Sediment accumulation rates and high-resolution stratigraphy of recent fluvial suspension deposits in various fluvial settings, Morava River catchment area, Czech Republic

    Sediment accumulation rates and high-resolution stratigraphy of recent fluvial suspension deposits in various fluvial settings, Morava River catchment area, Czech Republic Jan Sedl´acˇ ek, Ondˇrej B´abek, Ondˇrej Kielar PII: DOI: Reference:

S0169-555X(15)30208-7 doi: 10.1016/j.geomorph.2015.11.011 GEOMOR 5441

To appear in:

Geomorphology

Received date: Revised date: Accepted date:

13 July 2015 20 November 2015 21 November 2015

Please cite this article as: Sedl´aˇcek, Jan, B´ abek, Ondˇrej, Kielar, Ondˇrej, Sediment accumulation rates and high-resolution stratigraphy of recent fluvial suspension deposits in various fluvial settings, Morava River catchment area, Czech Republic, Geomorphology (2015), doi: 10.1016/j.geomorph.2015.11.011

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ACCEPTED MANUSCRIPT Sediment accumulation rates and high-resolution stratigraphy of recent fluvial suspension deposits in various fluvial settings, Morava River catchment area, Czech

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Republic

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Jan Sedláček a*, Ondřej Bábek a,b, Ondřej Kielar

Department of Geology, Palacký University of Olomouc, Tř. 17 listopadu 12, 77146 Olomouc,

Czech Republic

Department of Geological Sciences, Masaryk University, Kotlářská 2, 61137 Brno, Czech Republic

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* Corresponding author ([email protected], tel. +420 585 634 539)

Abstract

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We present a comprehensive study concerning sedimentary processes in fluvial sediment traps within the Morava River catchment area (Czech Republic) involving three dammed

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reservoirs, four meanders and oxbow lakes, and several natural floodplain sites. The objective of the study was to determine sediment accumulation rates (SAR), estimate erosion rates, 137

Cs method and historical data. Another

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calculating these using a combination of the

purpose of this study was to provide insight into changing erosion and accumulation rates over the last century. Extensive water course modifications were carried out in the Morava River catchment area during the twentieth century, which likely affected sedimentation rates along the river course. Other multiproxy stratigraphic methods (X-ray densitometry, magnetic susceptibility, and visible-light reflectance spectrometry) were applied to obtain additional information about sediment infill. Sediment stratigraphy revealed distinct distal-to-proximal patterns, especially in reservoirs. Granulometrically, silts and sandy silts prevailed in sediments. Oxbow lakes and meanders contained larger amounts of clay and organic matter, which is the main difference between them and reservoirs. Pronounced

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Cs peaks were

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ACCEPTED MANUSCRIPT recorded in all studied cores (maximum 377 Bq.kg−1), thus indicating Chernobyl fallout from 1986 or older events. Calculated sediment accumulation rates were lowest in distal parts of

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reservoirs (0.13–0.58 cm/y) and floodplains (0.45–0.88 cm/y), moderately high rates were

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found in proximal parts of reservoirs and oxbow lakes (2.27–4.4 cm/y), and the highest rates

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in some oxbow lakes located near the river (6–8 cm/y). The frequency of the inundation still can be high in some natural areas as in the Litovelské Pomoraví protected area, whereas the decreasing frequency of the inundation in other modified parts can contribute to a lower

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sedimentation rate. The local effects such as difference between SARs in oxbow lakes and

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reservoirs, different grain size distribution in both systems, and high variability in thickness of their proximal and distal parts play a crucial role in the analysis of regional accumulation rates. Local effects are much stronger than regional effects, such as rainfall and land use.

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Combined with the low resolution of time scales (usually only three datums are available: reservoir construction datum,

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Cs fallout event, and top of sediment), these effects may

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obscure the general trends of regionally increasing or decreasing net SARs, making the analysis of erosion rates from the sedimentary record an extremely difficult task.

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Keywords:

reservoirs; oxbow lakes; floodplain; 137Cs dating; sediment accumulation rates; trap efficiency 1. Introduction Human activity creates new sedimentation sites and influences or modifies the existing environment. Extensive changes can also be observed in fluvial systems. Human landscape modifications can be traced thousands of years back but, especially in the twentieth century, the pace of these changes was particularly high. Sedimentary records of such fluvial depositional systems as water reservoirs, oxbow lakes, and floodplains can be used to provide information about past processes in river basins, including sedimentation rates and erosion 2

ACCEPTED MANUSCRIPT rates within the catchment (Zolitschka, 1998; Owens et al., 1999; Lang et al., 2003; Shotbolt et al., 2005).

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In general, increased erosion rates caused by human activities have been detected in many

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catchments over recent decades (McCully, 1996; Morris and Fan, 1997; Batuca and Jordaan,

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2000; McCartney, 2009). Accelerated erosion may be caused by deforestation and intense agriculture activities (Kadlec et al., 2009; Pánek et al., 2013), such as during and after collectivization following World War II, which dramatically influenced landscape patterns

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and accelerated erosion rates in central and eastern Europe (Van Rompaey et al., 2003; Krása et al., 2005). The post-World War II period was the era of the most significant landscape

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changes in former Czechoslovakia (Bičík et al., 2001). The processes of collectivisation during the communist era transformed the original mosaic of small fields of arable land to the

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landscape structures composed of large parcels (Van Rompaey et al., 2003; Cebecauer and Hofierka, 2008). In many cases, land degradation was even more accelerated by the

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introduction of new unsuitable crops (Van Rompaey et al., 2003). On the other hand, reverse trends presumably affecting erosion rates, such as reforestation, have been reported from the

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hilly regions of the Carpathians in the eastern parts of the Czech Republic (Stacke et al., 2014). Another major change in land use and therefore erosion rates occured after the fall of the Iron Curtain in 1989. This change applied mainly to post-communist countries, including the former Czechoslovakia. The period after the 1990s is characterized by a decrease in the size of agricultural and arable land. On the other hand, the extent of meadows and pastures began to increase caused by some changes in agricultural management (Bičík et al., 2001). A common method to assess erosion rates within a watershed is to analyse present-day sedimentation rates in natural traps (Dendy and Boulton, 1976; Ritchie et al., 1983; Zolitschka, 1998; Davidson et al., 2004). Sediment load is of course not constant. Rather, it varies in space and time and is controlled especially by river discharge and also by sediment

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ACCEPTED MANUSCRIPT availability itself, especially in such regulated basins with a large percentage of embanked channels where most of the suspended sediment are produced from the soil erosion on basin

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hillslopes rather than from within the river channels. The highest loads are achieved during

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major floods, which, consequently, may cause rapid loss of storage capacity, especially in

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small reservoirs (Carvalho et al., 2000).

Natural erosion is highly variable; main factors controlling the erosion rate include in particular basin relief characteristics and runoff variability (Summerfield and Hulton 1994).

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The basic factors, which control erosion and sediment transport in the catchment area, are

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numerous and include, in particular, local geology, topography, climate, type of vegetation, size of the catchment area, and human land use (Morris and Fan, 1997; Bell, 1998; Verstraeten and Poesen, 2001). Sediment fluxes are sensitive to local land use changes in

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small catchments, while river sediments show rather regional trends and climate changes (Dotterweich, 2008). Many large river courses were modified. Flood management requires a

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number of engineering activities, which led to a change in frequency of the inundations and therefore influenced the sedimentation rates (Hudson et al., 2008).

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Artificial lakes and abandoned meanders represent ideal traps for suspended sediments along regulated water courses, providing important information about the present-day and near-past geomorphic behaviour of densely populated fluvial landscapes. The highest temporal resolution may be derived from lake and reservoir sediments. Owing to continuous sediment supply, they store signals of land use and erosion changes in decadal to seasonal resolution (Dearing 1991; Zolitschka, 1998; Dhivert et al., 2015). Moreover, oxbow lakes and floodplains are localised often in lowland areas, the greater amount of trapped sediments is expected there due to anthropogenic induced erosion. Small oxbow lakes, in particular, can be sensitive to changes in land use (Schneider et al., 2007).

