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

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.
Catena 36 1999 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,)
,Graham J.L.Leeks
b
a
Department of Geography,Uni Íersity of Exeter,Amory Building,Rennes Dri Íe,Exeter,DeÍon 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
137
Cs and unsup-
ported
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 cm
y2
yr
y1
.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 geological rtopographic 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 geological rtopographic 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:geography@exeter.ac.uk
0341-8162r99r$20.00 q 1999 Elsevier Science B.V.All rights reserved.
.
PII:S0341- 8162 99 00010- 7
( )
P.N.Owens et al.rCatena 36 1999 21±4722
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 and possibly changes in climate.These results enable estimates of contempo-
rary 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.q1999 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 cf.Walling and
.
Webb,1996.Such records frequently span only relatively short periods often -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 cf.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 e.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 e.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 e.g.,Rumsby and Macklin,1994;Walling and He,1994;Knox and
.
Kundzewicz,1997;Walling and He,1999.The selection of the most suitable deposi-
tional 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.rCatena 36 1999 21±47 23
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 cf.Walling et al.,1998a,b,1999a,b undertaken within the
.
Land±Ocean Interaction Study LOIS 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 cf.Wass and Leeks,1999 and the information on
.
contemporary suspended sediment sources reported by Walling et al.1999a,into a
.
longer-term ca.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.1997 and Foster and Lees 1999a,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 Fig.1.The River Ouse bears this name below the confluence of the
.
River Ure and Ouse Gill Beck near site 3,Fig.1.Its catchment area above the tidal
.
2
limit at Naburn Weir,near Acaster Malbis ca.site 1,is 3520 km.The River Wharfe,
which drains a catchment of 818 km
2
at the tidal limit at Tadcaster,is one of the main
.
tributaries of the River Ouse and joins the latter below the tidal limit Fig.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 Fig.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 Fig.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.
( )
P.N.Owens et al.rCatena 36 1999 21±4724
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.rCatena 36 1999 21±47 25
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

2
.
tube 98 cm surface area,which was driven into the sediment to depths of )50 cm.A
previous reconnaissance survey had shown that
137
Cs was unlikely to extend below 50
cm depth.The sediment cores were collected from representative locations,which were

identified as being both uncultivated because 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 Fig.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 Fig.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
137 210

using measurements of the environmental radionuclides Cs and unsupported Pb cf.
.
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 EG&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
210
.
226 210
difference between the total Pb activity at 46.5 keV and the Ra-supported Pb

214
.
activity calculated from the activity of its short-lived daughter Pb at 352 keV.
Samples were sealed for 21 days prior to analysis to allow for equilibrium between
226 222

210 222
Ra and Rn Pb is derived from the decay of gaseous Rn,the daughter of
226
..
Ra cf.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
137
Cs was used to estimate the average sedimentation
.
rate since 1963.The approach is described in detail in Walling and He 1997.In
essence,it is based on the known temporal pattern of atmospheric fallout of bomb-de-
rived
137
Cs,which peaked in 1963,and the assumption that the peak
137
Cs concentration
in the floodplain sediment profile can be equated with the 1963 fallout peak.Because of
the well documented slow downward migration of the
137
Cs peak in undisturbed soil
.
profiles,caused by processes such as bioturbation and leaching cf.Owens et al.,1996,
( )
P.N.Owens et al.rCatena 36 1999 21±4726
.
which can be assumed to also occur in floodplain sediments Walling and He,1997,the
sedimentation rates estimated from the depth of the
137
Cs peak were corrected for
post-depositional redistribution of sediment-associated
137
Cs within the profile.This
redistribution was estimated to result in downward migration of the
137
Cs peak by ca.
0.05 g cm
y2
yr
y1
.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
.
137
model proposed by He and Walling 1996 for floodplain sediments.Unlike Cs,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 exponen-
tial 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 i.e.,ca.five times
210
.
the 22.2 years half-life of 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 1996.
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.1999a see also Collins et al.,1997.In brief,over 160 samples )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 Yorkshire
...
Dales,Permian and Triassic Vale of York,and Jurassic North York Moors see Fig.
.
1.In each of these zones,representative samples were collected from the face of
.
eroding channel banks and ditches and from the surface ca.top 2 cm of woodland,
.
uncultivated pasture,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.1999a for determining the sources of contempo-
rary suspended sediment are suitable for tracing the sources of floodplain sediment.For
example,downcore variations in
137
Cs levels in floodplain sediment are primarily
controlled by temporal variations in atmospheric fallout,which complicate any interpre-
tation of variations in
137
Cs 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 x,x,and a
lf fd
 
