The development and application of a database of radiocarbon-dated Holocene fluvial deposits in Great Britain

Catena 66 (2006) 14 – 23 www.elsevier.com/locate/catena

The development and application of a database of radiocarbon-dated Holocene fluvial deposits in Great Britain Eric Johnstone *, Mark G. Macklin, John Lewin River Basin Dynamics and Hydrology Research Group, Institute of Geography and Earth Sciences, University of Wales, Aberystwyth, Ceredigion, SY23 3DB, UK

Abstract This paper reports on the development of a British database of 14C dated Holocene fluvial units over the last 15 years. Since its inception in 1989, the database has undergone substantial expansion, refinement and improvement together with considerable methodological development, so that today it serves as a powerful research tool for investigating the spatial and temporal dynamics of Holocene river development and flooding in Great Britain. The improved analytical method is here applied to the currently existing database by examining datasets of 14C dated fluvial units from river basins that lie within and beyond the limits of Late Devensian glaciation. This analysis indicates how the contrasted conditions in these two types of river environment have, to an extent, led to divergent records of flooding in response to Holocene climate and land-use change. However, even in the contrasting sedimentary records of upland glaciated, and lowland unglaciated river basins, six corresponding episodes of increased flood frequency are identified at c. 5730, c. 3540, c. 2730, c. 2280, c. 660 and c. 570 cal. BP. These represent widespread common responses from British river systems to large-scale changes in climate. The method of database construction and analysis outlined and demonstrated in this paper could be readily adopted in other parts of the world to improve our understanding of Holocene river behaviour at the continental- and global-scale. D 2005 Elsevier B.V. All rights reserved. Keywords: River floods;

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C dating; Fluvial sediments; Environmental change; Great Britain

1. Introduction Alluvial sediments represent the principal source of information concerning patterns of Holocene fluvial behaviour and provide key data for investigating cause and effect relationships between changing environmental conditions and river system activity (Baker et al., 1983; Starkel, 1983; Ely et al., 1993; Knox, 2000; Benito, 2003; Macklin and Lewin, 2003). A critical element in the investigation of longer term river activity is the need to obtain robust dating controls to provide a secure chronological framework for fluvial reconstruction (Maddy et al., 2001; Macklin and Lewin, 2003). During the last 20 years there has been substantial growth in the use of chronometric dating methods in Holocene fluvial research, which has contributed * Corresponding author. Tel.: +44 1970 622780. E-mail address: [email protected] (E. Johnstone). 0341-8162/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.catena.2005.07.006

significantly to an improved understanding of temporal patterns of river system behaviour. In Great Britain (GB), in particular, this growth has principally been reflected in the increased use of radiocarbon (14C) dating techniques (Fig. 1). Although there are still significant areas without dated Holocene alluvial sediments, the dataset that now exists does allow regional contrasts to be explored on a more reliable basis. In 1989, the growing use of both stratigraphic and chronometric dating controls in studies of British fluvial environments led Macklin and Lewin (because of editorial problems the publication of their paper was delayed until 1993) to initiate investigations of spatial and temporal patterns of Holocene river activity across GB. The work then reported represents the first stage in the construction of a database of dated fluvial deposits which, over the last 15 years, has evolved into a powerful research tool for investigating the controls of Holocene river dynamics and

E. Johnstone et al. / Catena 66 (2006) 14 – 23

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Fig. 1. Map of GB showing the increased use of 14C dating controls in Holocene fluvial studies between 1989 and 2004. The southern limit of Late Devensian Dimlington Stadial glaciation is also shown (after Bowen et al., 1986).

flooding in GB. This paper outlines the principal developments that this database has undergone since its inception, reports on the patterns of Holocene flood activity identified in the most recent iteration of database analysis and uses a new comparison of 14C dated fluvial units from river basins both within and beyond the limits of Late Devensian glaciation to illustrate the research potential of the database as it exists today.