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ACCEPTED MANUSCRIPT The amount of settled sediment in dammed reservoirs depends on the magnitude of the incoming sediment load and the trap efficiency, which means the percentage of sediment load

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settling in the trap while the remaining part escapes (Siyam et al., 2005). Abandoned

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meanders and oxbow lakes constitute another important element of river systems with

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significant sediment trap efficiency. A large proportion of sediment supply in oxbow lakes is again driven by flood discharge (Gasiorowski and Hercman, 2005; Bábek et al., 2008), with autochthonous sediment because of high organic productivity contributing to generally high

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sedimentation rates in abandoned meanders. High local sedimentation rates may provide finer

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temporal resolution of the resulting stratigraphic record (Foster et al., 1991; Shotbolt et al., 2001; Smol, 2008). As would be expected, the trap efficiency of abandoned meanders is lower than that of dam reservoirs inasmuch as they either acquire only a part of the discharge

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flowing in adjacent channels (semi-open meanders) or their sediment load is episodic and related to bank overflow during floods (oxbow lakes). However, little is known about the trap

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efficiency of oxbow lakes, and a comparative study with dammed reservoirs is needed. River floodplains comprise another geomorphic element in fluvial systems serving as

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sedimentary archive and providing information about the history of river catchment (e.g., Walling and He, 1998; Macklin et al., 2006; Phillips et al., 2007; Kadlec et al., 2009; Matys Grygar et al., 2011; Stacke et al., 2014). In contrast to reservoirs, sediment accumulation in floodplains is episodic and therefore prone to hiatuses, sediment alteration, and material uptake during pedogenesis (Sadler, 1981; Matys Grygar et al., 2011). Proximal-to-distal trends in floodplains, frequency of flood events, and the size of the inundation area all contribute to highly variable sedimentation rates in floodplains (Knox, 2006). In addition, river embankment has been documented to reduce the inundation area and, hence, sediment accumulation rates in floodplains (Grygar et al., 2010). Nevertheless, floodplains are much more widespread than reservoirs and oxbow lakes and often provide the only available source

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ACCEPTED MANUSCRIPT of stratigraphic information (Owens et al., 1999). Accelerated floodplain sedimentation rate was described worldwide from human-generated land use changes (e.g., Knox, 2006; Gell et

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al., 2009; Hoffmann et al., 2009). Consequently, assessment of regional erosion rates from the

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local sedimentary record can be connected with considerable error caused by local factors,

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and there is a need for deeper insight into local variability in sediment accumulation in different fluvial settings.

Reliable dating tools and depth–age profiles are critical in establishing chronology of

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fluvial archives, which are often based on the identification of important trends and events

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recorded in the sediments (Smol, 2002). Depositional events in reservoirs and oxbow lakes may serve as important correlation lines tied to historical events. In particular, this involves the onset of lacustrine sedimentation, which can be easily identified in sedimentary and

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historical records (Shotbolt et al., 2005; Nehyba et al., 2011; Sedláček et al., 2013). Dredging reservoir and abandoned meander sediments provide another type of timeline (Bábek et al.,

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2008; Brázdil et al., 2011). Among the most important dating tools is the 137Cs dating method. This has been widely used for studying erosion and sedimentation in many different

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depositional environments worldwide (DeLaune et al., 1978; Campbell, 1983; Walling and Bradley, 1989; Ritchie and McHenry, 1990). The

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Cs dating method is well constrained for

sediments with little post-depositional changes, reworking and bioturbation, in particular reservoir lakes and oxbow lakes. However, secondary caesium migration (Ciszewski et al., 2008) may affect the interpretation of

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Cs mass activities in floodplains. Comparative

studies of 137Cs distribution in various depositional fluvial environments are rare and a deeper insight can be helpful in 137Cs age interpretation of complex depositional systems. Several studies concerning fluvial systems were conducted in the Morava River catchment area. In our previous paper, we documented stratigraphy and pollution history in the Brno reservoir (Sedláček et al., 2013) — the research in the same reservoir published by Nehyba et

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ACCEPTED MANUSCRIPT al. (2011). In previous years, we have published two detailed studies about Certak oxbow lake (Bábek et al., 2008, 2011). Some authors focused on the Strážnické Pomoraví floodplain area

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(lower course of the Morava River). Brázdil et al. (2011) pointed out some floodplain changes

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over the past 130 years. Caused by some anthropogenic modifications of the river channel, the

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frequency and extent of floodplain inundation decreased. Kadlec et al. (2009) recognized several important periods recorded in floodplain sediments from the mediaeval warm period (overbank clay sediments), through the Little Ice Age (coarser sediments) to the period of

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increased floodplain sedimentation in the twentieth century. Grygar et al. (2010, 2011) used geochemical tools to determine the age of sediments and stratigraphic correlation. Stacke et

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al. (2014) described late Holocene evolution of the Bečva River floodplain (left tributary of the Morava River). During the twentieth century, massive reforestation and channel

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modifications resulted in a significant incision phase. The main aim of this paper is to provide an insight into local sediment accumulation rates

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in different fluvial settings and to assess the effects of local depositional settings on the regional sediment erosion-accumulation balance. The study is primarily based on 137Cs dating

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of various parts of reservoirs, abandoned meanders, and floodplains in the catchment of the River Morava, Czech Republic, a left-hand tributary to the River Danube. As by-products of this effort, we investigate the effects of proximality-distality in various sediment traps, the influence of artificial river embankment on sediment accumulation rates in newly-formed oxbow lakes and natural floodplains, and compare

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Cs distribution in those three essential

types of fluvial sediment depocentres. The key period is the second half of the twentieth century — there have been many dramatic land use changes and the goal is to determine how these changes are reflected in the SAR.

2. Geographical settings, hydrology, geology, and history 7

ACCEPTED MANUSCRIPT The study area is located within the Morava River (a left tributary to the Danube River) basin in the eastern part of the Czech Republic (Fig. 1). The catchment area of the river is

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26,658 km² and mean annual river discharge is 110 m3.s−1 close to its confluence with the

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Danube. Monthly discharge is highest in March and April and lowest in September and

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October (Brázdil and Kirchner, 2007). The main tributaries of the Morava River are the Svratka, Jihlava, Dyje, and Bečva rivers. The course of the Morava River was regulated and its length shortened by about 46% during the twentieth century. The initial impulse was to

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protect against floods and ensure constant discharge. But subsequently, flow velocity during floods increased (Brázdil et al., 2011). The catchment areas of individual tributaries are much

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smaller: 118.5 km2 for the Hloučela River catchment, which feeds the Plumlov reservoir; 1586.2 km2 for the Svratka River, which feeds the Brno reservoir; 4599.3 km2 for the Dyje

Nové Mlýny reservoir.

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catchment; and 11,731.4 km2 for the Svratka and Dyje catchments, which together feed the

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The catchment area is predominantly used for agriculture. The largest area is covered by arable land (42% of total), followed by forests (37%) and grassland (11%) (data for 2002–

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2006; Brázdil et al., 2011). Industry is concentrated mainly along the Morava and Svratka rivers together with several large- and medium-sized towns (Brno, Olomouc, Kroměříž) and a large number of small villages. The upper courses of the Morava River drain mainly metamorphic and igneous rocks of the Silesicum and Lugicum units of the Bohemian Massif and lower Carboniferous sandstones and shales of the Moravo–Silesian Culm Bbasin (Cháb et al., 2010). Downstream, the Morava River and its left tributaries drain the sandstones and shales of the Outer Western Carpathians. The Svratka, Jihlava, and Dyje rivers predominantly drain metamorphic and acid igneous rocks of the Moldanubicum, Moravicum, and Brunovistulicum units. The Hloučela River drains the sandstones and shales of the Moravo– Silesian Culm basin (Cháb et al., 2010). These bedrock units are overlain by unconsolidated

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ACCEPTED MANUSCRIPT Miocene marine deposits and Pliocene and Quaternary fluvial and lacustrine sediments. The surface is covered by various types of soils. Half of the area covers Cambisols and Podzols

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(hilly regions), followed by Chernozems (lowlands), brown soils, Fluvisols, and Luvisols.

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Three reservoirs (represented by 16 cores) were selected for this study: the Brno, Nové

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Mlýny, and Plumlov reservoirs, located on the Morava River tributaries (Table 1). There are no reservoirs on the Morava River itself, but this river has many natural and artificial meanders and oxbow lakes, including the four selected for this study (represented by seven

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cores): the Moravičany and Kurfürst meanders and the Zvole and Čerťák oxbow lakes. Three

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additional cores were taken from floodplains of the Litovelské Pomoraví protected landscape area, where the natural floodplain of the Morava River is preserved without anthropogenic regulation. All sites are shown in Fig. 1.

