Use of floodplain sediment cores to investigate recent historical changes in overbank sedimentation rates and sediment sources in the catchment of the River Ouse, Yorkshire, UK

Catena 36 Ž1999. 21–47

Use of floodplain sediment cores to investigate recent historical changes in overbank sedimentation rates and sediment sources in the catchment of the River Ouse, Yorkshire, UK Philip N. Owens a , Desmond E. Walling a

a,)

, Graham J.L. Leeks

b

Department of Geography, UniÕersity of Exeter, Amory Building, Rennes DriÕe, Exeter, DeÕon EX4 4RJ, UK b Institute of Hydrology, Wallingford, Oxfordshire OX10 8BB, UK Received 16 June 1998; received in revised form 26 January 1999; accepted 26 January 1999

Abstract Floodplain sediment cores collected from seven sites in the catchment of the River Ouse, in Yorkshire, UK, have been used to provide information on recent historical changes in both rates of overbank sedimentation and sediment sources. The environmental radionuclides 137Cs and unsupported 210 Pb have been used to establish chronologies for each core and to estimate average sediment accumulation rates for the last ca. 30 and 100 years, respectively. Average sedimentation rates for the individual cores ranged from 0.11 to 1.04 g cmy2 yry1. In all but one case, the estimates of average sedimentation rate during the last ca. 30 years for the individual cores are broadly similar to those for the last ca. 100 years, suggesting that overbank sedimentation rates have been essentially uniform over the longer time period. Composite fingerprints, based on a combination of geochemical and mineral magnetic properties, and a numerical mixing model have been used to investigate downcore changes in sediment source. In the case of source type, most of the cores reflect a primarily topsoil source, although there have been periods with increased contributions from subsoilrchannel bank sources. Within the Ouse basin in general, the period commencing in the late 19th and early 20th century and extending through to the 1960s, was characterised by increased contributions from topsoil sources. However, contributions from subsoilrchannel bank sources have increased over the last few decades. The source tracing results relating to sediment contributions from the three main geologicalrtopographic zones are in broad agreement with the proportion of the area of the catchment underlain by each rock type. Temporal variations in the contributions from the three geologicalrtopographic zones vary from site to site, but for the lower reaches of the River Ouse contributions from areas underlain by Permian and

)

Corresponding author. Tel.: q44-1392-263345; Fax: q44-1392-263342; E-mail: [email protected]

0341-8162r99r$20.00 q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 4 1 - 8 1 6 2 Ž 9 9 . 0 0 0 1 0 - 7

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Triassic rocks, which mainly outcrop in the Vale of York, have increased since the turn of the century. The changes in sediment source identified are probably a reflection of changes in land use and management Žand possibly changes in climate.. These results enable estimates of contemporary suspended sediment fluxes and sources to be placed into a historical context and provide a means of considering the likely impact of potential future changes in land use and climate in the study basin. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Suspended sediment; Floodplain sedimentation; Overbank deposits; Sediment sources; Source tracing; Environmental change

1. Introduction Information on suspended sediment yields from river basins is usually obtained by monitoring suspended sediment fluxes at downstream gauging sites Žcf. Walling and Webb, 1996.. Such records frequently span only relatively short periods Žoften - 20 years. and the temporal representativeness of the data is, in consequence, frequently uncertain. Similarly, studies of suspended sediment provenance, involving use of the fingerprinting approach, are commonly based on the collection of suspended sediment samples during high discharge events Žcf. He and Owens, 1995; Walling and Woodward, 1995; Walden et al., 1997; Collins et al., 1998. and thus only provide information relating to sources at the time of sampling. Given the possible effects of both climate change and changes in land use and management on catchment sediment budgets, there is a need to place information on contemporary suspended sediment fluxes and sediment provenance into a longer-term context, both to assess the representativeness of the data obtained from short-term monitoring programmes and to identify longer-term trends. In the absence of long-term river monitoring data, the sedimentary record found in depositional environments such as lakes, reservoirs and river floodplains, can be used to provide information on the past behaviour of a river basin. The use of lacustrine sediments to investigate the variation of sediment yields and sources over a range of different time periods is well established Že.g., Foster et al., 1985; O’Hara et al., 1993; Foster and Walling, 1994; Heathwaite, 1994; Owens and Slaymaker, 1994; Page and Trustrum, 1997; Zolitschka, 1998.. Similarly, the evidence provided by accumulating floodplain sediments deposited during overbank floods has also been used to reconstruct past changes in sediment sources Že.g., Passmore and Macklin, 1994; Foster et al., 1996; Collins et al., 1997; Foster et al., 1998. and to provide information on changing sediment fluxes Že.g., Rumsby and Macklin, 1994; Walling and He, 1994; Knox and Kundzewicz, 1997; Walling and He, 1999.. The selection of the most suitable depositional environment for such studies depends on the nature of the investigation, the character of the drainage basin and the time period over which sediment is likely to have accumulated. In the UK, there are numerous lakes and reservoirs in upland areas which can be used to investigate historical changes in the sediment response of upland basins, but floodplain sediments often represent the best source of information for lowland environments. This paper reports the use of sediment cores collected from river floodplains to provide information on changing suspended sediment sources and sedimentation rates at

P.N. Owens et al.r Catena 36 (1999) 21–47

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seven sites within the non-tidal component of the catchment of the River Ouse, and one of its main tributaries, the River Wharfe, in Yorkshire, UK. This investigation represents part of a larger project Žcf. Walling et al., 1998a,b, 1999a,b. undertaken within the Land–Ocean Interaction Study ŽLOIS. funded by the UK Natural Environment Research Council, which was concerned with monitoring and modelling material fluxes from the land to the ocean for parts of the UK. One of the aims of the investigation reported here was to place the estimates of contemporary suspended sediment flux obtained from the LOIS core monitoring programme Žcf. Wass and Leeks, 1999. and the information on contemporary suspended sediment sources reported by Walling et al. Ž1999a., into a longer-term Žca. 100 years. historical context. Historical reconstructions of sediment yield and sediment sources for upland areas within the study area, based on sediment records from small lakes and reservoirs, have also been undertaken and are presented in Lees et al. Ž1997. and Foster and Lees Ž1999a,b..

2. The study area and methods 2.1. The study area The River Ouse is one of the main rivers which drain into the Humber estuary in northeast England ŽFig. 1.. The River Ouse bears this name below the confluence of the River Ure and Ouse Gill Beck Žnear site 3, Fig. 1.. Its catchment area above the tidal limit at Naburn Weir, near Acaster Malbis Žca. site 1., is 3520 km2 . The River Wharfe, which drains a catchment of 818 km2 at the tidal limit at Tadcaster, is one of the main tributaries of the River Ouse and joins the latter below the tidal limit ŽFig. 1.. Unlike most of the other rivers which drain into the Humber estuary, the Ouse and Wharfe are largely unpolluted, gravel-bed rivers, which drain predominantly rural, agricultural areas with a low population density. Topographically, their catchments are dominated by the Pennine Hills in the west, which rise to over 700 m, and the western edge of the North York Moors to the east, which rise to over 300 m within the study area. The Vale of York, a relatively flat, low-lying area, separates these two areas of higher relief. The underlying solid geology of the study area is dominated by Carboniferous limestones and Millstone grit in the west, which progressively give way to Permian magnesian limestone and marls, and Triassic sandstones and marls towards the east ŽFig. 1.. Further east, limestones, shales and sandstones of Jurassic age dominate in the North York Moors. In many areas, particularly in the Vale of York, the solid geology is overlain by glacial drift deposits and lacustrine silts and clays ŽFig. 1., but the topography primarily reflects the underlying solid geology. In the uplands, the soils are dominated by peats and stagnohumic and stagnogley soils, while in the lowlands, stagnogleys, sandy gley soils and brown earths are common. The land use is strongly related to the underlying geology and topography. Pasture and rough grazing are dominant in the upland areas, whereas most of the land in the Vale of York is cultivated. There is relatively little woodland in the study area. Mean annual precipitation ranges from about 600 mm at the tidal limit to over 1800 mm in the headwaters in the Pennine Hills.