2. The development of a Britain: 1989 – 2003

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C fluvial database for Great

The first stage in the development of a database of 14C dated fluvial units reported by Macklin and Lewin (1993) aimed to: (1) identify significant episodes of Holocene

alluviation in British rivers; (2) establish regional or national events and their degree of synchrony; and (3) evaluate links between alluviation and climate, vegetation and land-use change. This was done by reviewing the published literature and compiling a database containing records of all fluvial units in GB that had been dated by 14C techniques, archaeological material, pollen biostratigraphy and soil stratigraphy. In this manner, Macklin and Lewin (1993) collected information on 123 dated units in river catchments across GB. The dates from each of the fluvial units were represented by a single, uncalibrated age and plotted in a histogram with 400-year class intervals. This led Macklin and Lewin (1993) to identify eight broadly defined phases of alluviation between c. 10,700 cal. BP and present (Fig. 2). Following an interim update, in which Macklin (1999) employed the same methods to analyze an expanded

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E. Johnstone et al. / Catena 66 (2006) 14 – 23

Fig. 2. Histogram plot of British dated alluvial units reported by Macklin and Lewin (1993) with alluviation phases identified in the same study. Data were plotted as uncalibrated ages; for comparison, calibrated age peaks are here listed on the right of the diagram.

database of dated fluvial units, the second major iteration of this work was reported by Macklin and Lewin (2003). The new analysis introduced a number of methodological developments. In the first instance, the number of records stored in the database had grown from 123 to 364 and incorporated fluvial units that had been dated solely by 14C techniques. Each entry in the database was also accompanied by additional information so that every dated sample was assigned to one of four depositional environments— channel-bed sediments, palaeochannel fills, floodplain sediments or flood-basin sediments. A further important methodological development was the identification of 14C samples that coincided with a modification in sedimentation style or rate, which allowed those dates marking geomorphologically significant changes in Holocene river activity to be picked out. Furthermore, 14C dates obtained from archaeological material were excluded from analysis due to the higher possibility of these being in a secondary context. This more rigorous process of data capture and

Fig. 3. Cumulative frequency plot of calibrated same study.

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interrogation meant that from the 364 entries recorded in the database, only 96 geomorphologically significant Fchange_ dates were singled out for use in analysis. The method of graphically representing and analyzing these Fchange_ dates was also revised by Macklin and Lewin (2003) so that cumulative frequency plots, showing the 2r age range of each chronologically ranked calibrated 14C date, were used instead of histogram plots (Fig. 3). Where three or more calibrated age ranges overlapped (shown by the vertical stacking of age ranges on the plot in Fig. 3) a mid-point was calculated to give a date for each significant phase of Holocene flood activity. Using these revised methods, Macklin and Lewin (2003) identified 14 flood episodes in GB during the Holocene (Fig. 3), and were able to pick out more detailed trends in regional patterns of flooding and variations associated with the four types of depositional environment. The 14 episodes of flooding were also compared to proxy climate records from GB (Hughes et al., 2000), central Europe (Haas et al., 1998) and the North

C dates (at the 2r age range) reported by Macklin and Lewin (2003) with flood episodes identified in the

E. Johnstone et al. / Catena 66 (2006) 14 – 23

Atlantic (Bond et al., 2001) to evaluate the role played by climate change in influencing patterns of British river activity. Such detailed comparisons were not possible earlier in Macklin and Lewin (1993) because of the lack of reliably reconstructed proxy climate records at that time.