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The Brno reservoir was constructed during 1936–1939 and filled in 1940 (Bayer et al., 1954). This valley-type reservoir is 10 km long and 800 m wide, with maximum depth of 18

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m near the dam (49°13'56.45"N, 16°31'8.53"E; WGS84). Its main tributary is the Svratka River with an annual average discharge of 7.68 m3s−1. The reservoir is long and narrow in its

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proximal part and relatively broad near the dam. The Nové Mlýny reservoir consists of three interconnected basins that were built in several steps during 1978 (upper basin), 1981 (middle basin), and 1989 (lower basin; 48°51'29.7"N, 16°43'20.34"'79450E; WGS84 at the lower basin dam). The basins are supplied by three rivers: the Dyje, which supplies the upper basin; and the Svratka and Jihlava, which have their confluence at and flow into the middle basin. The area of the reservoir is 32.32 km2, and because of its location in the lowlands it is very shallow with a maximum depth of 6 m. The Plumlov reservoir (49°28'11.79"N, 17°2'17.64"E; WGS84 at the centre of the dam) is small (0.68 km2) and relatively shallow (maximum depth of 12 m near the dam), but it has one of the oldest dams in the Morava River catchment. The

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ACCEPTED MANUSCRIPT reservoir is supplied from the Hloučela River, and it was put into operation in 1936. The dam was discharged and dredged during 2010–2011 (Table 1).

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More than 180 oxbow lakes, most of them artificial, can be found along the Morava River.

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The river was embanked in the first half of the twentieth century and the main channel was

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straightened and shortened, thus giving rise to a number of artificially abandoned meanders (Bábek et al., 2008). So, Morava River is a typical example of a meandering river that has been modified by regulation (Miřijovský et al., 2015). Several natural oxbow lakes, some of

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them still communicating with the river channel, are located in two protected landscape areas,

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‘Litovelské Pomoraví’ and ‘Strážnické Pomoraví’, where the naturally meandering character of the river is still preserved. The abandoned artificial oxbow lake near Zvole (49°50'23'' N, 16°55'40'' E; WGS84) was separated from the main channel in 1972 following the river

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embankment. Partly communicating with the river at its southern end (49°44'57''N, 16°58'53'' E; WGS84), the Moravičany meander is an artificial, semi-open meander. It was built in the

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1930s following the river embankment. The Kurfürst meander (49°39'42'' N 17°12'43'' E; WGS84) is a natural semi-open meander located in the southern part of the Litovelské

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Pomoraví. A part of the meander was separated from the active channel in the 1950s and 1960s and subsequently completely filled with sediment. Another part of the meander is still partly connected with the Morava River channel. Sediments of this active part were dredged in 1993 (Krejčí, 2009). The Čerťák oxbow lake (49°04'09''N, 17°26'08''E; WGS84), located near the town of Uherské Hradiště, is an artificial oxbow lake, built in the 1930s and later dredged in 1981 (Bábek et al., 2008). Natural floodplains with floodplain forests are preserved in the Litovelské Pomoraví protected landscape area, a 5–6 km wide and 30 km long zone located along the nonregulated course of the Morava River, north of the city of Olomouc. Various parts of the area are periodically inundated during floods.

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ACCEPTED MANUSCRIPT 3. Materials and methods Twenty–five sediment cores (Fig. 2) were collected from the three representative fluvial

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settings. Sixteen cores were collected from reservoirs, six from oxbow lakes, and three from

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floodplains. In our sampling strategy, we tried to select the samples from different parts of

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reservoirs/oxbow lakes in order to determine how sediments differ. Underwater, cores were recovered into transparent sampling tubes 1 m long and with 4 cm inner diameter using a rodoperated multisampler piston corer (Eijkelkamp, the Netherlands). On dry land, the cores

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were taken into foil liners 1 m long and with 4.5 cm diameter using a percussion drilling set

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(Eijkelkamp, the Netherlands). Several stratigraphic proxy methods were applied for better insight into sediment properties. The aim was to support our conclusions regarding sedimentation rate and sedimentation pattern. These methods have been used successfully in

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our previous studies (Bábek et al., 2008, 2010; Sedláček et al., 2013). X-ray densitometry was measured from X-ray radiographs in order to visualise sediment composition and structures

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(selected cores). All cores were cut lengthwise, described based on visual observation, photographed, and then sampled with 1- or 0.5-cm vertical intervals, depending on required

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resolution. Floodplain samples were taken directly in the field from shallow (<100-cm) pits dug out on dry land. The samples were air-dried at 50°C and stored in plastic bags. Samples from 13 cores and 3 floodplain pits were analysed for

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Cs mass activity with vertical

intervals of 3 cm (floodplain samples) and 4 cm (four pooled samples from sediment cores) using a PCAP laboratory gamma-ray spectrometer (Nucleus, USA) with an NaI (Tl) scintillation detector (detection limit 6 Bq.kg−1) and 30-m measurement time. Results were expressed as depth distribution in 137Cs mass activity and used to identify 137Cs fallout events. Sediment profiles were dated based on the 1954, 1963, and 1986 peaks. Grain size distribution of selected sediment samples was analysed by wet sieving and the under-sieve fraction <63 μm subsequently by laser particle sizer (CILAS 1064). Results were

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ACCEPTED MANUSCRIPT recalculated to sand, silt, and clay content. Total organic carbon (TOC) was measured for selected samples using a liquiTOC Vario cube (Elementar Analysensysteme, Germany;

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analytic precision is <1% TOC at >5 mg/L C).

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The colour of dry samples was measured by visible-light diffuse reflectance spectroscopy

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(Vis DRS) using a hand-held X-Rite SP62 sphere spectrophotometer (X-Rite, USA) with a gas-filled tungsten lamp light source, d/8 measurement geometry, and 8 mm aperture. Output data were represented as CIEL*a*b* colour data and percent reflectance relative to the white

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standard (white ceramic plate) at 400–700 nm wavelength with readings taken at 10-nm

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increments. Colour was measured on a dry, fresh, and flat surface of fine-ground sediment powder. Low-field magnetic susceptibility (MS) was measured using a KLY-4S Kappabridge apparatus (Agico, Czech Republic) with magnetic field intensity of 300 Am −1, operating

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frequency of 920 Hz, and sensitivity of 3.10−8 SI. Mass-specific data expressed in m3kg−1

4. Results

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were used.

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4.1. Lithology and stratigraphy of the cores Five cores were taken from the Brno reservoir: two cores from proximal, one core from middle, and two cores from distal parts (Fig. 2). The thicknesses of reservoir sediments varied from 2.15 m in proximal parts to only 8 cm in distal parts. The sediments in proximal parts of the reservoir (cores BP1 and BP2) revealed a distinct lamination of fine- and coarse-grained laminae of silt and sand (thickness about 0.2–1 cm). Core BP3 revealed no such lamination, except for its lower parts having faint silty and sandy layers. Granulometrically, these lacustrine sediments are comprised mainly from silts and sandy silts, with occasional silty sands and clayey silts (Fig. 4). Homogenous, clayey silts were recovered from the distal lake parts near the dam (cores BP4 and BP5). According to grain size analysis, sediments can be

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ACCEPTED MANUSCRIPT classified as sandy silts with a predominance of silt fraction (range 52.15–88%), followed by sand (range 12.46–46.79%), and a minor admixture of clay. Overall, larger amounts of clay

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appeared especially in the proximal parts. Sediments contained variable TOC percentages,

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from 2.2% to 7.7%.

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Relatively dense prelacustrine, compacted sands, silty sands, and silty-clayey sands with sparse gravel grains are preserved in the bottom parts of the cores, separated from the overlying lacustrine sediments by sharp grain size and colour boundary. These sediments had

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been deposited before the construction of the dam, presumably in an alluvial or colluvial

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environment (Sedláček et al., 2013).