24

P.N. Owens et al.r Catena 36 (1999) 21–47

Fig. 1. Location of the study area and the floodplain coring sites, and maps showing the general distribution of the three main geologies and the overlying Quaternary drift deposits.

P.N. Owens et al.r Catena 36 (1999) 21–47

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2.2. Core collection Floodplain sediment cores were collected from seven sites within the study area during 1994 and 1995, using a motorised percussion corer equipped with a steel core tube Ž98 cm2 surface area., which was driven into the sediment to depths of ) 50 cm. A previous reconnaissance survey had shown that 137Cs was unlikely to extend below 50 cm depth. The sediment cores were collected from representative locations, which were identified as being both uncultivated Žbecause cultivation would mix the accumulating overbank sediment. and susceptible to regular overbank flooding. Because attention focused on the Ouse basin, one core was collected from each of three sites along the floodplain of the River Ouse, while a single core was collected from the floodplain of the River Wharfe ŽFig. 1.. One core was also collected from the floodplains of each of the three main upstream tributaries of the River Ouse, namely, the Rivers Swale, Ure and Nidd ŽFig. 1.. Because only a single core was collected from each site, it has necessarily been assumed that the information obtained from that core is representative of conditions at that site. Problems associated with this assumption are, however, discussed later. The sampling sites were all located in the downstream reaches of the study rivers, in order to provide information on changes in the sediment response of the upstream catchments. The cores were sectioned into 1 or 2 cm increments, and these increments were air-dried and prepared for laboratory analysis. 2.3. Establishing core chronologies and sedimentation rates Floodplain core chronologies and associated sedimentation rates were determined using measurements of the environmental radionuclides 137Cs and unsupported 210 Pb Žcf. Walling and He, 1994; He and Walling, 1996; Walling and He, 1997.. Radionuclide activities were assayed simultaneously by g-ray spectrometry, using a low background, low energy, hyperpure n-type germanium coaxial detector ŽEG & G ORTEC LOAX HPGe. coupled to a multi-channel analyser. Cs-137 activities were determined from the 662 keV photopeak, while unsupported 210 Pb activities were determined from the difference between the total 210 Pb activity Žat 46.5 keV. and the 226 Ra-supported 210 Pb activity Žcalculated from the activity of its short-lived daughter 214 Pb at 352 keV.. Samples were sealed for 21 days prior to analysis to allow for equilibrium between 226 Ra and 222 Rn Ž210 Pb is derived from the decay of gaseous 222 Rn, the daughter of 226 Ra. Žcf. Joshi, 1987.. Count times were typically in the range 50,000 to 86,000 s, giving a measurement precision of between ca. "5% and "10% at the 95% level of confidence. Average rates of overbank sedimentation were determined for two different time periods. The depth distribution of 137Cs was used to estimate the average sedimentation rate since 1963. The approach is described in detail in Walling and He Ž1997.. In essence, it is based on the known temporal pattern of atmospheric fallout of bomb-derived 137Cs, which peaked in 1963, and the assumption that the peak 137Cs concentration in the floodplain sediment profile can be equated with the 1963 fallout peak. Because of the well documented slow downward migration of the 137Cs peak in undisturbed soil profiles, caused by processes such as bioturbation and leaching Žcf. Owens et al., 1996.,

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P.N. Owens et al.r Catena 36 (1999) 21–47

which can be assumed to also occur in floodplain sediments ŽWalling and He, 1997., the sedimentation rates estimated from the depth of the 137Cs peak were corrected for post-depositional redistribution of sediment-associated 137Cs within the profile. This redistribution was estimated to result in downward migration of the 137Cs peak by ca. 0.05 g cmy2 yry1 . The average overbank sedimentation rate for each core over the last ca. 100 years was determined using the unsupported 210 Pb measurements and the CICCS model proposed by He and Walling Ž1996. for floodplain sediments. Unlike 137Cs, the fallout of unsupported 210 Pb can be assumed to be effectively constant through time. Due to radioactive decay, accumulating overbank sediments tend to exhibit an exponential decrease in unsupported 210 Pb content with depth, and the rate of decrease enables the average sediment accumulation rate over the last ca. 100 years Ži.e., ca. five times the 22.2 years half-life of 210 Pb. to be estimated. The structure and relative simplicity of the CICCS model means that potential problems associated with post-depositional redistribution of the unsupported 210 Pb depth profile are effectively eliminated. Further details can be found in He and Walling Ž1996.. 2.4. Fingerprinting sediment sources Sediment source tracing employed the fingerprinting approach and was based on a comparison of the properties of sediment from individual sections of the floodplain cores with those of potential source materials. The approach used was similar to that used for contemporary source tracing in the study area, which is explained in detail in Walling et al. Ž1999a. Žsee also Collins et al., 1997.. In brief, over 160 samples Ž) 500 g mass. were collected throughout the study area, in order to characterise potential source materials. The study area was divided into three spatial zones, which correspond to the three main geologicalrtopographic zones, namely: Carboniferous ŽYorkshire Dales., Permian and Triassic ŽVale of York., and Jurassic ŽNorth York Moors. Žsee Fig. 1.. In each of these zones, representative samples were collected from the face of eroding channel banks and ditches and from the surface Žca. top 2 cm. of woodland, uncultivated Žpasture, rough grazing and moorland. and cultivated areas. The samples were air-dried at ca. 408C. In order to use the fingerprinting approach to establish the source of the floodplain sediment, sediment samples representative of floodplain sediment and potential source materials were analysed for a variety of diagnostic properties. However, not all of the properties employed by Walling et al. Ž1999a. for determining the sources of contemporary suspended sediment are suitable for tracing the sources of floodplain sediment. For example, downcore variations in 137Cs levels in floodplain sediment are primarily controlled by temporal variations in atmospheric fallout, which complicate any interpretation of variations in 137Cs concentration in terms of changing sediment sources. Similarly, organic constituents such as C, N and P are also unsuitable because of their non-conservative nature. Sediment source tracing was, therefore, based on the use of geochemical and mineral magnetic properties. Magnetic parameters were measured at Coventry University using a Bartington MS2B dual frequency sensor Ž x lf , x fd ., and a Molspin Pulse Magnetiser and a Minispin Fluxgate Magnetometer ŽSIRM Žs IRM at 0.8 T... Chemical elements Ži.e., Al, Ca, Cr, Cu, Fe, K, Mg, Mn, Na, Ni, Pb, Sr and Zn.