3. The British database of deposits in 2004

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C dated Holocene fluvial

Since 2003, the database of 14C dated fluvial deposits has undergone considerable expansion and further methodological development has also taken place. These changes have been made to: (1) update the database so that it contains data from the most recent fluvial research in GB (including Fgrey_ literature and unpublished Ph.D. material); (2) allow more detailed and varied analyses to be undertaken; and (3) improve the manner in which 14C dates are graphically represented and analyzed. The number of entries in the updated database is now 506 and includes information on 14 C dated fluvial units from a wide range of published and unpublished sources (for a full list of sources see www. aber.ac.uk/rivers/14c). Entries in the database have been expanded so that each 14 C date is accompanied by additional information that facilitates more detailed analyses (Fig. 4). This includes information on the geographical location of 14C sampling sites, details of the type of organic material sampled and a brief description of each sample’s sedimentary context. New dates continue to be assigned to a depositional environment although one further category has been added under this heading to incorporate debris flow and colluvial sediments. Interpretations have also been made for each date regarding the type of channel and floodplain pattern, or alluvial ensemble (Lewin, 2001), that existed at the time of deposition (Table 1; for a more detailed discussion of the depositional environment and alluvial ensemble categories see Lewin et al., 2005). To facilitate regional analysis each dated fluvial unit has been assigned to one of eight coherent precipitation variability regions, which have been shown to have responded in a consistent manner to fluctuations in precipitation and temperature over the length of the instrumental record (Gregory et al., 1991). The method used by Macklin and Lewin (2003) to pick out dates which mark geomorphologically significant changes in Holocene river activity has been retained, and this means that of the 506 14C dates in the database, 263 Fchange_ dates have been selected for use in the current iteration of database analysis. A major methodological change has been undertaken in the way in which datasets of 14C dates are graphically represented and analyzed. In previous iterations of this work 400-year classes for uncalibrated dates (Macklin and Lewin, 1993) or the 2r age range of calibrated dates (Macklin and Lewin, 2003) were used to identify episodes of Holocene flooding. In this latest version of database analysis (also

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reported in Lewin et al., 2005; Macklin et al., 2005), longterm records of riverine flooding were reconstructed using cumulative probability density functions (CPDFs) of all 263 14C dated fluvial units. Dates were calibrated and individual probability distributions were summed using OxCal (version 3.9; Bronk Ramsey, 1995, 2001) and plotted as CPDFs. This generated frequency curves that provide a good visual impression, or a best estimate, for the chronological distribution of calibrated 14C dates (Shennan, 1987; Bronk Ramsey, 2003). This method, which has been used in other studies where large numbers of 14C dates have been analyzed (e.g. Berendsen, 1984; To¨rnqvist, 1994; Meyer et al., 1995; Stouthamer, 2001; Wegmann and Pazzaglia, 2002; Michczyn´ska et al., 2003), was adopted in preference to previously used techniques as it takes account of the entire probability distribution associated with each calibrated 14C date. Recent work by Michczyn´ska and Pazdur (2004), who used Monte Carlo simulations to examine the representation of large datasets of 14C dates in CPDFs, also indicates that the increased number of dates used in the analysis of the expanded database produces statistically reliable results. The curve used in the 14C calibration process (INTCAL98; Stuiver et al., 1998), however, exerts an influence on the CPDFs so that dates coinciding with a plateau in the calibration curve may produce smaller, less well-defined peaks than dates which occur at steep sections of the curve. In an effort to address this issue a correction has been applied by generating a second CPDF plot using a simulated dataset of evenly distributed 14C dates and subtracting this from the CPDF plot of observed 14C dates (Fig. 5A). The resulting probability difference curve (PDC, Fig. 5B) shows the difference between the two CPDFs where peaks and troughs relate to apportioned and dispersed probabilities of several dated units, and changes in the height of the PDCs over the Holocene are interpreted to reflect variations in the occurrence of major flooding (Macklin et al., 2005). By comparing the PDC plot of 263 14C dated fluvial units with those plots produced in earlier iterations of this work, the effect imparted on analytical results by the development of the 14C database over the last 15 years can be seen. In the first instance, the skewed distribution of 14 C dates seen in the current PDC plot (Fig. 5B) is also apparent in the histogram plot produced in 1993 (Fig. 2) and the cumulative frequency plot produced in 2003 (Fig. 3). This appears to reinforce the importance assigned to the role of fluvial unit preservation potential in characterizing the nature of the Holocene sedimentary record (Lewin and Macklin, 2003). Beyond this important similarity, however, the three separate analyses have produced some differences, particularly in the temporal positioning of identified 14C date clusters and the numbers of flood episodes. Thus Macklin and Lewin (1993) were only able to discriminate eight broad (400-year) Fepisodes,_ and these have been subdivided in later analyses. The 14 phases of the Macklin