Eight cores were taken from the Nové Mlýny reservoir (Fig. 2). The maximum observed depth of reservoir sediments was 70 cm in the middle basin (core NM5) and 55 cm in the

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upper basin (core NM1). Sediments (sandy silts or silty sands) revealed a homogeneous character, but faintly visible sandy or vice versa finer (more clayey) laminae can be observed

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in some parts. Silty fractions prevailed in the sediments (range 42–85%; eight analyses), sometimes with high sand content (range 8–56%), and especially in subsurface layers, which

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suggests slightly upward coarsening trends. Sediments contained variable contents of organic matter and phytodetritus (TOC values between 1% and 5.48%), except for one extreme value of 13.7% (core NM4, depth 22–25 cm). The underlying prelacustrine unit consists of older fluvial and floodplain sediments composed of light to medium brown loamy silts and loamy sands with rare roots and plant remains. Three cores were taken from the Plumlov reservoir (Fig. 2). The longest of these had a length of 80 cm and onset of dam sedimentation was not obtained. Light brown and brownblack silts to sandy silts prevailed in the reservoir. Lamination was absent, and transitions into sandy coarser layers were gradual. Overall, silt was the predominant fraction (range 63.25– 86.11%; four analyses), but its content decreased at the expense of sand, with the maximum

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ACCEPTED MANUSCRIPT of sand occurring in subsurface layers (range 10.7–34.6%). Thus, a slightly coarsening upward trend was detected. Organic rich layers were indicated by black colour, but TOC

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values were low and characterized by a slow, long-term increasing trend (range 1.1–3.65%;

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16 analyses).

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All studied oxbow lakes and meanders (Fig. 2) revealed similar trends in lithology and sediment infill. Sediments were composed mainly of a silty fraction with variable contents of sandy fraction and common organic residues. In the Zvole oxbow lake, coarser laminae were

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rarely evident from flood events. The high content of organic matter suggests high productivity. Some layers contained partially decomposed organic matter, as well, whose

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occurrence may have been caused by changes in redox conditions. In the Moravičany meander, sediments were primarily fine-grained with abundant organic residues and

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phytodetritus. Here, three grain size analyses demonstrated the predominance of silt (range 87–89%) with a higher admixture of clay (range 10–13%) and rare sandy fractions (1%).

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Lamination was partially developed in certain horizons, but otherwise sediments revealed a homogeneous character. Two cores were taken in the Kurfürst meander. One core was taken

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from the active part of the meander and one core from the dry, completely silted part. Sediments in the active part represented a sedimentary record since 1993, when older sediments had been removed due to rapid silting of the meander. Silty sediments alternated with fine-grained sands. In general, silt prevailed in the sediments (range 80–90%). The contents of sand (range 0–10%) and clay (range 8–15%) were variable. Lamination was well developed in some parts, with laminae showing variable contents of organic matter and phytodetritus. Core CH4 displayed higher content of clay fraction (on the silted part of the meander). In the Čerťák lake three cores were taken, designated CRT1–CRT3. Basal sequence was composed of massive sandy silts with rare silty sand layers. Silty fraction appeared to be

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ACCEPTED MANUSCRIPT predominant; sandy admixture consisted of relatively well-sorted grains as well as rare organic remains. The overlying sequence consisted of alternating silty and sandy layers and

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the uppermost sequence was composed of silty loam with laminae of sandy silts rich in plant

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remains and coal matter.

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The depth of drilling into floodplain sediments was various (maximum 1.8 m). Two sites (cores L1 and H2) were located near the river and one site was located ~800 m from the main channel (core CH1). Floodplain sediments consisted of homogeneous clayey silts to sandy

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silts with little admixture of sand and rarely very fine-grained gravel. Sediments contained

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abundant brownish concretions of Fe oxy-hydroxides and sometimes muscovite flakes. Organic plant remains were common, especially in the upper and subsurface layers. The upper part of floodplain deposits usually hosted a well-developed, dark brown soil layer, which was

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usually ca. 20–35 cm thick and sometimes with visible gleyization. 4.2. Vertical patterns of magnetic susceptibility and sediment colour

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Each slide of all cores was subjected to stratigraphic proxy methods. Magnetic susceptibility (MS) of Brno reservoir sediments was relatively low (0.9–8.5 x 10−7 m3 kg−1).

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A major break in MS coincided with the boundary between prelacustrine colluvial/alluvial sediments and younger lacustrine sediments at depths of 2.15 m (cores BP1 and BP2), 2.0 m (BP3), and 0.15 m (cores BP4 and BP5). Above this boundary, a distinct fluctuation in MS can be observed, which corresponds to the alternation of coarse-grained and fine-grained laminae in the lacustrine sedimentary record (cores BP1 and BP2). The colour data (CIE L* and percentage of red reflectance) displayed similar patterns as did MS, which is to say a major break at the prelacustrine/lacustrine boundary and fluctuation in the lacustrine record due to fine lamination. The MS values at the Nové Mlýny reservoir varied within a wide range because of different inputs (1.08–8.11 x 10−7 m3 kg−1). An example of using proxy methods (core NM1, 15

ACCEPTED MANUSCRIPT Nové Mlýny reservoir) is shown in Fig. 3. The MS corresponded partly to the grain size variations, CIE L* data and TOC (Fig. 3). Minimal deflections of MS can be found especially

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in distal parts of the middle basin, which was supported by the uniform pattern of colour data.

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There is good correspondence in the MS signal in the upper basin (cores NM1 and NM2).

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The MS values at the Plumlov reservoir were relatively low for the bottom parts of cores ~70–65 cm beneath the surface (2.34–2.73 x 10−7 m3 kg−1). These were slowly increasing when moving upward in the core and reached a maximum at depths of 38–31 cm (2.94–4.19 x

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data) and was independent of grain size.

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10−7 m3 kg−1). The MS signal did not correspond to other proxy parameters (sediment colour

The MS signal partly reflected grain size changes in oxbow lakes and meanders. The MS tended to be without strong fluctuation in cores with homogenous characters, which was

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typical for the Zvole oxbow lake (mean MS value of 3.24 x 10−5 m3 kg−1). Faint fluctuations could be found in the Kurfürst meander, especially in the upper parts, and reflected changes in

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coarse silt and fine sand proportions. Mean value was 3.02 x 10−5 m3 kg−1. Local colour changes were influenced by organic matter content. A similar pattern can be found in Moravičany, where the MS signal displayed a more sawtooth pattern and mean value of 3.26

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x 10−5 x m3 kg−1. Variations in sand-sized fraction was the main factor affecting MS also at the Čerťák lake. Sandy layers tended to have high values of CIE L*, relatively high values of CIEa* (red hues), and lower MS values caused by the higher content of diamagnetic minerals (quartz and feldspars).

4.3. Mass activities and distribution of 137Cs Distinct peaks in mass activity of anthropogenic

137

Cs were observed in all studied cores

(Table 3). Peak values in individual cores or soil profiles varied from ca. 50 to nearly 400 Bq.kg−1 (the highest 137Cs value of 377 Bq.kg−1 was detected at the Brno reservoir), while the

16

ACCEPTED MANUSCRIPT normal background values found in deeper parts of the sediment profiles were typically below the instrument’s detection limit (6 Bq.kg−1). Peaks were mostly associated with a rapid

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increase followed by a sharp peak and slower decrease upward in the reservoirs and

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abandoned meanders. This pattern indicated more or less continuous sedimentation whereby

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the contaminated sediment was washed from the catchment and deposited in the lakes immediately after the fallout event. Selected depth profiles of

137

Cs mass activity are shown in Fig. 4. Multiple peaks were

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found in Brno reservoir cores BP1 and BP2 (similar depth pattern of

137

Cs profiles), which

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can be interpreted in part as fallout events (see section 5.1) and partly as artefacts caused by sediment contamination during recovery of the second, reintroduced core (see discussion in Bábek et al., 2008). Distinct 137Cs profiles were also obtained from the Nové Mlýny reservoir

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in the upper basin and proximal part of the middle basin, with one pronounced peak in each

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core and with maximum values of 90.2 and 127.8 Bq.kg−1. The depth positions of peaks were

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somewhat shallower than those in the Brno reservoir, typically 40–44 cm (upper basin) and 24–28 cm (middle basin). One pronounced peak was detected in each core of the Plumlov reservoir, located at depths of 32–36 cm (core P2), 16–20 cm (P1), and 8–12 cm (P3). Peak

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values exceeded 100 Bq.kg−1 in all three cores. Similar vertical patterns of

137

Cs mass activity were found in the abandoned meanders.