Molspin Pulse Magnetiser and a Minispin Fluxgate Magnetometer SIRM sIRMat 0.8
...
T.Chemical elements i.e.,Al,Ca,Cr,Cu,Fe,K,Mg,Mn,Na,Ni,Pb,Sr and Zn
( )
P.N.Owens et al.rCatena 36 1999 21±47 27
were analysed using a Unicam 939 Atomic Absorption Spectrophotometer after diges-
.
tion of the sediment in concentrated hydrochloric and nitric acid cf.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.1999a.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

i.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 one 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 R for the n tracer
es
properties and m source groups involved,using
2
m
C y C P

f i s i s
n
/
ss1
R s 1
.

es
C
f i
is1
 0
where C is the concentration of tracer property i in the floodplain sediment sample,C
f i s i
is the mean concentration of tracer property i in source group s and P is the relative
s
contribution of source group s.The model must satisfy two linear constraints,namely:
.
a the relative contribution from each source must lie within the range 0 to 1,i.e.,
0FP F1 2
.
s
.
b the sum of the relative contributions from all sources must equal 1,i.e.,
n
P s1.3
.

s
ss1

An assessment of the goodness-of-fit provided by the optimised mixing model cf.
.
Collins et al.,1997;Walling et al.,1999a gave mean relative errors for the predicted
( )
P.N.Owens et al.rCatena 36 1999 21±4728
concentrations of individual fingerprint properties which typically ranged between"7%
.
and"14% i.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 oÍer the last ca.100 years
Fig.2 illustrates the contrasting depth distributions of
137
Cs and unsupported
210
Pb in
floodplain cores collected from sites characterised by different sedimentation rates.In
.
137
the case of the core collected from site 1 Fig.2a the 1963 Cs peak is located at ca.

y2
.
21 cm depth 31 g cm accumulated mass,while the peak for the core collected from
.
y2
.
site 2 Fig.2b is located at ca.10 cm depth 9 g cm 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 unsup-
ported
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
210

in the unsupported Pb content of deposited sediment due 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
137
Cs and unsupported
210
Pb concentrations evident between the two cores reflect

several factors including contrasts in the radionuclide fallout flux and thus precipitation
.
amounts,in the radionuclide content of the deposited sediment,and in the sedimenta-
tion 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
137
Cs-based sedimentation rate presented in Table 1 was based on another core
.
collected nearby as part of the study reported in Walling et al.1998b.The average
sediment accumulation rates show considerable variation between sites and range from
.
y2 y1
.
0.11 site 7 to 1.04 g cm yr site 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 cf.Walling et al.,1998b,
.
1999b.
( )
P.N.Owens et al.rCatena 36 1999 21±47 29
137 210
.
Fig.2.The depth distribution of Cs and unsupported Pb in the floodplain cores collected from sites 1 a
.
and 2 b.
When the average sedimentation rates estimated for the two different time periods for
.
each core are compared,they are in some cases i.e.,sites 2 and 6 almost the same.In
.
other cases i.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.
( )
P.N.Owens et al.rCatena 36 1999 21±4730
Table 1
Average floodplain sediment accumulation rates for the last ca.32 and 100 years estimated using
137
Cs and
unsupported
210
Pb measurements
137
Sitercatchment National grid Cs-based Unsupported
210
reference sedimentation rate Pb-based
y2 y1
.
g cm yr sedimentation rate
y2 y1
.
g cm yr
.
1 Lower Ouse SE597460 0.95 1.04
.
2 Middle Ouse SE512578 0.24 0.23
.
3 Upper Ouse SE467620 0.64 0.68
.
4 Swale SE363799 0.50 ±
.
5 Ure SE315731 0.18 0.42
.
6 Nidd SE487559 0.17 0.17
.
7 Wharfe SE231457 0.13 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 Íariations 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 finger-
prints.
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
.
2
.
collected from site 1 Fig.3a,Fe shows a significant r s0.87 positive correlation