18 E. Johnstone et al. / Catena 66 (2006) 14 – 23 Fig. 4. An extract from the 14C database as it exists today, detailing the additional information that is stored alongside each 14C date entry. The extract shows information that accompanies 14C dates reported by Brazier and Ballantyne (1989), Hooke et al. (1990), French et al. (1992), Needham and Longley (1980), Macklin et al. (1994) and Johnstone (2004).

E. Johnstone et al. / Catena 66 (2006) 14 – 23 Table 1 Categories of depositional environment and alluvial ensemble used in the database of 14C dated fluvial units Depositional environment

Alluvial ensemble

A B C D E

1 2 3 4 5

Channel bed sediments Palaeochannel fills Floodplain sediments Flood basin sediments Debris flow/colluvial sediments

Alluvial fan Upland stream/gully Braided/wandering system Actively meandering system Inactive meandering system

and Lewin (2003) analysis also differ from the Macklin et al. (2005) 16 phases, listed here in Table 2, particularly in the earlier Holocene. For example, a quite prominent step in the cumulative frequency plot at c. 7720 cal. BP is not picked out by the PDC analysis (compare Figs. 3 and 5B). Small peaks in the early Holocene are eliminated by the PDC analysis because they may reflect 14C calibration Fbunching._ On the other hand, the PDC analysis does recognize several peaks in the c. 5730– 4550 cal. BP period that were not identified in the 2003 analysis. Despite the major developments and improvements that have been undertaken in the last 15 years, some restrictions associated with the analysis of the 14C database remain. Many of these mirror data gaps and shortcomings that exist in Holocene fluvial research as a whole. Problems include the inherent limitations associated with the temporal resolution of 14C dating techniques (Brown, 2003; Macklin and Lewin, 2003; Maddy et al., 2003) and of fluvial unit preservation (Lewin and Macklin, 2003), which mean that sedimentary archives of Holocene alluviation are incomplete and, indeed, may have Fmore gap than record_ (Lewin, 2001). As a corollary to this last point, Macklin and Lewin

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(1993) also indicated that analysis of the database of 14C dates only highlights depositional episodes in the Holocene fluvial record and does not identify phases of increased lateral channel movement, floodplain incision or erosion. Each of these may relate to flood episodes which are not recorded by sediment deposition at particular sites. Notwithstanding these limitations, the enlarged database of 14C dates and the revised method of analysis do allow a more precise and comprehensive examination of the overall character of the British Holocene fluvial sedimentary record to be undertaken than has previously been possible.

4. Holocene flood activity in British upland glaciated, and lowland unglaciated river environments To illustrate further the utility of the 14C database as an effective research tool, the entire enlarged database of 263 Fchange_ dates has been divided into two separate datasets. The first consists of 14C dates from river catchments in upland parts of northern and western GB that were glaciated during the Late Devensian Dimlington Stadial, whilst the second was constructed using 14C dates obtained from lowland drainage basins in southern and eastern parts of the country which lie at the margins of, or beyond, Dimlington Stadial ice limits (Fig. 1). This classification scheme has been employed before to distinguish between two broad types of river environment in GB: upland glaciated river systems which are characterized by higher catchment relief, steeper valley floor gradients and higher unit discharges, and lowland unglaciated drainage basins that generally have lower catchment relief, lower valley floor gradients and

Fig. 5. Graphical plots illustrating the derivation of PDCs used in the analysis of the GB Holocene flood record (after Macklin et al., 2005, p. 939). The plots show the CPDF of 263 observed 14C dates from GB flood units along with the CPDF of 234 evenly distributed simulated 14C dates (A) and the PDC that results when the simulated CPDF is subtracted from the observed CPDF (B). Probability and probability difference are measured in arbitrary units (a.u.).