The maximum value of 318 Bq.kg−1 was measured in the Zvole oxbow lake (core ZV1) at a depth of 31 cm, with a subsequent rapid decrease of mass activity upward. Several 137Cs mass activity maxima were detected in the Kurfürst meander (core CH4). Minor peaks (10 and 10.4 Bq.kg−1 at depths of 162 and 149 cm) could be found at the core’s base, followed by a minor peak (54 Bq.kg−1) located at a depth of 70 cm and another major peak (74.2 Bq.kg−1) at a depth of 23 cm. One pronounced peak was found in the Moravičany meander (core MOR1) with maximum mass activity of 135.6 Bq.kg−1 (depth of 38 cm) followed by fluctuating

17

ACCEPTED MANUSCRIPT activity values but a generally decreasing upward trend. Two cores (CRT2 and CRT3) were measured at the Čerťák oxbow lake, with one distinct peak located at depths of 140 cm (core CRT2) and of 150 cm (core CRT3). The maximum mass activity measured in the Čerťák

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oxbow lake was 254 Bq.kg−1 (core CRT3). Another minor peak was found in core CRT2

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(depths of 204 to 216 cm) but this is interpreted as an artefact caused by sediment washed down during sequential drilling (as in the case of the Brno reservoir). For more discussion about 137Cs dating at this location, see Bábek et al. (2008).

137

137

Cs mass

Cs showed a prominent, sharp peak (93 Bq.kg−1) at depths of

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activities. In the L1 profile,

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All three profiles from floodplains yielded distinct vertical patterns of

6–9 cm, followed by values steadily decreasing upward. Vertical distribution of

137

Cs in the

two remaining sections showed rather diffuse patterns with no sharp peaks. The maxima were 137

Cs and overlain by equally slow decreases up-section;

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underlain by a slow increase in

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maximum values were still high, however, ranging from 50 (core CH1) to 83 Bq.kg−1 (core

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5. Discussion

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H2).

5.1. Age interpretation of the 137Cs record and historical data Several events were recorded in the sediments that can be used for age determination. The onset of lacustrine sedimentation in a reservoir is an important depth level that was observed in a majority of the cores as a first-order lithological boundary, based on lithological description, X-ray images, and depth profiles of sediment colour and MS (Fig. 3). Four dated levels were identified in the cores based on the onset of lacustrine sedimentation (reservoir filling). These ranged from 1940 at Brno to 1989 at Nové Mlýny. Dated dredging levels are associated with similar overturns in lithology and physical properties of the sediment.

18

ACCEPTED MANUSCRIPT Dredging was identified in three cores, ranging in onset from 1981 (Čerťák oxbow lake) to 1993 (Kurfürst meander).

137

Cs mass activity. The oldest

137

Cs dates were found in Brno reservoir sediments,

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of

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The overlying lacustrine sequence was dated in most cores based on the depth distribution

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corresponding to fallout events related to 1954 and the 1962–1963 maximum of aboveground nuclear weapon testing (Oldfield et al., 1983; Appleby, 2001; Heim et al., 2004). The most prominent peak in 137Cs activity is interpreted as the fallout event from the Chernobyl nuclear

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power plant disaster in 1986 (Walling et al., 1999; Appleby, 2001; Heim et al., 2004; Wildi et al., 2004; Bábek et al., 2008; Sedláček et al., 2013). The intensity of this peak is greater than

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that of the weapon testing events, partly because of the coring sites’ rather short distance from the Chernobyl site (ca. 1000 km in a straight line) and partly because of the rather short half-

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life (30.17 y ± 0.03 y) of 137Cs (Ritchie and McHenry, 1990; Appleby, 2001). The 137Cs peaks were sharper and of higher intensity in the lacustrine context (reservoir and abandoned

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meanders) than in floodplains. Similar patterns have been found elsewhere in Europe (Walling and Bradley, 1989; Ritchie and McHenry, 1990; Foster and Walling, 1994; Johnson-

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Pyrtle et al., 2000; Ilus and Saxén, 2005). Lacustrine peaks are linked with direct atmospheric fallout and collection of polluted sediment from riverine runoff immediately after fallout events. The mobility of

137

Cs in water-saturated lacustrine sediment is probably limited, thus

minimizing the effect of post-depositional migration and peak diffusion. There is no relationship between catchment size and

137

Cs supply into the reservoir because the

distribution of radionuclide substances was very irregular and influenced by air currents and rainfall (Appleby et al., 2001). The 137Cs mass activity profiles in floodplains displayed much lower peaks, and these are underlain by slowly and gradually increasing values. This can be attributed in part to the intermittent nature of floodplain sedimentation (because of floods), whereby the contaminated

19

ACCEPTED MANUSCRIPT sediment is redistributed by a series of floods following fallout events (cf. Matys Grygar et 137

al., 2014) and with the amount of redistributed 137

Cs isotope downward in the aerated soil

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addition, secondary mobilization of deposited

Cs depending on the flooded area. In

137

Cs mass activities in

Cs in floodplain sediments can be caused by burrowing soil

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floodplains. Redistribution of

137

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profiles further contributes to the rather diffused peaks and lower

fauna such as earthworms or because of frequent infiltration of flood waters (Ciszewski et al., 2008).

137

Cs mass activity in depth profiles within Europe. One pronounced peak has been

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of

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The results from this study are in a good agreement with the general trends in distribution

described in many studies from environments such as reservoirs, lakes (e.g., Walling and Bradley, 1989; Ritchie and McHenry, 1990; Foster and Walling, 1994; Johnson-Pyrtle et al.,

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2000; Dhivert et al., 2015), and floodplains (He and Walling, 1992; Grygar et al., 2010). In contrast, some studies indicate post-depositional mobility in alluvial and sandy sediments

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(Ciszewski et al., 2008) and in forests soils from microbial activity (Clint et al., 1992; Simkiss, 1993). It is appropriate, therefore, when determining the sedimentation rate using Cs to consider also the sediment grain size characteristics (Hobo et al., 2010).

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137

5.2. Sedimentation rates and age models Calculated average sedimentation rates for all locations are shown in Table 4 and Fig. 5. Sedimentation rates were highly variable within any given location as well as across locations. The minimum values were observed in distal parts of reservoirs: 0.25 cm/y in the Brno reservoir (BP5) and 0.58 cm/ y in the Nové Mlýny reservoir (NM8 in the lower basin). The Nové Mlýny’s lower basin has no significant tributary and therefore much of the sediment is trapped in the middle and upper basins before entering the lower basin. Higher sediment accumulation rates were calculated for reservoirs’ proximal parts: 2.97–3.94 cm/y in

20

ACCEPTED MANUSCRIPT cores BP1 and BP2 in the Brno reservoir, 1.83–2.33 cm/y in the Nové Mlýny reservoir, and 0.9–1.8 cm/y in the Plumlov reservoir. The riverine zones of reservoirs (cores BP1, BP2,

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NM1, and NM2) had clearly higher sedimentation rates than did the lacustrine zones (cores

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BP5, NM4, and NM8; cf. Kalff, 2003; Smol, 2008). Lower sedimentation rates were

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measured close to the shore and higher rates near the reservoir axes (former river thalweg). This all indicates that reservoirs tend to develop wedge-like sediment bodies, pinching out in the proximal-to-distal direction; and they have the tendency to seal the original fluvial

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topography especially in the distal, lacustrine zone. In the Plumlov reservoir, however, sediment thickness and accumulation rates were high near the dam, thereby corresponding to

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a so-called ‘muddy lake area’ (c.f. Shotbolt et al., 2005; Dhivert et al., 2015). This difference compared to other reservoirs lies probably in the smaller size of the Plumlov reservoir (see

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section 5.3). Sedimentation rates in some abandoned meanders are somewhat lower (1–1.38 cm/y in the Zvole oxbow lake, 2.06 cm/y in the Moravičany meander). The meanders’ distal

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parts had higher organic matter content, which suggests calmer conditions and prevalence of organic sedimentation. On the other hand, meanders and oxbow lakes located in the middle

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and lower Morava River course displayed higher sedimentation rates. The composite Kurfürst meander has a rather complicated infill history. The active part of the meander still connected with the main channel is characterized by rapid sedimentation rates. Quick accumulation was the reason for sediment dredging in 1993 (Krejčí, 2009). After the dredging, between 1993 and 2009, sedimentation rates reached up to 8 cm/y per year. The rear, now inactive part of the meander was separated from the main channel during the 1960s and subsequently filled with fine-grained material having high organic matter content, which suggests high organic productivity. Sedimentation rates reached 1.4 cm/y in the final stages of meander infill, between 1986 and 2009. The Čerťák oxbow lake displayed sediment accumulation rates ranging from 1.2 to 6.1 cm/y , increasing from distal parts to proximal parts of the lake

21

ACCEPTED MANUSCRIPT (Bábek et al., 2011). The meanders, too, tended to develop wedge-like sediment bodies, with a maximum sedimentation rate concentrated close to their connection with the active channel.

