking of sediment during floods and fast local deposition, with thicknesses of 20–30 cm occurring locally after a single spring flood. The relatively fast aggradation rate and the prevailing deposition of fine, cohesive sediments in the floodplain near Strážnice were probably factors responsible for the existence of an anastomosed, multichannel system and common avulsions (Makaske, 2001). Anastomosed rivers in temperate, humid climates usually have mean sedimentation rates above 1 mm/year (Makaske, 2001), lower than the mean aggradation rates of Morava River in the studied area. Our data indicate that the sufficiently rapid aggradation of the Morava floodplain has been ongoing for at least the last 4 ky. 5.3. Historical changes of the sedimentation and channel pattern Sedimentation in studied area was not catastrophically affected by the large Slavonic settlement in the lowlands of lower reach of the Morava River at the end of the first millennium AD, as hypothesised by Prudič (1978) and Opravil (1983). The sediments from that period (M215, M127, M110, and M312, Fig. 5) do not record any synchronous, abrupt change. The Slavonic state collapsed in the early 10th century. Pollen evidence to support the proposed floodplain forest clearance at that time is lacking in core PW-1. Past reorganisations of the channel systems are documented by the facies changes in the sediment profiles. Rather abrupt changes prevail in most profiles, such as switchovers between distal and floodplain sediments, or loams/channel sediments and distal floodplain sediments

with several dm transition zones. Stepwise, gradual sediment gradation is rather rare. The deposits of anastomosing rivers undergoing to avulsion should have more dramatic depth variations than meandering rivers, because avulsions favour the facies alteration rather than lateral channel movement with continuous gradation of floodplain fines (Veerts and Bierkens, 1993). The facial changes in most profiles can be correlated and we therefore consider them indicators of the reorganisations of the channel system. One of the reorganisations of the river channel system has been reflected in the segment B of the studied section, where a change from channel sediments or loams to distal floodplain sediments occurred around ~1200 AD (Fig. 5). The 14C ages pre-dating this change are 967 to 1229 AD in M331 and 1169 to 1269 AD in M107 and the age post-dating it is 1280 to 1442 AD in M332. Interestingly the 13th century change of land use from arable crops to pasture is recorded in pollen assemblage in PW-1 (Table 4). The simplest explanation of this evolution would be substantial reorganisation of the channel system in the segment B at that time. The synchronicity of the observed changes is within the accuracy of the age model for these strata. Because no major change in deposition occurred simultaneously in segment A, the shift in segment B likely mirrored local reorganisation of the channel network. Another, more extensive reorganisation of channel system occurred between 1550 and 1600 AD (Fig. 5). This date is much better established and the change, namely sediment coarsening, (distal to proximal floodplain) has been recorded in many profiles in both segments A and B of the studied channel belt. That event is pre-dated at 1391 to 1641 AD and post-dated at 1445 to 1814 AD in M326, and it was also identified in other outcrops in erosion banks in our previous reports and dated to 1550 AD (Grygar et al., 2010; Kadlec et al., 2009). Apparently the sediment simultaneously fined upward in the current distal floodplain at Muchárov (Fig. 6); this can be rationalised as a consequence of an

T. Matys Grygar et al. / Catena 86 (2011) 192–207

205

Table 5 ANOVA table for correlation of mean aggradation rates with the mean ΔCu of the sediment between the dating points and the mean age of the sediment (mean of the dating points). 15 values from 14 profiles with proximal and distal floodplain sediments, levee sediments are not included. Parameter

Value

Error

t-value

Prob N |t|

Y-intercept ΔCu Mean age

0.35 − 1.70 6.6 · 10− 5

0.09 0.77 2.1 · 10− 5

4.06 − 2.20 3.12

0.00136 0.04626 0.00807

this change, e.g., the area denoted as distal floodplain has been much reduced.

5.4. Possible influence of past hydrological extremes

Fig. 9. Examples of historical avulsions to the south west of Strážnice from a series of maps of the Austrian military surveys covering the period from the second half of the 18th to the first half of the 19th century. A: the topographic situation of the floodplain before avulsion occurred showing forest and road network (Section No. 115, original scale 1:28,800), B: the topographic situation after the avulsion showing new channel connecting the Morávka branch with the main Morava channel (Section No. O_12_V, original scale 1:28,800), C: the avulsion channel established as a new, main course of the Morávka branch by the end of the first half of the 19th century, remnants of the original channel are visible at the bottom of the picture (Section No. 4458_2, original scale 1:25,000). The maps are from Austrian State Archive/Military Archive, Vienna; Geolab Ústí nad Labem; Ministry of Environment of the Czech Republic.

increase in the speed and volume of water flowing through the current Morava channel at the expense of parallel channel (called Morávka on 18th and 19th century maps). At that time the Morávka branch passed through the town of Strážnice; in the 20th century it was straightened and canalised (forming the Baťa Channel). According to both historical maps and sediment records, there was no dramatic change in the sedimentation regime in segments A and B of the current channel belt since the 17th century. The most relevant recent changes of the Morava River occurred in the 20th century due to the introduction of flood defences. They limit the width of the inundated area considerably (Fig. 1). The sedimentation must reflect