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E. Johnstone et al. / Catena 66 (2006) 14 – 23

Table 2 Episodes of major Holocene riverine flooding identified from the GB (Macklin et al., 2005, Fig. 5B), upland glaciated, and lowland unglaciated (Fig. 6) PDCs of 14C dated fluvial units All British rivers

Upland glaciated rivers

11,160

Lowland unglaciated rivers 11,190 10,600 10,170 9300 7540 6780

6820 6130 5730 5540 4840 4520

5730 5540 4840 4490

5920 5690

4160 4030 3540

3580

2730 2550 2280

2730 2560 2280

1950 1650 1290

1950

860 660 570

850 660 570

3830 3510 3150 2800 2320 2150 1650

1290 1160 650 570 390

Flooding episodes recorded in all three PDCs are shown in bold (ages cal. BP).

lower unit discharges (Macklin and Lewin, 1986, 1993; Gregory, 1997; Macklin, 1999; Lewin and Macklin, 2003). By creating separate datasets from these two broad types of British river environment, the analysis method outlined

Fig. 6. PDCs of 14C dates from upland glaciated (207 measured in arbitrary units (a.u.).

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above can be used to compare patterns of Holocene riverine flooding in upland glaciated, and lowland unglaciated catchments. The relative impacts of Holocene environmental change on river behaviour in these environments can then be evaluated in the light of the variations and similarities observed in their respective sedimentary records. In Fig. 6 the PDC plots created using the 14C dates from upland glaciated, and lowland unglaciated river catchments have been overlaid. The upland glaciated PDC was constructed using a dataset of 207 14C dates and shares some striking similarities with the British PDC plot produced by Macklin et al. (2005, Fig. 5B). Given that 79% of the 263 14C dates used in the British PDC come from drainage basins that were glaciated during the Late Devensian, the similarities are not surprising and the convergent curves serve to illustrate the strong influence that 14C dates from river catchments of this type exert on the current overall picture of British Holocene flood episodes. The record from upland glaciated catchments displays no major peaks during the early Holocene, with only limited evidence for flood phases at c. 6130, c. 5730 and c. 5540 cal. BP. Prominent peaks in the PDC first appear at c. 4840 cal. BP and steep increases in the magnitude of peaks can be seen during the late Holocene at c. 2730 cal. BP and again at c. 660 cal. BP. The overall record in formerly glaciated parts of northern and western GB is dominated by large peaks that are clustered within a 2000 year period between c. 2750 and c. 550 cal. BP, when 101 (49%) of the 207 14C dates used to construct the plot occur. The 56 14C dates that make up the dataset from lowland unglaciated river systems in southern and eastern GB have produced a PDC plot that is notably different in character, although the small number of dates in the dataset means that the record may be affected by sampling restrictions and, as such, any interpretations need to be made with caution. Despite this, the lowland unglaciated PDC appears to provide evidence for a longer record of Holocene flooding