2
.
with depth,Ca exhibits a significant r s0.87 negative correlation with depth,while

2
.
Al exhibits no significant r s0.01 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 cf.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
( )
P.N.Owens et al.rCatena 36 1999 21±47 31
.
Fig.3.Downcore variations in various geochemical properties for selected floodplain cores:a depth
.
distributions of Al,Ca and Fe in the core collected from site 1;b depth distributions of Ca in the cores from
.
sites 1,2 and 6;and c depth distributions of Pb and Zn for the core from site 2.
( )
P.N.Owens et al.rCatena 36 1999 21±4732
independent of source.However,in this study Al is included in the composite finger-
print 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 represented 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 see 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 transforma-
tions 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-deposi-
tional transformations and the use of composite fingerprints,incorporating a combina-
tion of mineral magnetic and geochemical properties,should reduce the potential
Table 2
.
Correlation coefficients 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 Floodplain core
1 2 3 4 5 6 7
Geochemical n 32 31 33 27 32 25 25
U U U U U
Al 0.10 0.44 0.79 0.28 0.40 0.71 0.52
U U U U U
Ca 0.93 0.24 0.49 0.40 0.10 0.89 0.72
U U U U U U U
Cr 0.82 0.74 0.41 0.70 0.66 0.72 0.60
U U U U
Cu 0.28 0.96 0.46 0.22 0.10 0.88 0.48
U U U U U U
Fe 0.93 0.51 0.89 0.17 0.60 0.54 0.72
U U U U U U
K 0.49 0.37 0.87 0.56 0.10 0.52 0.59
U U U U U
Mg 0.95 0.36 0.65 0.32 0.40 0.63 0.81
U U U
Mn 0.32 0.85 0.17 0.73 0.30 0.40 0.45
U U U U U
Na 0.66 0.46 0.75 0.60 0.55 0.37 0.10
U U U U U U
Ni 0.42 0.39 0.87 0.33 0.46 0.60 0.87
U U U U
Pb 0.14 0.28 0.10 0.40 0.40 0.93 0.94
U U U
Sr 0.01 0.03 0.44 0.10 0.40 0.06 0.75
U U U U U U
Zn 0.54 0.59 0.24 0.50 0.55 0.85 0.89
Mineral magnetic n 20 31 33 27 32 25 25
U U U U U
x 0.20 0.96 0.73 0.10 0.87 0.59 0.92
lf
U U U U U
SIRM 0.10 0.95 0.59 0.66 0.92 0.57 0.33
.
nsNumber of samples floodplain sediment increments.
U
.
Significant p-0.05.
( )
P.N.Owens et al.rCatena 36 1999 21±47 33
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 i.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 g
y1
,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 Pb sulphide and blende
.
Zn 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 cf.Macklin et al.,1997.Large-scale mining in the Pennines

was restricted to the early to middle 19th century although 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 concentra-
tions 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
137
Cs and unsupported
210
Pb measure-
ments.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
210