P.N. Owens et al.r Catena 36 (1999) 21–47

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were analysed using a Unicam 939 Atomic Absorption Spectrophotometer after digestion of the sediment in concentrated hydrochloric and nitric acid Žcf. Allen, 1989.. Both floodplain sediment samples and source materials were analysed for each determinand using the same procedures to permit direct comparison. Furthermore, in order to facilitate direct comparison of the tracer property concentrations associated with the various materials, all analyses were undertaken on the - 63 mm fraction. Additional correction for differences in particle size composition between floodplain sediment and source materials was based on the specific surface areas of the samples. These were estimated from the absolute particle size composition of the mineral fraction, determined using a Coulter LS130 laser diffraction granulometer, after standard chemical and ultrasonic pretreatment. The statistical and numerical procedures used for source tracing are described in Walling et al. Ž1999a.. In essence, a two-stage statistical procedure was employed to determine suitable composite fingerprints. First, the Mann–Whitney U-test or Kruskal– Wallis H-test was used to identify which geochemical and mineral magnetic sediment properties were able to differentiate the different source groups. Secondly, Multivariate Discriminant Function Analysis was used to determine the best composite fingerprint Ži.e., the set of tracer properties that affords optimum discrimination between source groups., comprising a selection of those properties identified in stage one. A numerical mixing model was then used to establish the relative contribution of the different sources to the sediment sample. For each of the tracer properties i in the composite fingerprint, a linear equation is constructed that relates the concentration of property i in the floodplain sediment sample to that in the mixture representing the sum of the relative contributions from the different source groups. Thus, the composite fingerprint is represented by a set of linear equations Žone for each of the properties in the composite fingerprint.. Instead of solving the set of linear equations directly, the least-squares method was used, and the relative contributions of the individual sources s are established by minimizing the sum of squares of the residuals Ž R es . for the n tracer properties and m source groups involved, using 2

m n

R es s Ý is1



Cf i y

ž

Ý Cs i Ps ss1

Cf i

/

0

Ž 1.

where Cf i is the concentration of tracer property i in the floodplain sediment sample, Cs i is the mean concentration of tracer property i in source group s and Ps is the relative contribution of source group s. The model must satisfy two linear constraints, namely: Ža. the relative contribution from each source must lie within the range 0 to 1, i.e., 0 F Ps F 1 Ž 2. Žb. the sum of the relative contributions from all sources must equal 1, i.e., n

Ý Ps s 1.

Ž 3.

ss1

An assessment of the goodness-of-fit provided by the optimised mixing model Žcf. Collins et al., 1997; Walling et al., 1999a. gave mean relative errors for the predicted

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P.N. Owens et al.r Catena 36 (1999) 21–47

concentrations of individual fingerprint properties which typically ranged between "7% and "14% Ži.e., of the order of "10%.. This indicated that the mixing model is able to provide an acceptable prediction of the concentrations of the fingerprint properties associated with individual sediment samples associated with each floodplain core.

3. Results 3.1. Changes in sedimentation rate oÕer the last ca. 100 years Fig. 2 illustrates the contrasting depth distributions of 137Cs and unsupported 210 Pb in floodplain cores collected from sites characterised by different sedimentation rates. In the case of the core collected from site 1 ŽFig. 2a. the 1963 137Cs peak is located at ca. 21 cm depth Ž31 g cmy2 accumulated mass., while the peak for the core collected from site 2 ŽFig. 2b. is located at ca. 10 cm depth Ž9 g cmy2 accumulated mass.. The unsupported 210 Pb profiles for the two cores exhibit an essentially uniform reduction in concentration with depth, which is primarily a function of radioactive decay of unsupported 210 Pb in the deposited sediment, with differences in the shape of the profiles from the two sites mainly reflecting differences in sedimentation rate. The relatively uniform nature of the reduction in unsupported 210 Pb with depth in the two cores indicates that overbank sedimentation is likely to have been quasi-continuous and that there is no evidence of major discontinuities in sedimentation at these two sampling locations. Small downcore fluctuations in unsupported 210 Pb concentrations may reflect variations in the unsupported 210 Pb content of deposited sediment Ždue to variations in sediment source or the particle size composition of the deposited sediment., variations in the depth of sedimentation associated with individual overbank floods and in the frequency of such floods, or uncertainties associated with the analytical procedure. The differences in the 137Cs and unsupported 210 Pb concentrations evident between the two cores reflect several factors including contrasts in the radionuclide fallout flux Žand thus precipitation amounts., in the radionuclide content of the deposited sediment, and in the sedimentation rate, between sites 1 and 2. Table 1 presents estimates of average sedimentation rate over the last ca. 30 and 100 years for each of the seven floodplain cores. Unfortunately, the radionuclide depth distribution for the core from site 4 indicated that this coring site had been disturbed and the 137Cs-based sedimentation rate presented in Table 1 was based on another core collected nearby as part of the study reported in Walling et al. Ž1998b.. The average sediment accumulation rates show considerable variation between sites and range from 0.11 Žsite 7. to 1.04 g cmy2 yry1 Žsite 1.. This variation reflects differences in the magnitude, duration and extent of flooding and also differences in the suspended sediment concentrations associated with overbank flows for the various rivers and sites. Also, the cores were collected at different distances from the channel, and previous work by the authors in the study area has demonstrated a general trend of decreasing sedimentation rate with increasing distance from the channel Žcf. Walling et al., 1998b, 1999b..

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Fig. 2. The depth distribution of 137Cs and unsupported 210 Pb in the floodplain cores collected from sites 1 Ža. and 2 Žb..

When the average sedimentation rates estimated for the two different time periods for each core are compared, they are in some cases Ži.e., sites 2 and 6. almost the same. In other cases Ži.e., sites 1, 3 and 5., there is evidence to suggest that average sedimentation rates over the last ca. 30 years were less than those for the last ca. 100 years, while for site 7, sedimentation rates appear to have increased in recent times. For most of the sites, the sedimentation rates estimated for the last ca. 30 years are within "10% of the average for the last ca. 100 years, and thus rates of overbank deposition can be seen as being essentially constant over the entire period. The only major exception is the core taken from the River Ure at site 5, where the average sedimentation rate for the last ca.

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Table 1 Average floodplain sediment accumulation rates for the last ca. 32 and 100 years estimated using unsupported 210 Pb measurements Sitercatchment

Ž1. Lower Ouse Ž2. Middle Ouse Ž3. Upper Ouse Ž4. Swale Ž5. Ure Ž6. Nidd Ž7. Wharfe

137

National grid reference

Cs-based sedimentation rate Žg cmy2 yry1 .