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With a final cutoff of the meander, sedimentation rates drop but still may remain as high as

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1.4 cm/y.

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The main factors influencing the sediment accumulation rates in reservoirs and oxbow lakes are position (proximal–distal) within the water body and its accommodation space and morphometric parameters. Sedimentation rates are highest in the proximal parts, as clearly

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seen in our data from the Brno and Nové Mlýny reservoirs as well as from the Čerťák oxbow

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lake and Kurfürst meander and as would be expected.

The lowest calculated sedimentation rates were measured in floodplain sediments. The greatest accumulation, meanwhile, was found at site CH1 (located in proximity to the river)

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D

with mean sediment accumulation of 0.88 cm/y. Slightly lower rates were observed at L2 with a value of 0.67 cm/y. The lowest rate of 0.45 cm/y was found for core L1.

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Knowing the depth positions of specific events and mean sedimentation rates enables the creation of reliable age models for each location (Fig. 6), whereby depth reflects sediment

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increment. Several time intervals were distinguished, especially in the Brno reservoir. The first period (1940-1952) is characterised by fast sedimentation rates. This fact can be caused by initial filling and land use changes in the watershed after World War II (Bičík et al., 2001; Van Rompaey et al., 2003), which could be highlighted by several small floods (Nehyba et al., 2011). According to the age models, there has been a slight increase in the erosion and sedimentation rates since 1986 for Brno reservoir, Nové Mlýny reservoir, and Zvole. Conversely, a slight decrease can be observed in the Morava River course (Certak).

5.3. Processes of sediment accumulation

22

ACCEPTED MANUSCRIPT Silt is the dominant grain size fraction carried by rivers into reservoirs (Walling and Moorehead, 1989), and this is consistent with the majority of our grain size distribution

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results. A relatively high content of sand fraction was observed in numerous samples from the

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Brno (range 37–43%) and Nové Mlýny (range 26–42%) reservoirs. Clay content was lower,

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whereas samples from the meanders displayed higher admixtures of clay, usually about 10%. The higher content of clay particles seems to be the main grain size difference between reservoirs and abandoned meander sediments (see Fig. 7) and suggests calmer settling

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conditions for the latter. Higher clay content in some oxbow lake sediments has been

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described in several studies (Gasiorowski and Hercman, 2005; Chalupová and Jánský, 2007; Zachmann et al., 2013). Higher hydrodynamic energy levels in reservoir sediments were also supported by frequent lamination and heterogeneous grain size distribution (Nehyba et al.,

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2011; Sedláček et al., 2013), which were mostly absent in the meanders and oxbow lakes. Sedimentation in the meanders was therefore predominantly governed by processes of

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suspension fallout of clay and silt particles (without considerable contribution from underwater currents). Some of them may display more dynamic water action, as suggested by

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the laminated nature of the sedimentary record in the Čerťák oxbow lake (Bábek et al., 2011). The reservoirs displayed more heterogeneous grain size and sediment thickness patterns than did the abandoned meanders, which are generally governed by the distance from influx, hyperpycnal currents and their velocities, and accommodation space. The thickest layers of lacustrine sediment were found in the proximal parts of studied reservoirs (e.g., the Brno reservoir) and the so-called riverine zone (cf. Kallf, 2003; Smol, 2008; Sedláček et al., 2013). In general, proximal parts tend to accumulate large amounts of sediments caused by high trap efficiency (Shotbolt et al., 2005). Sediments are mainly transported by hyperpycnal and hypopycnal currents, many of which relate to flood events. In the riverine zone, sediment accumulation showed a tendency to lateral accretion toward the reservoir axis in cross section.

23

ACCEPTED MANUSCRIPT Bottom morphology, sediment thickness, frequent lamination, and sediment layer geometry, as seen in ground-penetration radar profiles (Nehyba et al., 2011; Sedláček et al., 2013),

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suggest that this holds true for the Brno reservoir. The sediments are slightly inclined in the

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proximal-to-distal direction indicating downstream accretion of sediment in the reservoir

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(Sedláček et al., 2013). Owing to higher sediment accumulation, a deltaic form can be created near river inflows into the reservoir (Nové Mlýny) with progradation. Toward the dam, sediment thickness decreased and processes of sedimentation fallout began to prevail.

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The distribution of sediment thickness may also have been influenced by underwater currents. Sediment accumulation may have been higher near the dam, constituting a so-called

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‘muddy lake area’ (Morris and Fan, 1997), in particular in small reservoirs where sediments can easily reach the dam. If the underflow reaches the dam, it can rise up against it and create

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an area with high concentrations of suspended, fine-grained sediment (Shotbolt et al., 2005). The results indicate that reservoirs as well as meanders build wedge-like sediment bodies.

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Their internal architecture, as indicated by ground-penetrating data and inclination of ageequivalent surfaces (Bábek et al., 2008; Sedláček et al., 2013) correspond to low-angle delta

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foresets. Total sediment accumulation also depends on the production of autochthonous materials (Foster and Lees, 1999), which can be high in summer months, especially in small oxbow lakes (Galbarczyk-Gąsiorowska et al., 2009). This production contributes together with little water mixing to the creation of reducing conditions (Chalupová and Janský, 2007). The flood events could be recorded in oxbow lake sediments (Gasiorowski and Hercman, 2005) as was documented in the Čerťák oxbow lake. According to the energy of the floodwater, the lake sediments could be eroded or buried by fluvial sediments also (Gasiorowski and Hercman, 2005). Terrestrialization of former river branches caused by river regulation and disconnection from main channel has led to changes in sedimentary conditions as can be seen in the Zvole oxbow lake. Then, reduced hydrological activity can result in

24

ACCEPTED MANUSCRIPT increased sedimentation of autochtonous and allochtonous material (Kirschner et al., 2001; Hudson et al., 2008). Hudson et al. (2008) reported that 67% (average) of the total lake

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surface area has been converted to wetlands since the year of cutoff. During floods,

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sedimentation into oxbow lakes via the cutoff channels may have increased. Furthermore,

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oxbow lakes within the embanked floodplain have undergone much higher rates of infilling (Hudson et al., 2008).

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5.4. Reservoir trap efficiency and estimate of net erosion rates

The amount of sediment trapped within reservoirs is closely related to the trap efficiency

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— the percentage of total particles that settle down into a sediment trap (Dendy, 1974; Kallf, 2003; Smol, 2008). For small Czech reservoirs, mean trap efficiency is estimated at 82%,

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while mean annual sediment supply is about 359,000 m3 (Vrána and Beran, 2002). The remaining suspended, mostly fine-grained silt and clay particles are transported downstream

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through the outlet. Estimating the total amount of sediment trapped within reservoirs and oxbow lakes from just several cores is a difficult task. Our approximate findings of sediment

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volume trapped within reservoirs are based on calculations of sediment thickness and reservoir geometry. The volume of sediment was obtained by the volume of a geometric figure that resembles the threedimensional shape (Romero-Díaz et al., 2007). The results are presented in Table 5. We calculated that 1,541,700 m3 of sediment was deposited in the Brno reservoir, which means ca. 23,707 m3/y (1940–2008) of sediment was trapped within the Brno reservoir. Almost 3,715,000 m3 of sediment is deposited in all three basins of the Nové Mlýny reservoir. Similar volumes were calculated for the upper (1,500,000 m3) and middle (1,470,000 m3) basins. The smallest quantity, about 737,000 m3, was deposited in the lower basin. Using sediment volume and operation time, this means ~50,200 m3/y (upper basin), 54,500 m3/y (middle basin), and 33,500 m3/y (lower basin) of sediments were received

25

ACCEPTED MANUSCRIPT annually. Erosion rate (expressed in m3/km2/y) can be calculated for Brno and Nové Mlýny reservoir from our findings considering the size of the catchment area. This value is highest

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for the Brno reservoir (14.95), slightly lower for the upper basin (10.92), and lowest for the

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middle and lower basins (7.43).