extrapolation of the unsupported Pb-based sedimentation rate see 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 Table 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 concentra-
tions 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 oÍer 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
.
i.e.,geology,topography and land use of the Ouse and Wharfe basins,the source
( )
P.N.Owens et al.rCatena 36 1999 21±4734
material signatures for both basins have been pooled.The cores from all seven locations
.
were examined for changes in the main source types topsoil or subsoil rchannel bank
and in source areas,as represented by the three main geological rtopographic zones
.
Carboniferous,Permian and Triassic,and Jurassic.The only exception was the core
.
collected from the River Wharfe site 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 termed 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 subsoil rchannel 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 cf.Fig.3c means that they are unsuitable for establishing downcore variations in
Table 3
.
Significance levels from the Mann±Whitney U-test topsoil,subsoilrchannel bank and Kruskal±Wallis
.
H-test geological zones used to establish the ability of each tracer property to discriminate between source
groups
Tracer Topsoil vs.subsoilr Geological
a b
property channel bank zones
U U
x 0.001 0.002
lf
U U
x 0.001 0.001
fd
U
SIRM 0.001 0.281
U
Al 0.300 0.001
U
Ca 0.824 0.001
Cr 0.202 0.566
Cu 0.757 0.766
U U
Fe 0.001 0.012
U U
K 0.001 0.001
U
Mg 0.618 0.001
U U
Mn 0.001 0.001
U
Na 0.796 0.027
U U
Ni 0.001 0.011
U
Pb 0.194 0.004
U U
Sr 0.016 0.001
U
Zn 0.002 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 p-0.05.
( )
P.N.Owens et al.rCatena 36 1999 21±47 35
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
137
Cs 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 for 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 pasture and
..
arable in the study area.However,for the cores from sites 2 and 3 River Ouse and site
.
7 River 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
a
Topsoil Ís.subsoil rchannel bank
x 61.49
lf
Fe 79.09
K 80.91
Sr 84.55
x 83.64
fd
Mn 84.55
Ni 90.00
b
Geological zones
Mn 57.46
Mg 60.45
Al 62.69
K 61.94
Ca 59.70
Sr 63.43
Ni 67.16
Fe 70.15
a
Topsoil represents the top ca.2 cm soil from uncultivated,cultivated and woodland land,while subsoil rchan-
nel 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.
( )
P.N.Owens et al.rCatena 36 1999 21±4736
Fig.4.Downcore changes in the relative contributions of topsoil and subsoil rchannel bank sources for the
seven floodplain cores.
.
at Acaster Malbis site 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
( )
P.N.Owens et al.rCatena 36 1999 21±47 37
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 which is dated at ca.1984,38±44
..
cm 1940s and below 60 cm 1920s 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 see 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 site 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 middle
.
and upper River Ouse contributions from subsoil rchannel 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 River Swale,5
..
River Ure and 6 River 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 prior to the 1920s,50 cm prior to the 1850s and 20 cm
.
prior 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 middle 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 and ror
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 site 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 subsoil rchannel 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 1950s.
( )
3.3.2.Spatial location geological rtopographic zones
In order to fingerprint sediment originating from different parts of the study area,and
more particularly from different geological rtopographic zones,representative source
materials were collected from the three main geological zones in the study area,which
( )
P.N.Owens et al.rCatena 36 1999 21±4738
.
broadly coincide with the three main topographic areas,i.e.,Carboniferous Pennines,
..
Permian and Triassic Vale of York and Jurassic North York Moors.Although there

are areas where the solid geology is overlain by Quaternary glacial drift deposits such
..
as boulder clay and lacustrine deposits see Fig.1,these mainly occur in the Vale of
York and the land immediately bordering the Vale.Thus the three geological rtopo-
graphic 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 90%,may partly
.
reflect the fact that glacial drift deposits unlike 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 the 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 contribu-
tions from different geological zones over the time period associated with each core.For
..
example,the core from site 6 River Nidd is dominated ca.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 ca.60%,and areas underlain by Permian and Triassic
.
rocks providing an important secondary contribution ca.40%.There is no or 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 contribu-

tions from the main geological zones.In the case of the core collected from site 5 River
.
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
( )
P.N.Owens et al.rCatena 36 1999 21±47 39