SE597460 SE512578 SE467620 SE363799 SE315731 SE487559 SE231457

0.95 0.24 0.64 0.50 0.18 0.17 0.13

137

Cs and

Unsupported Pb-based sedimentation rate Žg cmy2 yry1 . 210

1.04 0.23 0.68 – 0.42 0.17 0.11

30 years is only 43% of the longer-term value. This recent reduction in overbank sedimentation at site 5 could reflect either local changes in overbank sedimentation at this specific sampling point, or basin-wide changes in sediment delivery and deposition in the Ure basin. 3.2. Downcore Õariations in sediment properties Prior to using the geochemical and mineral magnetic properties of the floodplain sediment cores to reconstruct past sediment sources, it is important to examine the downcore variations in these sediment properties. This is necessary in order to establish whether any of the properties exhibit downcore variations which could reflect controls other than changes in sediment source, such as post-depositional transformation in response to pedogenesis, which would compromise their effectiveness as source fingerprints. Significant downcore variations in the various geochemical and mineral magnetic properties are evident within the seven floodplain cores, and these probably reflect a combination of changes in sediment source, variations in particle size composition and organic matter content and, possibly, in situ changes in sediment properties. However, when all of the properties are examined together, there is no evidence of a common downcore trend, because some properties remain approximately constant with depth, while others either increase or decrease with depth. For example, for the floodplain core collected from site 1 ŽFig. 3a., Fe shows a significant Ž r 2 s 0.87. positive correlation with depth, Ca exhibits a significant Ž r 2 s 0.87. negative correlation with depth, while Al exhibits no significant Ž r 2 s 0.01. relationship with depth. The downcore variations in property concentrations for the seven cores cannot be explained solely in terms of variations in sediment particle size or organic matter content. In some investigations, element concentrations have been standardised relative to Al, in order to reduce the effects of downcore variations in particle size composition and organic matter content, and possible post-depositional transformations Žcf. Horowitz, 1991.. However, if the resulting downcore variations in standardised element concentrations are to be used to decipher changes in sediment source, it is necessary to assume that Al concentrations are

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Fig. 3. Downcore variations in various geochemical properties for selected floodplain cores: Ža. depth distributions of Al, Ca and Fe in the core collected from site 1; Žb. depth distributions of Ca in the cores from sites 1, 2 and 6; and Žc. depth distributions of Pb and Zn for the core from site 2.

P.N. Owens et al.r Catena 36 (1999) 21–47

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independent of source. However, in this study Al is included in the composite fingerprint for geologicalrtopographic source ascription, and its use for standardisation is, therefore, precluded. Furthermore, in the mixing model, the potential problem associated with the effect of downcore variations in particle size on property concentrations has been reduced by correcting for contrasts in particle size composition between material associated with individual source groups Žrepresented by a mean value for a particular source group. and each floodplain sediment increment. There are also contrasts in the behaviour of specific properties between the cores, such that a given property may either increase, decrease or remain constant with depth in different cases Žsee for example Fig. 3b.. Table 2 lists the results of correlating the concentrations of individual sediment properties with depth for the seven floodplain cores. For all cores, there are significant correlations for many of the sediment properties, which could reflect either a progressive change in sediment source through time or progressive in situ property transformation through time. It is not possible to interpret precisely the relationship between property concentration and depth for each property for each core, but the results presented in Table 2 suggest that in some cases downcore variations in sediment properties may be due to post-depositional transformations within the overbank sediment. However, it is unlikely that all of the mineral magnetic and geochemical properties used in this study will be affected by post-depositional transformations and the use of composite fingerprints, incorporating a combination of mineral magnetic and geochemical properties, should reduce the potential

Table 2 Correlation coefficients Ž r . associated with the relationship between property concentration and depth for the floodplain sediment cores collected from the seven sites in the Ouse basin Property

Geochemical

Mineral magnetic

Floodplain core

n Al Ca Cr Cu Fe K Mg Mn Na Ni Pb Sr Zn n x lf SIRM

1

2

3

4

5

6

7

32 0.10 0.93U 0.82U 0.28 0.93U 0.49U 0.95U 0.32 0.66U 0.42U 0.14 0.01 0.54U 20 0.20 0.10

31 0.44U 0.24 0.74U 0.96U 0.51U 0.37U 0.36 0.85U 0.46U 0.39U 0.28 0.03 0.59U 31 0.96U 0.95U

33 0.79U 0.49U 0.41U 0.46U 0.89U 0.87U 0.65U 0.17 0.75U 0.87U 0.10 0.44U 0.24 33 0.73U 0.59U

27 0.28 0.40U 0.70U 0.22 0.17 0.56U 0.32 0.73U 0.60U 0.33 0.40U 0.10 0.50U 27 0.10 0.66U

32 0.40U 0.10 0.66U 0.10 0.60U 0.10 0.40U 0.30 0.55U 0.46U 0.40U 0.40U 0.55U 32 0.87U 0.92U

25 0.71U 0.89U 0.72U 0.88U 0.54U 0.52U 0.63U 0.40 0.37 0.60U 0.93U 0.06 0.85U 25 0.59U 0.57U

25 0.52U 0.72U 0.60U 0.48U 0.72U 0.59U 0.81U 0.45U 0.10 0.87U 0.94U 0.75U 0.89U 25 0.92U 0.33

ns Number of samples Žfloodplain sediment increments.. Significant Ž p- 0.05..

U

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problems associated with such transformations. However, these problems must be borne in mind when interpreting the source tracing results presented below. Fig. 3c presents the depth distributions of Pb and Zn in the core from site 2 and the pattern seen in this core is broadly similar to that found in the cores collected from most of the other sites Ži.e., sites 1, 3, 4, 5 and 6.. In this core, Pb and Zn concentrations increase dramatically upcore, from relatively low background levels at depth to peak levels of ca. 2500 and 1000 mg gy1 , respectively, at ca. 38 to 45 cm depth. Above this level, concentrations gradually decrease towards the surface. The patterns illustrated in Fig. 3c cannot be explained by downcore variations in particle size composition and organic matter content and instead reflect the effects of metal mining for Pb and Zn in the headwaters of the Ouse basin, where veins carrying galena ŽPb sulphide. and blende ŽZn sulphide. are found in the Lower Carboniferous rocks. Smelting and dumping of mine waste and the uncontrolled discharge of fine-grained metalliferous waste into rivers resulted in widespread metal contamination of river sediment from the Pennines down to York and beyond Žcf. Macklin et al., 1997.. Large-scale mining in the Pennines was restricted to the early to middle 19th century Žalthough the peak period is likely to vary locally within the study area., after which most of the readily accessible ores were worked-out. Three important observations can be made in relation to the Pb and Zn profiles associated with the floodplain sediment cores. First, the enhanced Pb and Zn concentrations provide evidence that contaminated sediment, originally derived from the mining areas in the upstream headwaters, has been transported to the lower reaches of the study rivers, and then deposited on the floodplains during overbank events. Secondly, it is possible to use the Pb and Zn profiles in the floodplain cores to provide an independent check on the core chronologies provided by the 137Cs and unsupported 210 Pb measurements. In all cases, there is a close agreement between the age of the sediment derived using the radionuclides and the Pb and Zn profiles and this correspondence adds confidence to the sedimentation rates presented in Table 1. Thus, for example, the peak in Pb and Zn concentrations in the core from site 2 can be equated with the period of peak mining activity, and this is consistent with the sediment chronology based on extrapolation of the unsupported 210 Pb-based sedimentation rate Žsee later comments on extrapolation., which provides a tentative date of ca. 1810s for the Pb peak at ca. 45 cm depth and ca. 1840s for the Zn peak at ca. 38 cm depth. Thirdly, although Pb and Zn concentrations are significantly correlated with depth for many cores ŽTable 2., the close correspondence between the shape of the Pb and Zn profiles and the history of mining activity in the Pennines suggest that significant correlations between property concentrations and depth are not necessarily due to property transformations. The implications of the downcore variations in Pb and Zn for source tracing are discussed below. 3.3. Variations in sediment sources oÕer the last ca. 100 years Geochemical and mineral magnetic properties have been used with the fingerprinting approach to examine downcore, and thus temporal, changes in sediment sources for the cores collected from the seven locations. Due to the similar nature and characteristics Ži.e., geology, topography and land use. of the Ouse and Wharfe basins, the source