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According to Krása et al. (2005), 4,150,000 m3 of sediment is trapped within the Brno reservoir (theoretical calculation using advanced ArcGIS tools), while its storage capacity has decreased by about 22% since the dam was constructed. At the Plumlov reservoir, 204,265 m 3

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(which is 2,723 m3/y from the period 1936–2011) of sediment was dredged during reservoir

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revitalization in 2011, although this was not the total volume of deposited sediment (national company Povodí Moravy, unpublished data). In the middle basin of the Nové Mlýny reservoir, Říha et al. (2010) estimated the total volume of accumulated sediment at 1,000,000

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D

m3, which is very similar to our calculations. The trap efficiency of abandoned meanders is expectably much lower than in reservoirs because only a part of the total water volume enters

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the stationary water body to settle down. Generally, sedimentation rates were only slightly lower in reservoirs (ranging from 0.29 to

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4.4 cm/y) than in abandoned meanders (ranging from 1.01 to 8 cm/y), although there was a large overlap between both rates. Much greater variability existed between proximal and distal parts of individual sediment bodies; in most cases, sedimentation rates decreased from proximal to distal by factors ranging between ca. 3 (between NM1 and NM8, CRT2 and CRT3) and ca. 10 (BP1 and BP5). The trap efficiency of floodplains is more difficult to interpret. Our sediment accumulation rate data for floodplain sites were similar to those from other studies carried out in the Morava River floodplain (Grygar et al., 2010; Matys Grygar et al., 2011). The earlier authors had presented sedimentation rates for the past 1300 years in the Strážnice floodplain area ranging from 0.23 cm/y to 0.31 cm/y for proximal floodplain. Similar values were presented in a more

26

ACCEPTED MANUSCRIPT recent study (Nováková et al., 2013) from the Litovelské Pomoraví (0.51 cm/y calculated from a core located south of the city of Litovel) and the Strážnice (0.37 cm/y) floodplains.

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Kadlec et al. (2009) pointed to a substantial increase in the sedimentation rate in the second

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half of the twentieth century to a value of 0.8 cm/y (Strážnice) caused by intensive agriculture

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activities. However, aggradation rates in floodplains depend greatly on distance from the active channel, floodplain morphology, and connectivity between water bodies (Matys Grygar et al., 2014; Dhivert et al., 2015). The general assumption that sedimentation rates decrease

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with increasing distance from the river channel (e.g., Hobo et al., 2010) hold true for our

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results from the Litovelské Pomoraví floodplain. Therefore, to calculate the total volume of accumulated sediment in floodplains is a highly difficult task that is beyond the scope of this paper. Nevertheless, vertical aggradation rates in floodplains are not at all negligible (being

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lower than those in the reservoirs and abandoned meanders only by a factor of ca. 2 to ca 8), and floodplains cover vast areas along the middle and lower courses of the Morava River

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catchment area. The trap efficiency of floodplains must therefore be enormous, and this probably affects the total volume of sediment deposited in reservoirs located downstream. The

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decreasing frequency and extent of floodplain inundation (lower course of the Morava River) was reported in recent decades. This fact can be related to anthropogenic changes into the drainage network rather than to climate forcing (Brázdil et al., 2011) because sediment fluxes are especially sensitive to changes in local land use (Dotterweich, 2008). As a result, most of the floodplain areas are flooded now only during major flooding episodes in the Morava River catchment area (Stacke et al., 2014). We should mention that much of the alluvial deposits might be temporary and remobilizable. The residence time generally decreases with increasing size of the catchment area (Phillips et al., 2007). Some authors (e.g., Gomez et al., 1999; Lauer and Parker, 2008) have suggested that some sediment losses are caused by

27

ACCEPTED MANUSCRIPT transport associated with meander migration. Thus, anthropogenic modifications of the Morava River courses can prevent these changes and may influence the residence time.

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Sediment suspended load data are scarce for the Morava River catchment. Rosendorf

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(1998) reported a mean concentration of suspended sediment to vary between 60 and 107

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mg.l−1, while mean annual sediment supply varied between 106,556 and 453,673 tonnes in the monitored profiles (Table 3). During a catastrophic (D1000) flood in July 1997, the amount of sediment transported by the Morava River increased by a factor of 35. This demonstrates the

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importance of floods for the sediment supply, and we should note that the Morava River

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drainage area is affected by frequent flooding (Bábek et al., 2011). So this catastrophic flood could lead to the accumulation of enormous transported material amount. The results show that there are large differences in total volumes of annually accumulated

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sediment rates among locations. Some of the main factors affecting sediment load are catchment area size, land use, and hydrological regime. The sediment pathways change

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through time and also depend on the magnitude of rainfall events (Lang et al., 2003). Sedimentation rates in the Morava River catchment area were interpreted as to be strongly

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affected by such anthropogenic measures as river course modification and river bank consolidation. The highest sedimentation rates were found in the oxbow lakes located within the Litovelské Pomoraví protected landscape area. Higher sediment loads can be deposited especially during extreme floods in the oxbow lakes as was documented elsewhere (Kirschner et al., 2001; Gasiorowski and Hercman, 2005). The explanation of extremely high sediment accumulation rates in the Kurfürst meander lies in its position within the protected area, which is not influenced by human modifications and is often flooded. The sedimentation in oxbow lakes is affected by distance from a river, with increasing distance decreasing the thickness of sediments. The sedimentary record is probably more complete in reservoirs owing to higher trap efficiency, while sedimentation is more affected by floods in oxbow lakes. Finer (clay)

28

ACCEPTED MANUSCRIPT particles are trapped within oxbow lakes, while reservoirs provide sufficient time for settling to the bottom and the finest-grained particles are not trapped.

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The main factors affecting erosion and sedimentation rates in the Morava River catchment

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area are probably topography and land use type. In addition, the erosion rate is high in large

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parts of the Czech Republic, one of the highest in Europe. Cerdan et al. (2010) reported the mean erosion rate 2.6 (t ha-1 y-1) for the Czech Republic. In the territory of the Czech Republic more than 50% of agricultural soils are exposed to water erosion (according to

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Vopravil et al., 2007). As mentioned above, a higher erosion rate is related to some land use

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changes during the communist era. After the Iron Curtain fall, some changes in agricultural management led to changes in land use; however, erosion rate remains high as can be inferred from our age models. This may be caused by gradual changes. The extent of meadows and

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pastures began to increase, but large parcels of fields remain in some areas. Arable land contributes to the total erosion considerably, and the proportion to the overall erosion is

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estimated to be 72.4% (Cerdan et al., 2010). In addition, the response of fluvial systems to land use changes may not be instaneous. It can vary from days to years depending upon the

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internal configuration and the catchment size (Lang et al., 2003; Dotterweich, 2010). We can assume that this change will be fastest in small catchment areas such as the Hloučela River catchment area (Plumlov reservoir). Sediment yields are directly linked to the degree of forest cover also (Zolitschka,1998), so deforestation events may be recorded in sediments. The magnetic susceptibility signal may reflect the elevated erosion rate in the Hloučela catchment. According to 137Cs dating, the deforestation event occurred in the late 1980s. Another important factor is the river course modifications because they influence the sediment fluxes and the places where the sediments can be deposited. Considering its catchment area size (Table 5), the amount of sediments trapped in the Nové Mlýny reservoir could have been expected to be higher. The explanation for this discrepancy could be that this

29

ACCEPTED MANUSCRIPT reservoir is located on the river’s lower course. A considerable amount of sediment may have been deposited in the floodplains located upstream and only a part of the eroded material is

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transported farther downstream into the reservoir. The Brno and Plumlov reservoirs are

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located in the upper courses of their catchments without floodplains. Therefore, these

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reservoirs can receive more sediment. Because of their high trap efficiency, those reservoirs also affect the amount of sediments transported downstream and contribute to lower

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sedimentation rates.