Fig.5.Downcore variations in the relative contributions from the three main geological zones Carboniferous,
.
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 River Ouse and site 4 River 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 dated at ca.1940s to 1990s
( )
P.N.Owens et al.rCatena 36 1999 21±4740
.
and below 30 cm depth 1920s.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 ca.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 post-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 i.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 see Fig.1,and the broad similarity in the
.
geological source results for site 3 with those for site 4 River 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 see
.
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.OÍerbank sedimentation and suspended sediment fluxes
There are several points to consider when interpreting the information on sedimenta-
tion 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 cf.Walling et al.,1998b,1999b;Walling,1999.Secondly,it is also important
( )
P.N.Owens et al.rCatena 36 1999 21±47 41
to recognise that the similarity in the overbank sedimentation rates for the last ca.30 and
.
100 years Table 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
.
137
deposit at between ca.31 and 35 cm depth cf.Walling et al.,1998a.Using the Cs
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 Williams,
.
1957.This single flood,therefore,deposited several centimetres of sediment,whereas
Fig.6.Depth distributions of
137
Cs and d for the floodplain sediment core collected from the River Swale at
50
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 of the overbank sediment deposited at this site after the core was collected in 1994
50
.
during the floods in the Ouse basin in January and February 1995 cf.Walling et al.,1998a.
( )
P.N.Owens et al.rCatena 36 1999 21±4742
the average sedimentation rate at this sampling location since the mid-1960s has been
y2 y1

y1
.
estimated to be ca.0.50 g cm yr ca.0.5 cm yr.
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 cf.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 sites 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 landown-
ers 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
137
Cs fallout
occurs approximately midway through the period of increased flood magnitude and
.
frequency evidenced by the River Ouse at York which 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.1998.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 1994,1999 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 1999a,
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
137
Cs 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 last ca.100±200 years.Interestingly,the biggest
.
change in sediment yield documented by Foster and Lees 1999a for the study basin
occurred in a small upland reservoir in the Wharfe catchment,where the average
( )
P.N.Owens et al.rCatena 36 1999 21±47 43
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 representa-
tive 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 subsoil rchannel bank sources are also found at

all sites and these reflect,at least in part,the well developed channel banks locally )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 cf.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 cf.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 1999 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 cf.Morgan,1988,and in the
.
LOIS study area in particular cf.Foster and Lees,1999a,studies in other agricultural
.
catchments in the UK e.g.,Heathwaite et al.,1990;Foster and Walling,1994 have
( )
P.N.Owens et al.rCatena 36 1999 21±4744
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 inten-
sively 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 site 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 River 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 both 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 cf.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 cf.Robson et al.,1998,and clearly requires further
research.
5.Perspective
Floodplain sediment cores have been used to examine changes in overbank sedimen-
tation 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 cf.Wass and
..
Leeks,1999 and sources cf.Walling et al.,1999a undertaken in this basin,and to
place that information into a longer-term context.Furthermore,this historical perspec-
tive 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 Jones,1993,
they will probably include an increase in the seasonality of precipitation and runoff
.
Beven,1993;Marsh and Sanderson,1997,a marked increase in the frequency,
.
magnitude and,possibly,seasonality of flooding Beven,1993,and an increase in both