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material signatures for both basins have been pooled. The cores from all seven locations were examined for changes in the main source types Žtopsoil or subsoilrchannel bank. and in source areas, as represented by the three main geologicalrtopographic zones ŽCarboniferous, Permian and Triassic, and Jurassic.. The only exception was the core collected from the River Wharfe Žsite 7. for which only changes in source type were considered, because the catchment contributing to this site is underlain solely by Carboniferous rocks. 3.3.1. Source type For the purpose of source type ascription, source materials were originally classified into three groups, representing topsoil from woodland, uncultivated and cultivated areas, and also a group representing material from eroding channel banks, ditches and subsoil sources Žtermed subsoilrchannel bank.. However, the composite fingerprint selected by the Discriminant Function Analysis classified - 70% of the source materials correctly. Because of the relatively poor discrimination between the four source groups, source materials were subsequently classified simply as either topsoil or subsoilrchannel bank material. Table 3 presents the results of applying the Mann–Whitney test to assess the ability of the individual geochemical and mineral magnetic properties to discriminate these two source groups. In the case of Pb and Zn, the influence of historic metal mining in the upland parts of the basin on their depth distributions in most of the floodplain cores Žcf. Fig. 3c. means that they are unsuitable for establishing downcore variations in Table 3 Significance levels from the Mann–Whitney U-test Žtopsoil, subsoilrchannel bank. and Kruskal–Wallis H-test Žgeological zones. used to establish the ability of each tracer property to discriminate between source groups Tracer property

Topsoil vs. subsoilr channel bank a

Geological zones b

x lf x fd SIRM Al Ca Cr Cu Fe K Mg Mn Na Ni Pb Sr Zn

0.001U 0.001U 0.001U 0.300 0.824 0.202 0.757 0.001U 0.001U 0.618 0.001U 0.796 0.001U 0.194 0.016U 0.002U

0.002U 0.001U 0.281 0.001U 0.001U 0.566 0.766 0.012U 0.001U 0.001U 0.001U 0.027U 0.011U 0.004U 0.001U 0.064

a

Topsoil represents the top ca. 2 cm soil from uncultivated, cultivated and woodland fields, while subsoilrchannel bank represents material collected from eroding channel banks, ditches and subsoil sources. b The three main geological zones are Carboniferous, Permian and Triassic, and Jurassic. U Significant Ž p- 0.05..

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sediment source, and they have been excluded from the source tracing analysis. Furthermore, because of the effects of mining, the concentrations of Pb and Zn in most of the floodplain cores are considerably greater than the range of values for the potential sources, even when enrichment effects due to particle size differences are taken into account, and this further compromises the use of these two elements for source tracing. Table 4 presents the results of the Discriminant Function Analysis which was used to select the best composite fingerprint and Fig. 4 presents the results of applying the mixing model to the seven cores. The sediment chronology for each core has been derived from the sedimentation rate estimates provided by the 137Cs and unsupported 210 Pb measurements, with levels between 1963 and 1994r1995 established using the 137 Cs-based sedimentation rate, and levels prior to 1963 being estimated by either interpolation Žfor the period 1963 to ca. 100 years B.P.. or extrapolation of the unsupported 210 Pb-based sedimentation rate. Due to the problems and errors associated with extrapolating sedimentation rate estimates beyond the time period for which they were derived, dates in excess of 100 years B.P. should be treated with caution. In general, topsoil is seen to be the dominant sediment source for most of the sites and this reflects the importance of soil erosion from agricultural land Žpasture and arable. in the study area. However, for the cores from sites 2 and 3 ŽRiver Ouse. and site 7 ŽRiver Wharfe., a significant amount of the overbank sediment has been derived from subsoilrchannel banks. For the core collected at the downstream limit of the River Ouse

Table 4 Results from the stepwise Multivariate Discriminant Function Analysis used to identify the optimum combination of tracer properties for use as a composite fingerprint for discriminating source groups Tracer property

Cumulative % samples classified correctly

Topsoil Õs. subsoilr channel bank a x lf Fe K Sr x fd Mn Ni

61.49 79.09 80.91 84.55 83.64 84.55 90.00

Geological zones b Mn Mg Al K Ca Sr Ni Fe

57.46 60.45 62.69 61.94 59.70 63.43 67.16 70.15

a

Topsoil represents the top ca. 2 cm soil from uncultivated, cultivated and woodland land, while subsoilrchannel bank represents material collected from eroding channel banks, ditches and subsoil sources. b The three main geological zones are Carboniferous, Permian and Triassic, and Jurassic.

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Fig. 4. Downcore changes in the relative contributions of topsoil and subsoilrchannel bank sources for the seven floodplain cores.

at Acaster Malbis Žsite 1., the mixing model results suggest that, in general, there has been little change in sediment source over the last ca. 100 years represented by this core. Topsoil has remained the dominant source throughout this period, although there is some