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6. Conclusions

This study demonstrated that the studied reservoirs have high trap efficiency and serve as

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ideal sediment traps. Particularly their proximal parts tend to accumulate large amounts of

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sediment, which may constitute a technical problem for a reservoir’s lifetime together with the risk of storage capacity loss. This can be clearly seen in the very young Nové Mlýny

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reservoir (upper basin) prior to the onset of a higher accumulation rate and low water depths. The trap efficiency of oxbow lakes and meanders is not as high as in the reservoirs, although

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the proximal part of oxbow lakes show the highest local SARs of all sites that we studied. Their sediment supply is very irregular and driven by floods, but accommodation space is much smaller and this contributes to higher sedimentation rates. In addition, oxbow lakes tend to accumulate much finer-grained sediment than reservoir lakes. The sedimentary record is only episodic in floodplains. The lowest accumulation rates were found in floodplains and distal parts of reservoirs. Sediment transport is strongly affected by human modifications in the Morava River catchment area. Some human activities have led to the increased erodibility of the catchment area, but higher sediment flux is influenced by the magnitude of rainfalls and catchment-wide floods such as the catastrophic (D1000) flood in July 1997. In particular, river bank regulation

30

ACCEPTED MANUSCRIPT could cause faster local sedimentation rates. Those changes can be easily reflected in sedimentary traps such oxbow lakes. The highest sedimentation rate, between ~6 and ~8 cm/y

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were measured in the artificial and natural abandoned meanders, suggesting that local SARs

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can be strongly influenced by human activities such as river embankment. In addition, land

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use changes made in the last century may have a significant impact on the frequency of flooding and thereby on the sedimentation rate. So far, comparative studies of this extent have never been carried out in European countries. The results confirm that the main studied

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environments constitute good sedimentary archives with high trap efficiency. The main difference is in the continuity of the sedimentary record, which is more complete in 137

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reservoirs. Using the

Cs method combined with historical data in various fluvial traps can

Acknowledgements

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be a good tool for the estimation of erosion rate, especially in larger watersheds.

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This work was supported by the Czech Science Foundation (GAČR) research project P210/12/0573. The authors would like to thank Lenka Němčíková for providing a data set

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from floodplain samples. The authors wish to thank the three anonymous reviewers and journal editor Richard A. Marston for their stimulating comments and suggestions that improved the original manuscript.

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ACCEPTED MANUSCRIPT Figure captions:

Fig. 1. Position of studied locations within the Morava River catchment area (Czech

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Fig. 2. Detailed map of locations and core extraction sites.

Fig. 3. Example of physical stratigraphy proxies: X-ray image of a core (NM1, Nové Mlýny

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reservoir), showing grain size, magnetic susceptibility, TOC, and CIEL* (brightness) values. Fig. 4. Mass activity from 137Cs dating of selected cores and interpretation of datum levels. Fig. 5. Sediment accumulation rates. Ring size is directly proportional to sedimentation rate.

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Fig. 6. Age models for each location.

Fig. 7. Ternary plot diagram for sand, silt, and clay content; samples from reservoirs, oxbow

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ACCEPTED MANUSCRIPT Table 1 Basic hydrological characteristics of studied reservoirs

9.76 170.7 3.91 0.63 4,599.3 168.3 13.33

23.68 167.1 45.77 14.5 11,853.1 430 8 163.5 41.06

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17.54 170 3.51 11 11,713.4 167.25 40.86

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2.08 219 13.02 2.6 1,586.23 155 1.37 225.8 7.6

Lower

Plumlov 0.35 266.38 2.72 1.632 118.5 12 0.009 276.43 0.58

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Storage area (mil m3) Variable storage (mil m3) Catchment area (km2) Harmless outflow (m3/s) Minimal outflow (m3/s) Emergency spillway (m) Annual discharge (m3/s)

Nové Mlýny Upper Middle

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Parameter Permanent storage (mil m3) Level of permanent storage (m)

Brno

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ACCEPTED MANUSCRIPT Table 2 Results from 137Cs dating Core

Sampling year

Brno reservoir Brno reservoir Brno reservoir Plumlov reservoir Plumlov reservoir Plumlov reservoir Nové Mlýny reservoir Nové Mlýny reservoir Zvole oxbow lake Moravičany meander Kurfürst meander Čerťák Oxbow lake Čerťák Oxbow lake Litovelské Pomoraví Litovelské Pomoraví Litovelské Pomoraví

BP1 BP2 BP5 P1 P2 P3 NM1 NM4 ZV1 LIT2 CH4 CRT2 CRT3 L1 H2 CH1

2008 2008 2009 2010 2010 2010 2008 2008 2010 2010 2010 2007 2007 2012 2012 2012

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137

Cs maximum (Bq/kg) 377 222 61 115.9 100.2 130.2 90.2 127.8 318 135.6 74.2 193 254 93 83 50

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Chernobyl fallout – growth of peak (depth, cm) 82–86 62–65 11–13 20–24 40–44 24–28 48–52 24–28 31–38 39–44 30–37 148–156 162–166 13.5–16.5 22.5–25.5 22.5–25.5

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ACCEPTED MANUSCRIPT Table 3 Concentration of suspended sediment and total sediment load data for selected water-gauge stations (Rosendorf ed. 1998)

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9,146

58

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213,135

14,427

528,406

197,331

12,527

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8,113

Monthly suspended Monthly load, D1000 Average annual average flood, suspended load suspended June 1997 (1986–1995, t) load (t) (t) 106,556 8,072 290,532 453,673 37,803 870,311

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Catchment area (km2) 3,322 7,014

Maximum concentration of suspended sediment, D1000 flood in 1997 532 1,146

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Water gauge station Olomouc Kroměříž Uherské Hradiště Strážnice

Mean concentration of suspended sediment (mg/l) 72 107

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ACCEPTED MANUSCRIPT Table 4

Core

Time interval

Brno reservoir Brno reservoir Brno reservoir Brno reservoir Brno reservoir Brno reservoir Brno reservoir Brno reservoir Brno reservoir Brno reservoir Nové Mlýny reservoir Nové Mlýny reservoir Nové Mlýny reservoir Nové Mlýny reservoir Nové Mlýny reservoir Nové Mlýny reservoir Plumlov reservoir Plumlov reservoir Plumlov reservoir Zvole oxbow lake Zvole oxbow lake Moravičany meander Kurfürst meander Kurfürst meander Čerťák oxbow lake Čerťák oxbow lake Čerťák oxbow lake Čerťák oxbow lake Čerťák oxbow lake Litovelské Pomoraví floodplain Litovelské Pomoraví floodplain Litovelské Pomoraví floodplain

BP1 BP1 BP1 BP1 BP2 BP2 BP2 BP2 BP5 BP5 NM1 NM1 NM4 NM4 NM6 NM8 P1 P2 P3 ZV1 ZV1 MOR1 CH1 CH4 CRTI CRT2 CRT2 CRT3 CRT3 L1

1940–1952 1952–1963 1963–1986 1986–2008 1940–1952 1952–1963 1963–1986 1986–2008 1940–1986 1986–2008 1978–1896 1986–2009 1981–1986 1986–2009 1981–2010 1989–2010 1986–2010 1986-2010 1986–2010 1972–1986 1986–2010 1986–2010 1993–2009 1986–2009 1981–1986 1981–1986 1986–2008 1981–1986 1986–2008 1986–2012

H2

1986–2012

0.67

CH1

1986–2012

0.88

SC R

NU

MA

D

TE

CE P

AC

Mean sedimentation rate (cm/y) 4.62 0.54 3.26 4.2 4.58 1.64 3.61 3.1 0.13 0.29 0.63 2.27 1.4 1.18 2.33 0.58 0.9 1.68 1.04 1.01 1.38 2.06 8 1.4 6.1 6.2 3.7 2.4 1.2 0.45

IP

Location

T

Calculated average sedimentation rates

50

ACCEPTED MANUSCRIPT Table 5 Calculated sediment volumes for the Nové Mlýny and Brno reservoirs

T

Catchment area (km2) 11,853 4,599 11,855 11,856 1,586

IP

Total sediment volume received annually (m3/y) 123,850 50,222 54,505 33,510 23,718

Erosion rate (m3/km2/y) 10.45 10.92 4.60 2.83 14.95

AC

CE P

TE

D

MA

NU

Nové Mlýny (total) Upper basin Middle basin Lower basin Brno

Sediment volume (m3) 3,715,520 1,506,660 1,471,630 737,230 1,541,670

SC R

Reservoir

51

ACCEPTED MANUSCRIPT Highlights Studied reservoirs and oxbow lakes can have high trap efficiency and serve as ideal sediment traps.

T

The reservoirs displayed more heterogeneous grain-size and sediment-thickness patterns than

IP

did the abandoned meanders.

SC R

The lowest accumulation rates were found in floodplains and distal parts of reservoirs and the highest sedimentation rates in proximal parts of reservoirs and oxbow lakes. The main difference is in the continuity of the sedimentary record, which is more complete in

AC

CE P

TE

D

MA

NU

reservoirs.

52