rates of soil erosion and sediment yields Boardman and Favis-Mortlock,1993;Wilby et
( )
P.N.Owens et al.rCatena 36 1999 21±47 45
.
al.,1997.These changes are likely to coincide with changes in land use and land
.
management practices Hulme 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 research
.
grant GSTr02r774 investigation within the UK NERC Land±Ocean Interaction Study
.
LOIS and this paper is publication number 642 of the LOIS Community Research
.
Programme.We would like to thank Qingping He and Joan Lees Coventry 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 both 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.
( )
P.N.Owens et al.rCatena 36 1999 21±4746
He,Q.,Owens,P.,1995.Determination of suspended sediment provenance using caesium-137,unsupported
lead-210 and radium-226:a numerical mixing model approach.In:Foster,I.D.L.,Gurnell,A.M.,Webb,
.
B.W.Eds.,Sediment and Water Quality in River Catchments.Wiley,Chichester,pp.207±227.
He,Q.,Walling,D.E.,1996.Use of fallout
210
Pb measurements to investigate longer-term rates and patterns
of overbank sediment deposition on the floodplains of lowland rivers.Earth Surface Processes and
Landforms 21,141±154.
Heathwaite,A.L.,1994.Chemical fractionation of lake sediments to determine the effects of land-use change
on nutrient loading.Journal of Hydrology 159,395±421.
Heathwaite,A.L.,Burt,T.P.,Trudgill,S.T.,1990.Land use controls on sediment production in a lowland
.
catchment,southwest England.In:Boardman,J.,Foster,I.D.L.,Dearing,J.A.Eds.,Soil Erosion on
Agricultural Land.Wiley,Chichester,pp.69±86.
Horowitz,A.J.,1991.A Primer on Sediment-trace Element Chemistry.Lewis,MI,USA,pp.134.
Hulme,M.,Hossell,J.E.,Parry,M.L.,1993.Future climate change and land use in the United Kingdom.
Geographical Journal 159,131±147.
Jones,D.K.C.,1993.Global warming and geomorphology.Geographical Journal 159,124±130.
Joshi,S.R.,1987.Nondestructive determination of lead-210 and radium-226 in sediments by direct photon
analysis.Journal of Radioanalytical and Nuclear Chemistry Articles 116,169±182.
Knox,J.C.,Kundzewicz,Z.W.,1997.Extreme hydrological events,palaeo-information and climate change.
Hydrological Sciences Journal 42,765±779.
Lawler,D.M.,Grove,J.,Couperthwaite,J.S.,Leeks,G.J.L.,1999.Downstream change in river bank erosion
rates in the Swale±Ouse system,Northern England.Hydrological Processes,in press.
Lees,J.,Foster,I.,Jones,D.,Owens,P.,Walling,D.,Leeks,G.,1997.Sediment yields in a changing
environment:a historical reconstruction using reservoir bottom-sediments in three contrasting small
.
catchments,North York Moors,UK.In:Walling,D.E.,Probst,J.-L.Eds.,Human Impacts on Erosion
and Sedimentation.IAHS Publication No.245,IAHS Press,Wallingford,UK,pp.169±179.
Longfield,S.A.,Macklin,M.G.,1999.The influence of recent environmental change on flooding and sediment
fluxes in the Yorkshire Ouse basin.Hydrological Processes,in press.
Longfield,S.,Macklin,M.,Rumsby,B.,1995.Historic flooding at York.LOIS Newsletter 7,4±5.
Macklin,M.G.,Hudson-Edwards,K.A.,Dawson,E.J.,1997.The significance of pollution from historic metal
mining in the Pennine orefields on river sediment contaminant fluxes to the North Sea.The Science of the
Total Environment 194r195,391±397.
Marsh,T.J.,Sanderson,F.J.,1997.A review of hydrological conditions throughout the period of the LOIS
monitoring programmeÐconsidered within the context of the recent UK climatic volatility.The Science of
the Total Environment 194r195,59±69.
Morgan,R.P.C.,1988.Soil Erosion and Conservation.Longman,London.
O'Hara,S.L.,Street-Perrott,F.A.,Burt,T.P.,1993.Accelerated soil erosion around a Mexican highland lake
caused by prehispanic agriculture.Nature 362,48±51.