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evidence of an increase in topsoil contributions from ca. 60% at over 70 cm depth to ca. 80% at the surface. Superimposed on this general trend, there are periods of increased subsoilrchannel bank contributions at ca. 6–8 cm Žwhich is dated at ca. 1984., 38–44 cm Ž1940s. and below 60 cm Ž1920s. depth, which may reflect increased contributions from channel banks and ditches during high-magnitude flood events. The relatively constant contributions from the two source types at this site probably reflect its downstream location, because the effects of changes in sediment sources in the upstream catchment associated with one subcatchment or a specific area are likely to be masked by mixing with sediment contributions from other parts of the basin. Thus, although the individual subcatchments contributing to the River Ouse may experience temporal variations in the relative contribution from different sources Žsee Fig. 4., synchronous basin-wide changes would be required to produce a pronounced shift in the sediment source record at the downstream tidal-limit of the River Ouse Žsite 1.. In contrast, the information on changes in sediment source obtained for the cores from sites 2 to 7 is more variable. For example, in the case of the cores collected from sites 2 and 3 Žmiddle and upper River Ouse. contributions from subsoilrchannel bank material are generally dominant in the lower sections of each core, but there is a noticeable increase in the contribution from topsoil sources between depths of 6 and 22 cm and between ca. 16 and 50 cm, respectively, which can be dated to the period extending from the 1900s to the 1960s. The temporal variations of sediment source type shown by sites 4 ŽRiver Swale., 5 ŽRiver Ure. and 6 ŽRiver Nidd. are broadly similar to those shown by sites 2 and 3 on the River Ouse, with contributions from the two main sources being approximately constant below ca. 30 cm Žprior to the 1920s., 50 cm Žprior to the 1850s. and 20 cm Žprior to the 1870s., respectively. Above these depths, there are pronounced and steady increases in the contributions from topsoil sources, although contributions from subsoilrchannel bank material increase again in the post-1963 period. The broad correspondence of the timing of source changes between sites 2 and 3 Žmiddle and upper River Ouse. and those recorded for sites 4, 5 and 6 on the three tributaries is encouraging and suggests that there may have been similar changes in land use andror hydrological conditions across the Swale, Ure and Nidd catchments. Slight differences in the timing of source changes recorded for these upstream subcatchments would, however, tend to attenuate the pattern of changes in sediment source recorded at the downstream limit Žsite 1.. The sediment core collected from the River Wharfe at site 7 exhibits a more complex pattern of changing source contributions over time, with pronounced shifts in the relative importance of topsoil and subsoilrchannel bank sources. Generally, the contributions from the two source groups are approximately equal, although there are several peaks in the contribution from topsoil sources, with the most recent occurring at ca. 6–8 cm depth Ž1950s.. 3.3.2. Spatial location (geologicalr topographic zones) In order to fingerprint sediment originating from different parts of the study area, and more particularly from different geologicalrtopographic zones, representative source materials were collected from the three main geological zones in the study area, which

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broadly coincide with the three main topographic areas, i.e., Carboniferous ŽPennines., Permian and Triassic ŽVale of York. and Jurassic ŽNorth York Moors.. Although there are areas where the solid geology is overlain by Quaternary glacial drift deposits Žsuch as boulder clay. and lacustrine deposits Žsee Fig. 1., these mainly occur in the Vale of York and the land immediately bordering the Vale. Thus the three geologicalrtopographic zones identified above are broadly representative of the main zones of the study area, as defined by solid geologyrdrift and topography, and, for convenience, are referred to as geological zones. Table 3 presents the results of applying the Kruskal– Wallis test to assess the ability of each property to distinguish between geological source groups. It is important to note that those properties that were successful in this test were able to distinguish between source groups, classified in terms of the underlying solid geology, irrespective of the effects of overlying Quaternary glacial drift and lacustrine deposits. The composite fingerprint, as selected by Stepwise Discriminant Function Analysis, is given in Table 4, and this fingerprint was able to classify 70% of the source materials into the correct source group. This lower level of source group discrimination, compared to that achieved for the source type composite fingerprint Ž90%., may partly reflect the fact that glacial drift deposits Žunlike sand and gravel, and lacustrine deposits. are not confined solely to the Permian and Triassic source group, as they also overlie Carboniferous and Jurassic rocks in places. Despite this complication, a level of discrimination of 70% for the three geological zones can still be seen as high, and indicates that the geological source group results can be treated as meaningful. The results of applying the mixing model to the floodplain sediment cores collected from sites 1 to 6 are presented in Fig. 5 Žthe mixing model has not been applied to the core collected from site 7 as the upstream catchment contains Carboniferous rocks only.. For some sites, there are no major changes in the relative importance of the contributions from different geological zones over the time period associated with each core. For example, the core from site 6 ŽRiver Nidd. is dominated Žca. 80%. by contributions from areas underlain by Carboniferous rocks, which outcrop in the Pennine Hills to the west, with contributions from areas underlain by Permian and Triassic strata averaging ca. 20%. Because most of this subcatchment is underlain by Carboniferous rocks and there are no outcrops of Jurassic rocks, these results are consistent with the relative proportion of the catchment occupied by each geological zone. Similarly, for the core collected from the River Ouse at site 2, the contributions from the three geological zones are again effectively constant through time, with sediment contributions from areas underlain by Carboniferous rocks dominating Žca. 60%., and areas underlain by Permian and Triassic rocks providing an important secondary contribution Žca. 40%.. There is no Žor so low that it is not recognised by the mixing model. contribution from Jurassic rocks to this site for any time period, and this is thought to reflect the fact that the core was collected close to the confluence with the River Nidd. Thus, it appears that at site 2, most overbank sediment originates from the catchment of the River Nidd. For other sites, there are more pronounced temporal variations in sediment contributions from the main geological zones. In the case of the core collected from site 5 ŽRiver Ure. there are no Jurassic rocks in the upstream catchment, and thus sediment is derived from areas underlain by either Carboniferous or Permian and Triassic rocks. In this subcatchment, the areas underlain by Carboniferous rocks are the dominant sediment

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Fig. 5. Downcore variations in the relative contributions from the three main geological zones ŽCarboniferous, Permian and Triassic, and Jurassic. in the study area for the floodplain cores collected from sites 1 to 6.

source and these results are consistent with the areal dominance of this rock type in the upstream catchment. There is evidence of two major peaks of increased contributions from the areas underlain by Carboniferous rocks which can be approximately dated to the 1850s and 1940s, and of increasing contributions from the areas of Carboniferous rocks since ca. 1960. For the cores from sites 1 and 3 ŽRiver Ouse. and site 4 ŽRiver Swale., the temporal variations in sediment contributions from the different geological source areas are more complex. In the case of the core from the River Swale, contributions from areas underlain by Permian and Triassic rocks generally dominate and this is likely to reflect the proximity of this geological zone to the sampling site. There are, however, periods with increased contributions from areas of Carboniferous and Jurassic rocks, which tend to occur simultaneously, between ca. 2 and 22 cm depth Ždated at ca. 1940s to 1990s.

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and below 30 cm depth Ž1920s.. For the cores collected from the River Ouse at sites 1 and 3, there are again contributions from all three geological source groups. However, at site 3, contributions from areas underlain by Carboniferous rocks are found only below ca. 50 cm depth Žca. 1900s.. For this site, sediment derived from areas of Permian and Triassic rocks has tended to increase over the last ca. 100 years, with recent increases in sediment derived from areas of Jurassic rocks in the upper ca. 16 cm Žpost-1963.. The lack of sediment contributions from areas underlain by Carboniferous rocks above 50 cm depth at site 3 is inconsistent with the results for the River Ure at site 5, where sediment contributions for the same time period Ži.e., post-1900s. are derived mainly from areas of Carboniferous rocks. It is possible, that this inconsistency may reflect incomplete mixing of suspended sediment below the confluence of the Rivers Ure and Swale during overbank flood events and that the sediment deposited at site 3 has been derived mainly from the River Swale. This explanation is consistent with the location of site 3 relative to the Rivers Swale and Ure Žsee Fig. 1., and the broad similarity in the geological source results for site 3 with those for site 4 ŽRiver Swale.. The unexpected results for site 3, and also those for site 2 described earlier, highlight the potential problems associated with the collection of sediment cores adjacent to confluences and indicate that careful thought needs to be given to sample collection for future work. It is also important to indicate that the composite fingerprint used for geological source identification was only able to classify ca. 70% of the source materials correctly Žsee Table 4. and this relatively low discrimination should be recognised as a possible source of uncertainty when interpreting the results for sites 2 and 3, described above. The increased contribution from areas underlain by Permian and Triassic rocks over the last ca. 100 years recorded at site 3 is also reflected in the core from site 1, as the latter site exhibits a gradual reduction in the contribution from areas underlain by Carboniferous rocks towards the present. The contribution from areas of Jurassic rocks to the overbank sediment collected from site 1 is approximately constant over the last ca. 100 years. The pattern recorded for Permian and Triassic areas in the floodplain cores from sites 1 and 3 could be interpreted as reflecting a recent increase in the contribution from cultivated land, which mainly occurs in the Vale of York.