The occurrence of avulsions within the smaller channels of the original anastomosing Morava River probably reflects their greater risk to the effects of hydrological extremes, because such narrow and shallow channels are more easily blocked by falling trees during floods (Lewin, 2010). The mechanism of avulsions in aggrading rivers caused by crevasse splays and water flow redirecting toward the former distal floodplain areas (backswamps) could have been triggered by extreme discharge events. Because of the commencement of systematic hydrological records on the Morava River in 1881 (Brázdil et al., 2011, in press), highresolution flood information from the pre-instrumental period can be obtained only from secondary, documentary data (Brázdil et al., 2006). Although the oldest recorded flood of the Morava River is from August 1500, an acceptable flood chronology is only available from AD 1691 (Brázdil et al., 2005, 2011). This means that the flood frequency for the previous period has to be taken from other Central European rivers. Even in other rivers, the records from 12th–14th centuries are not covered by systematic flood data. Old Bohemian documentary sources record seven floods of the Vltava (Moldau) River in Prague between AD 1250 and 1281 and four floods between AD 1315 and 1321 (Brázdil et al., 2005), but for the greater part of the Medieval time no other data are available. Missing documentary records could be substituted by sedimentary archives: Careful statistical analysis of Great Britain, Spanish, and Polish fluvial archives revealed a systematic statistically significant increase of abrupt modification of sediment styles at 570, 660, and 860–880 year BP (Macklin et al., 2006), i.e. in 15th, 14th, and 12th centuries AD. In Polish rivers the 12th century peak of flooding activity was by far the most prominent in the last millennium. These exceptional floods in Vltava River as well as fluvial activity maxima in Polish Rivers roughly correspond with the Morava River reorganisation in the studied area estimated to have occurred in the 13th century, assuming uncertainty of dating sediment profiles in the studied area. A study of the frequency and severity of floods in Central European rivers during the 16th century showed their prevalence during the second half of the century (Brázdil et al., 1999). This same tendency was confirmed by the River Traun floods at Wels (Upper Austria) recorded since AD 1441, particularly for the summer (Rohr, 2006, 2007) as well as for several rivers with good data series in Germany such as the Pegnitz at Nuremberg or the Main (Glaser, 2001, 2008). Also Mudelsee et al. (2003) found a maximum in the rate of flooding of the Middle Elbe in the 16th century, presumably in its latter half and better expressed for the summer than for the winter floods. More frequent summer floods show importance of spells of heavy precipitation for their origin. In the context of the past 500 years this period of higher flood frequency was specified to 1540–1600 AD (Glaser et al., 2010). Of course, it does not mean that similar periods of higher flood frequency were missing after 1600 AD (see e.g. Brázdil et al., 2005; Glaser et al., 2010), but the coincidence of the 16th century

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extreme and prominent reorganisation of the Morava River channel pattern documented by sedimentary records is noteworthy. 5.5. General evaluation The correlateable overbank siliciclastic sediments deposited before 700 AD have yet not been found in Strážnice area. This might be either because the negligible deposition of siliciclastics before this date, but more likely because they have not been preserved over a sufficient area of the floodplain because of low preservation potential of the actual floodplain. The latter could be a consequence of changing ratios of deposition and erosion of floodplain sediments, i.e. aggradation or incision, as has previously been described in the Upper Rhine Graben (Erkens et al., 2009) and further German river valleys (Schirmer, 1995; Schirmer et al., 2005). The floodplain of Morava River has most probably mosaic terrace pattern in Schirmer's terminology, with remnants of older deposition stages in the form of row or fill-in-fill terraces in terms of Schirmer (1995), past meander belts in terms of Aslan and Autin (1999) or paleo-meanders in terms of Erkens et al. (2010). Section M130 (Fig. 5) confirms this hypothesis and indicates that the overbank fines were indeed already being deposited a few millennia ago. This conclusion is also in agreement with previous reports on the Morava floodplain (Havlíček and Smolíková, 1994) from other parts of lower reach of Morava River. Very substantial, either linear or even exponential increase in deposition of siliclastics in floodplains during the last one or two millennia has recently been reported by Notebaert and Verstraeten (2010), Macklin et al. (2010) and Lewin (2010), who reviewed studies on European rivers. The results obtained in Strážnice Area are in noteworthy contradiction with these studies. The reason of this difference is not clear with respect to the intense agricultural practises in south east Moravia during the last 1300 years. Perhaps the land use practises in Moravia do not have so strong erosional impact as in other European watersheds studied so far. 6. Conclusions Floodplain sediments are an excellent archive of changes to the Morava River in the Strážnice area, from which the period of the last 1300 years has yet been exploited. Proxy analyses of the sediment lithology combined with basic sedimentological examination in the field and spatial relations of the lithological units allows their essential facies assignment. In addition to 14C dating, the base of industrial contamination in floodplain fines provides a chronostratigraphic date point across the studied area and allows an evaluation of net aggradation rate in the floodplain during the 20th century. The main changes in the sediment fill can be attributed to avulsion and reorganisations of the past channel system likely due to climatic extremes, which are inherent to a system with such a large net aggradation of silty-clayey sediments. The obtained net aggradation rate has not increased dramatically during the Medieval or modern periods, which is in contradiction with findings in other European floodplains. Acknowledgement The field work, pollen and diatom analysis, geography study and C dating were funded by project IAAX00130801 (Grant Agency of the Academy of Sciences of the Czech Republic) and geochemical analyses were partly performed thanks to institutional funding of Institute of Inorganic Chemistry AS CR, project No. AV0Z40320502. The work would have never been performed without enthusiastic and inspiring work of Jaroslav Kadlec (Geological Institute AS CR) and his willingness to share the gathered results of 14C dating. PW1 core has been kindly retrieved by Filip Stehlík from Geological Institute AS CR. Sample processing in laboratory and laboratory analyses (cation exchange by [Cu(trien)]2+ and EDXRF element analyses) have been 14

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