Owens,P.N.,Slaymaker,O.,1994.Post-glacial temporal variability of sediment accumulation in a small
.
alpine lake.In:Olive,L.J.,Loughran,R.J.,Kesby,J.A.Eds.,Variability in Stream Erosion and Sediment
Transport.IAHS Publication No.224,IAHS Press,Wallingford,UK,pp.187±195.
Owens,P.N.,Walling,D.E.,He,Q.,1996.The behaviour of bomb-derived caesium-137 fallout in catchment
soils.Journal of Environmental Radioactivity 32,169±191.
Page,M.J.,Trustrum,N.A.,1997.A late Holocene lake sediment record of the erosion response to land use
change in a steepland catchment,New Zealand.Zeitschrift fur Geomorphologie 41,369±392.
Passmore,D.G.,Macklin,M.G.,1994.Provenance of fine-grained alluvium and late Holocene land-use
change in the Tyne basin,Northern England.Geomorphology 9,127±142.
Robson,A.J.,Jones,T.K.,Reed,D.W.,Bayliss,A.C.,1998.A study of national trend and variation in UK
floods.International Journal of Climatology 18,165±182.
Rumsby,B.T.,Macklin,M.G.,1994.Floodplain and channel response to recent abrupt climate change:the
Tyne basin,Northern England.Earth Surface Processes and Landforms 19,499±515.
Walden,J.,Slattery,M.C.,Burt,T.P.,1997.Use of mineral magnetic measurements to fingerprint suspended
sediment sources:approaches and techniques for data analysis.Journal of Hydrology 202,353±372.
Walling,D.E.,1999.Linking land use,erosion and sediment yields in river basins.In:Man and River
Systems,Developments in Hydrobiology.Kluwer,Dordrecht,in press.
( )
P.N.Owens et al.rCatena 36 1999 21±47 47
Walling,D.E.,He,Q.,1994.Rates of overbank sedimentation on the flood plains of several British rivers over
.
the past 100 years.In:Olive,L.J.,Loughran,R.J.,Kesby,J.A.Eds.,Variability in Stream Erosion and
Sediment Transport.IAHS Publication No.224,IAHS Press,Wallingford,UK,pp.203±210.
Walling,D.E.,He,Q.,1997.Use of fallout
137
Cs in investigations of overbank sediment deposition on river
floodplains.Catena 29,263±282.
Walling,D.E.,He,Q.,1999.Changing rates of overbank sedimentation on the floodplains of British rivers
.
during the past 100 years.In:Brown,A.G.,Quine,T.A.Eds.,Fluvial Processes and Environmental
Change.Wiley,Chichester,in press.
.
Walling,D.E.,Webb,B.W.Eds.,1996.Erosion and Sediment Yield:Global and Regional Perspectives.
IAHS Publication No.236,IAHS Press,Wallingford,UK.
Walling,D.E.,Woodward,J.C.,1995.Tracing sources of suspended sediment in river basins:a case study of
the River Culm,Devon,UK.Marine and Freshwater Research 46,327±336.
Walling,D.E.,Owens,P.N.,Leeks,G.J.L.,1998a.The characteristics of overbank deposits associated with a
major flood in the catchment of the River Ouse,Yorkshire,UK.Catena 32,309±331.
Walling,D.E.,Owens,P.N.,Leeks,G.J.L.,1998b.The role of channel and floodplain storage in the suspended
sediment budget of the River Ouse,Yorkshire,UK.Geomorphology 22,225±242.
Walling,D.E.,Owens,P.N.,Leeks,G.J.L.,1999a.Fingerprinting suspended sediment sources in the
catchment of the River Ouse,Yorkshire,UK.Hydrological Processes,in press.
Walling,D.E.,Owens,P.N.,Leeks,G.J.L.,1999b.Rates of contemporary overbank sedimentation and
sediment storage on the floodplains in the main channel system of the Yorkshire Ouse and River Tweed,
UK.Hydrological Processes,in press.
Wass,P.D.,Leeks,G.J.L.,1999.Suspended sediment fluxes from the LOIS River Monitoring networks.
Hydrological Processes,in press.
Wilby,R.L.,Dalgleish,H.Y.,Foster,I.D.L.,1997.The impact of weather patterns on historic and contempo-
rary sediment yields.Earth Surface Processes and Landforms 22,353±363.
Williams,H.B.,1957.Flooding characteristics of the River Swale.Unpublished PhD thesis,University of
Leeds,UK.
Zolitschka,B.,1998.A 14,000 year sediment yield record from western Germany based on annually laminated
lake sediments.Geomorphology 22,1±17.