4. Discussion 4.1. OÕerbank sedimentation and suspended sediment fluxes There are several points to consider when interpreting the information on sedimentation rates presented in Section 3.1. First, the results relate to changes in overbank sedimentation rates, which are assumed to be indicative of changes in suspended sediment fluxes at downstream sites, rather than upstream erosion rates. Thus, although the overbank sedimentation results imply that there have been no major changes in suspended sediment fluxes over the last ca. 100 years in the study area, it is possible that erosion rates in upstream areas have changed significantly, but that such changes have been buffered by sediment storage associated with sediment delivery to the downstream reaches Žcf. Walling et al., 1998b, 1999b; Walling, 1999.. Secondly, it is also important

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to recognise that the similarity in the overbank sedimentation rates for the last ca. 30 and 100 years ŽTable 1. will partly reflect the fact that the two periods are not mutually exclusive, in that they both incorporate the period since 1963. Thirdly, the overbank sedimentation rates are average values for periods of ca. 30 and 100 years, and there is likely to have been considerable variation in overbank sedimentation rates over shorter periods of time related to the incidence of large flood events, which may deposit considerably more sediment than the longer-term average. Thus, for example, in the core collected from the River Swale at site 4, illustrated in Fig. 6, there is a very coarse deposit at between ca. 31 and 35 cm depth Žcf. Walling et al., 1998a.. Using the 137Cs profile, this layer of coarse sediment can be dated to the late 1940s and ascribed to the major flood which occurred in the Swale catchment on 22 March 1947 ŽWilliams, 1957.. This single flood, therefore, deposited several centimetres of sediment, whereas

Fig. 6. Depth distributions of 137Cs and d 50 for the floodplain sediment core collected from the River Swale at site 4. The layer of coarse sediment centred at ca. 33 cm depth is probably due to a single major flood in 1947. Also shown is the d 50 of the overbank sediment deposited at this site Žafter the core was collected in 1994. during the floods in the Ouse basin in January and February 1995 Žcf. Walling et al., 1998a..

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the average sedimentation rate at this sampling location since the mid-1960s has been estimated to be ca. 0.50 g cmy2 yry1 Žca. 0.5 cm yry1 .. Unfortunately, there are no long-term records of the suspended sediment loads transported by the study rivers over the last ca. 100 years, to which the overbank sedimentation rates presented in Table 1 can be related. However, there is a long record of peak flood levels for the River Ouse at York Žcf. Longfield et al., 1995; Longfield and Macklin, 1999., which, although characterised by considerable temporal variability, shows clear evidence of an increase in both the magnitude and frequency of flooding over the last ca. 120 years, particularly since the 1940s. Such an increase in flooding in recent years might be expected to be associated with an increase in rates of overbank sedimentation. The apparent discrepancy in trends between the sedimentation rate data for the River Ouse Žsites 1, 2 and 3., which show no evidence of an increase in sedimentation rate towards the present, and the gauging records for York, could reflect a number of factors. First, the trend of increasing flood frequency and magnitude with time may have been offset by reduced suspended sediment concentrations in floodwaters during overbank events in recent decades. Secondly, many local residents and landowners believe that the study rivers have become far more flashy in recent decades, probably due to expansion of land drainage. Thus, although flood magnitude and frequency may have increased, the duration of overbank flows, and thus the time available for deposition, could have decreased through time. Thirdly, the 1963 peak in 137Cs fallout occurs approximately midway through the period of increased flood magnitude and frequency evidenced by the River Ouse at York Žwhich commenced in the early 1940s., and, as indicated above, comparison of time periods which are not mutually exclusive necessarily limits the resolution of the comparison. The essentially constant overbank sedimentation rates evidenced by the Ouse basin over the last ca. 100 years are, however, in general agreement with the detailed statistical analysis of the flood records available for UK rivers since the 1940s undertaken by Robson et al. Ž1998.. These authors analysed trends and variations in pooled annual data for 890 gauging stations in the UK and found no statistically significant trend of increasing flood magnitude or frequency over the period 1941 to 1990. They suggested that the lack of any overall trend in the flood data available for UK rivers means that there is no evidence that climate change has affected flood behaviour since the 1940s. Similarly, the evidence for essentially constant overbank sedimentation rates over the past 100 years described above for the Ouse basin is in broad agreement with results presented by Walling and He Ž1994, 1999. for the floodplains of other UK lowland rivers. It is also consistent with the reconstructed historical sediment yields for upstream areas presented by Foster and Lees Ž1999a., based on analysis of sediment cores collected from small lakes and reservoirs in headwater catchments in the LOIS study area, for which chronologies were established using 137Cs and unsupported 210 Pb measurements. For those reservoirs within or adjacent to the Ouse basin, the average sediment yields for the period covered by the 137 Cs depth profile were in most cases similar to the longer-term averages for the period since the reservoirs were impounded Žlast ca. 100–200 years.. Interestingly, the biggest change in sediment yield documented by Foster and Lees Ž1999a. for the study basin occurred in a small upland reservoir in the Wharfe catchment, where the average

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sediment yield between 1953 and 1995 was almost half the average sediment yield for the period since 1907. Such local variations in the timing and direction of changes in suspended sediment fluxes in headwater catchments, may obscure any trends recorded in overbank sediments in downstream reaches. 4.2. Changes in sediment sources Prior to discussing the source tracing results presented above, it is important to recognise that they relate to suspended sediment deposited during overbank flood events. However, since there is no reason to expect that suspended sediment transported by in-channel and overbank events should exhibit major differences in source, and because high magnitude events will account for a major proportion of the total suspended sediment flux, it has been assumed that the floodplain deposits are generally representative of the suspended sediment transported by the study rivers. In most cases, the source type results identify topsoil material as the dominant source of the overbank deposits. This demonstrates the importance of soil erosion, probably on both pasture and arable land, as a sediment source in the contributing catchments. However, significant contributions from subsoilrchannel bank sources are also found at all sites and these reflect, at least in part, the well developed channel banks Žlocally ) 2 m high. in the downstream reaches of the study rivers and the appreciable rates of erosion that characterise channel banks in the Ouse basin Žcf. Lawler et al., 1999.. There have been significant changes in the relative contribution of the two source types over time, which are likely to reflect changes in land use, and in particular the influence of land drainage, conversion of pasture to arable land and agricultural intensification. Metal mining in the headwaters may also have had an effect by locally disturbing land, but these effects are likely to have declined following the peak of mining activity in the mid-19th century. Land drainage activity in both upland and lowland areas in the basin is well-documented Žcf. Longfield and Macklin, 1999., with the most intensive periods of drainage occurring in the mid-19th century and again in the mid-20th century. Although the initial disturbance caused by drainage works could be expected to increase subsoil contributions, in the longer-term land drainage is likely to have encouraged cultivation and increased grazing pressure on pasture land, thereby increasing topsoil erosion. Land use records for the study area compiled by Longfield and Macklin Ž1999. show that between the 1860s and ca. 1940 there was a trend of decreasing arable land and increasing permanent grassland. This trend changed abruptly during the Second World War, and was followed by a rapid increase in the amount of arable land and a decrease in the relative proportion of permanent grassland. This latter trend continued after the mid-1940s, albeit at a reduced rate. The period of increased contributions from topsoil sources, which approximately corresponds to the late 19th and early and mid-20th centuries, therefore, largely coincided with a period of increasing permanent pasture and decreasing arable land. Although rates of topsoil erosion have been shown to be generally higher for arable land both in the UK in general Žcf. Morgan, 1988., and in the LOIS study area in particular Žcf. Foster and Lees, 1999a., studies in other agricultural catchments in the UK Že.g., Heathwaite et al., 1990; Foster and Walling, 1994. have