C dates), and lowland unglaciated (56

14

C dates) river catchments in GB. Probability difference is

E. Johnstone et al. / Catena 66 (2006) 14 – 23

than the upland glaciated dataset, with peaks present as early as c. 11,190 and c. 10,600 cal. BP. Larger magnitude peaks occur in the lowland unglaciated PDC from c. 4160 cal. BP but are followed by a period with very few flood units between c. 2050 and c. 900 cal. BP. A major difference between the two plots is evident in the timing of the most pronounced periods of flood activity. In the lowland unglaciated PDC the most prominent cluster of large peaks precedes that seen in the upland glaciated record by 1500 years and extends from c. 4250 to c. 2050 cal. BP, when 29 (52%) of the 56 14C dates are found. Peaks in flood phases recorded in the British PDC plot (Macklin et al., 2005, Fig. 5B) and the upland glaciated, and lowland unglaciated PDC plots (Fig. 6) have been listed in Table 2. The flood phase peaks presented in the table reflect the divergent records of Holocene riverine flooding that characterize the upland glaciated, and lowland unglaciated PDC plots but the table also indicates a number of periods when more frequent flooding appears to have affected the whole of GB. Where peaks all fall within a 100-year range, a major nationwide episode of increased flood frequency has been picked out and is shown in bold. In this manner, six episodes of major Holocene flooding affecting both upland glaciated, and lowland unglaciated river catchments in GB can be identified at c. 5730, c. 3540, c. 2730, c. 2280, c. 660 and c. 570 cal. BP. The occurrence of six mid-late Holocene phases of increased flood frequency suggests that river systems in GB have, at these times, responded in a similar manner to changing environmental conditions and, in particular, to large-scale variations in climate. Each flooding episode, for example, corresponds with a period of increasing (c. 5730, c. 3540, c. 660, c. 570 cal. BP) or high (c. 2730, c. 2280 cal. BP) atmospheric 14C production rates, which have been shown to be associated with phases of decreasing or low solar activity (Masarik and Beer, 1999; Bond et al., 2001). The marine record of North Atlantic ice-rafted debris events, reflecting periods when cool, ice-bearing waters from north of Iceland were advected as far south as GB, also indicates that the c. 2730 and c. 2280 cal. BP flood episodes coincided with periods of cold ocean surface temperatures, whilst flooding at c. 5730 and c. 660 cal. BP occurred when ocean surface waters were cooling (Bond et al., 1997, 2001). Terrestrial palaeoclimate indicators provide further evidence to suggest that large-scale changes in climate were the principal forcing mechanism for the nationwide flooding episodes. Periods of higher groundwater levels (and by inference wetter hydroclimatic conditions) have been identified, for example, from the mean lifespan of bog oaks in Germany, the Netherlands and Ireland, which correspond closely to British flooding episodes at c. 5730, c. 3540, c. 2730 and c. 2280 cal. BP, though the bog oak record does not extend far enough for comparisons to be made with the c. 660 and c. 570 cal. BP flood periods (Leuschner et al., 2002). Wet shifts recorded in mires across north-western Europe also provide evidence for widespread phases of

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climatic deterioration that correspond well with all but the earliest of the British flood phases (Hughes et al., 2000). For example, the c. 2280 cal. BP phase of increased flood activity coincides with mire wet shifts reported in northern England (Barber et al., 1994; Hughes et al., 2000), southern Scotland (Chambers et al., 1997), Denmark (Aaby, 1976) and Norway (Nilssen and Vorren, 1991). Aside from the six corresponding phases of increased flood frequency, the major differences that exist between the two PDC plots in Fig. 6 reflect the variable responses of upland glaciated, and lowland unglaciated river systems in GB to changes in climate and land-use. A major factor in these differences is likely to have been the divergent catchment conditions that characterize the two types of fluvial environment. River systems in upland parts of northern and western GB that were exposed to glacial activity during the Late Devensian are broadly characterized by greater catchment relief, steeper valley floor gradients and higher unit discharges, whilst lowland drainage basins in southern and eastern parts of the country, which lie beyond the limits of Late Devensian glaciation, generally have lower catchment relief, lower valley floor gradients and lower unit discharges (Macklin and Lewin, 1993). These differences in catchment characteristics have in particular served to condition rates of fluvial unit preservation, which has had a major impact on the shape of the PDC plots. Lewin and Macklin (2003) suggested that prior to c. 6000 cal. BP upland glaciated river basins experienced relatively high rates of channel incision and lateral reworking so that very few early Holocene fluvial units were preserved. In contrast, lowland unglaciated river basins have been characterized by episodic aggradation throughout the Holocene, which has led to higher rates of fluvial unit preservation. This difference in fluvial sedimentation style is reflected in Fig. 6 where no flood episodes are recorded in upland glaciated catchments prior to c. 6130 cal. BP, whilst the record from lowland unglaciated basins suggests that six flooding episodes occurred between c. 11,190 and c. 6780 cal. BP. Rather than representing major differences in the number of flood phases that took place, the contrasting early Holocene records may reflect the divergent rates of fluvial unit preservation in upland glaciated, and lowland unglaciated river basins, which have been brought about due to their varying catchment characteristics. During the second half of the Holocene, rates of fluvial unit preservation in upland glaciated river catchments increased as episodic valley floor aggradation was superimposed on the longer-term trend of channel incision (Lewin and Macklin, 2003). Despite these increased rates of fluvial unit preservation in upland glaciated catchments, the two types of British river environment continue to exhibit divergent records of flood activity. One likely explanation for the persistence of variable fluvial behaviour is the impact of anthropogenic land-use change in the midlate Holocene. Forest clearance and the expansion of

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permanent agriculture would have served to increase runoff and sediment supply and would also have made river basin sedimentation considerably more responsive to changes in climate (Macklin and Lewin, 2003). In conjunction with higher rates of preservation potential, this may help to explain the increased number and magnitude of flood peaks present in the sedimentary records of both upland glaciated, and lowland unglaciated river systems during the second half of the Holocene. The variable timing of flood episodes that persisted into the mid-late Holocene may reflect differing patterns and rates of anthropogenic land-use change between north-western and south-eastern GB. In particular, large-scale forest clearances and agricultural expansion in lowland areas of southern and eastern GB generally took place earlier than it did in upland parts of northern and western GB, with clearance rates in the lowlands peaking between c. 3000 and c. 2000 cal. BP and between c. 2500 and c. 1500 cal. BP in the uplands (Roberts, 1998). This apparent time-lag in the northward and westward spread of forest clearances may manifest itself in the PDC plots with the dominant period of 14C date clusters in lowland unglaciated river systems (c. 4250 – c. 2050 cal. BP) preceding that which is seen in the record of upland glaciated river environments (c. 2750– c. 550 cal. BP) by c. 1500 years.

5. Conclusions The British database of 14C dated fluvial units has undergone substantial expansion and considerable methodological development since its inception in 1989. These changes have led to the results from database analyses being refined and improved so that today it serves as a powerful research tool for investigating the spatial and temporal dynamics of Holocene river development and flooding in GB. The effectiveness of the database as a research tool has been demonstrated by examining the 14C dated sedimentary record of riverine flooding in upland glaciated, and lowland unglaciated river catchments. This analysis indicated how the intrinsic catchment characteristics associated with these two types of river environment have led to the development of divergent records of flooding in response to climate and land-use change during the Holocene. The six corresponding episodes of increased flood frequency that were identified in upland glaciated, and lowland unglaciated river basins during the mid-late Holocene represent periods when large-scale changes in climate have served to override differences in intrinsic catchment conditions, and variable patterns of anthropogenic land-use change, to generate a common fluvial response in both types of environment. The analysis of the two British 14C datasets also highlights the need for the use of Fchange_ dates, as outlined by Macklin and Lewin (2003), to be more widely adopted in Holocene fluvial research. This is particularly

true for river catchments in lowland areas of southern and eastern GB where, at present, very few 14C Fchange_ dates exist. Our relatively poor understanding of early Holocene river dynamics and flooding in upland parts of northern and western GB has also been identified as an area in need of further investigation. Finally, the method of database construction and analysis we have developed is generic and could be readily adopted to reconstruct patterns of Holocene river dynamics and flooding in other parts of the world (see Macklin et al., 2006-this volume). Following such a methodology, the construction and analysis of 14C dated fluvial databases has already been undertaken in Poland (Starkel et al., 2006-this volume) and Spain (Thorndycraft and Benito, 2006-this volume), and initial results suggest that the database methods could be employed elsewhere in the world to improve understanding of the behaviour of Holocene river systems at the continental- and even global-scale.

Acknowledgements This work was supported by grants awarded to Johnstone and Macklin from ICSU and Historic Scotland, and to Macklin and Lewin from NERC (NER/A/S/2001/00454). The authors wish to thank Dr. Paul Brewer, Dr. Simon Gittins, Dr. Marco van de Wiel and Professor Andreas Lang for their help in developing the database method on which this paper is based.

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