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demonstrated that rates of topsoil erosion on heavily overgrazed pasture land may be similar or even greater than those from arable land, due to the decreased infiltration rates and increased runoff and the reduced vegetation cover density associated with intensively grazed areas. The decrease in the contribution from topsoil sources in recent decades may reflect an increased awareness of the on-site and off-site problems associated with topsoil erosion on agricultural land and the effects of improved land management practices, such as the use of riparian buffer zones, in reducing the amount of topsoil reaching the river system. The changes in the relative contribution of sediment derived from the three main geological zones documented for each of the six sites examined are likely to reflect changes in land use and management, with the precise impact varying both through time and from site to site. For example, the increased contributions from the areas underlain by Carboniferous rocks in the 1850s, 1940s and since the 1960s, evidenced by the core collected from the River Ure Žsite 5., may reflect the effects of metal mining, land drainage in upland areas and increased stocking densities on pasture land in the Pennine Hills, respectively. Equally, the increased contributions from the areas underlain by Permian and Triassic rocks documented for sites 1 and 3 ŽRiver Ouse. since the early 1900s, may be due to the expansion of lowland drainage and an increase in the amount of arable land in the Vale of York. Although the changes in sediment sources Žboth type and spatial location. described above can be related to changes in land use and the influence of land drainage, it is important to recognise that changes in climate over the last ca. 100 years may also have caused temporal changes in sediment sources Žcf. Foster and Lees, 1999a,b; Longfield and Macklin, 1999.. However, identification of the relative importance of climate or land use change in influencing the relative importance of specific sediment sources involves a number of difficulties Žcf. Robson et al., 1998., and clearly requires further research.

5. Perspective Floodplain sediment cores have been used to examine changes in overbank sedimentation rates and sediment sources over the last ca. 100 years at seven locations within the drainage basin of the River Ouse. Despite a number of potential limitations associated with the approaches and procedures employed, the results provide useful information to complement investigations of contemporary suspended sediment loads Žcf. Wass and Leeks, 1999. and sources Žcf. Walling et al., 1999a. undertaken in this basin, and to place that information into a longer-term context. Furthermore, this historical perspective could assist in predicting the geomorphological impact of possible future climate and land use change scenarios. Although the effects of future climate change due to global warming are uncertain, and likely to vary from region to region ŽJones, 1993., they will probably include an increase in the seasonality of precipitation and runoff ŽBeven, 1993; Marsh and Sanderson, 1997., a marked increase in the frequency, magnitude and, possibly, seasonality of flooding ŽBeven, 1993., and an increase in both rates of soil erosion and sediment yields ŽBoardman and Favis-Mortlock, 1993; Wilby et

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al., 1997.. These changes are likely to coincide with changes in land use and land management practices ŽHulme et al., 1993., which may exacerbate the effects of climate change. In the absence of long-term records of suspended sediment loads, the use of the sedimentary record contained within depositional environments, such as floodplains, lakes and reservoirs, coupled with historical river flow and land use data, provide a valuable source of information for reconstructing historical changes in sediment fluxes and sources, and examining their response to intrinsic and extrinsic forcing variables.

Acknowledgements The work described in this paper was undertaken as part of a Special Topic Žresearch grant GSTr02r774. investigation within the UK NERC Land–Ocean Interaction Study ŽLOIS. and this paper is publication number 642 of the LOIS Community Research Programme. We would like to thank Qingping He and Joan Lees ŽCoventry University. for help with the collection of floodplain cores, Art Ames and Lee Bottrill for assistance with laboratory analyses, and Terry Bacon and Helen Jones for producing the figures. Special thanks are extended to Ian Foster and Joan Lees Žboth Coventry University. for undertaking mineral magnetic analyses. Comments made by Olav Slaymaker and an anonymous referee have helped to improve the paper.

References Allen, S.E., 1989. Chemical Analysis of Ecological Materials. Blackwell, Oxford, 369 pp. Beven, K., 1993. Riverine flooding in a warmer Britain. Geographical Journal 159, 157–161. Boardman, J., Favis-Mortlock, D.T., 1993. Climate change and soil erosion in Britain. Geographical Journal 159, 179–183. Collins, A.L., Walling, D.E., Leeks, G.J.L., 1997. Use of the geochemical record preserved in floodplain deposits to reconstruct recent changes in river basin sediment sources. Geomorphology 19, 151–167. Collins, A.L., Walling, D.E., Leeks, G.J.L., 1998. Use of composite fingerprints to determine the provenance of the contemporary suspended sediment load transported by rivers. Earth Surface Processes and Landforms 23, 31–52. Foster, I.D.L., Lees, J.A., 1999a. Changing headwater suspended sediment yields in the LOIS catchments over the last century: a palaeolimnological approach. Hydrological Processes, in press. Foster, I.D.L., Lees, J.A., 1999b. Physical and geochemical properties of suspended sediments delivered to the headwaters of the LOIS River Basins over the last 100 years: an analysis of lake and reservoir bottom-sediments. Hydrological Processes, in press. Foster, I.D.L., Walling, D.E., 1994. Using reservoir deposits to reconstruct changing sediment yields and sources in the catchment of the Old Mill reservoir, South Devon, UK, over the last 50 years. Hydrological Sciences Journal 39, 347–368. Foster, I.D.L., Dearing, J.A., Simpson, A., Carter, A.D., Appleby, P.G., 1985. Lake catchment studies of erosion and denudation in the Merevale catchment, Warwickshire, UK. Earth Surface Processes and Landforms 10, 45–68. Foster, I.D.L., Owens, P.N., Walling, D.E., 1996. Sediment yields and sediment delivery in the catchments of Slapton Lower Ley, South Devon, UK. Field Studies 8, 629–661. Foster, I.D.L., Lees, J.A., Owens, P.N., Walling, D.E., 1998. Mineral magnetic characterisation of sediment sources from an analysis of lake and floodplain sediments in the catchments of the Old Mill reservoir and Slapton Ley, South Devon, UK. Earth Surface Processes and Landforms 23, 685–703.

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