Earth Surface Processes and Landforms
Earth Surf. Process. Landforms 27, 403–423 (2002)
Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/esp.327
CHANGES IN SEDIMENT SOURCES AND FLOODPLAIN DEPOSITION
RATES IN THE CATCHMENT OF THE RIVER TWEED, SCOTLAND,
OVER THE LAST 100 YEARS: THE IMPACT OF CLIMATE AND LAND
USE CHANGE
PHILIP N. OWENS* AND DESMOND E. WALLING
School of Geography and Archaeology, University of Exeter, Amory Building, Rennes Drive, Exeter, Devon, EX4 4RJ, UK
Received 12 February 2001; Revised 3 September 2001; Accepted 17 September 2001
ABSTRACT
Evidence from floodplain cores collected from three sites in the middle reaches of the Tweed basin in Scotland is used
to reconstruct changes in sediment sources and overbank floodplain deposition rates over the last c. 100 years. Core
chronologies and sedimentation rates are established using 137 Cs and unsupported 210 Pb measurements. The average
sedimentation rates since 1963 range from 1Ð9 š 0Ð2 to 2Ð2 š 0Ð2 kg m2 a1 and are lower than the average rates for
the period 1894/95 to 1963, which range from 2Ð7 š 0Ð6 to 5Ð9 š 0Ð9 kg m2 a1 . There is also evidence of significant
downcore variations in sediment source, defined in terms of both type (i.e. topsoil or channel bank/subsoil material)
and spatial location (i.e. main geological/topographic zones). There is no clear link between the changes in overbank
sedimentation rates and sediment sources and the trends shown by precipitation, weather pattern and river flow records
over the past 100 years, suggesting that changes in climate alone cannot explain the downcore trends. Instead, the temporal
changes in overbank sedimentation rates and sediment source appear to be linked more closely to changes in land use
and land management over the past 100 years and, more particularly, the introduction of land drainage at the end of the
19th century, the rapid increase in afforestation since the 1940s and the post-war conversion of grassland to arable land.
Copyright  2002 John Wiley & Sons, Ltd.
KEY WORDS: floodplains; overbank sedimentation; sediment sources; climate change; land use change; River Tweed
INTRODUCTION
The UK has a long history of land use change associated with human occupation. Much of the UK was covered
by woodland until about 5000 years ago, when Mesolithic man started to clear small areas of woodland. The
erosional effects of this small-scale land clearance were usually localized (Evans, 1990). Since this time,
there have been phases of land clearance and cultivation, often associated with periods of population growth
and/or technological advance, which have resulted in accelerated erosion and shifts in geomorphic activity
both on the land (e.g. colluviation in dry valleys and upslope of field boundaries) and within river systems
(e.g. alluviation on floodplains) (cf. Bell, 1982; Brown and Barber, 1985; Evans, 1990; Smyth and Jennings,
1990; Tipping, 1992; Foster et al., 2000). Equally, there have also been periods when rates of erosion and the
intensity of geomorphic processes declined in response to land use change. In the Dark Ages, for example,
there was a marked drop in population, much of the land reverted to woodland and scrub, and available
evidence suggests that rates of both erosion and alluviation declined (Robinson and Lambrick, 1984).
There is now widespread recognition that rates of soil erosion on agricultural land in the UK have increased
over the last 100 years (Speirs and Frost, 1985; Boardman, 1990; Evans, 1990). This reflects growth in
the amount of land under cultivation, the intensification of farming practices and changes in the timing of
farming activities (e.g. a switch from spring- to autumn-sown crops). Furthermore, in recent years, European
* Correspondence to: P. N. Owens, National Soil Resources Institute, Cranfield University, North Wyke, Okehampton, Devon EX20
25B, UK. E-mail: [email protected]
Contract/grant sponsor: UK NERC; Contract/grant number: GST/02/774.
Copyright  2002 John Wiley & Sons, Ltd.
404
P. N. OWENS AND D. E. WALLING
Community agricultural policies have had an important impact on land use and management in the UK, and
in turn on soil erosion, colluviation and alluviation.
The effects of changes in land use and management on rates of soil erosion, and valley and floodplain
sedimentation, outlined above, have been superimposed on the effects of changes in climate. Many studies
have linked historical changes in climate and weather patterns to significant changes in rates of soil erosion,
river flooding and overbank sedimentation (e.g. Rumsby and Macklin, 1994; Tipping, 1994; Wilby et al., 1997;
Foster et al., 2000). Furthermore, it is likely that future changes in climate will increase soil erosion rates and
sediment yields (Boardman and Favis-Mortlock, 1993; Wilby et al., 1997), and increase the frequency and
magnitude of flooding (Beven, 1993). Separating the effects of changes in land use and climate on geomorphic
activity is, however, a difficult task, particularly given the general absence of long-term records of geomorphic
activity. Under certain conditions, the sedimentary record preserved in depositional environments can be used
to provide surrogate information on the past behaviour of a river basin. The evidence provided by cores
collected from overbank floodplain deposits has, for example, been used to reconstruct past changes in
sediment flux and sediment sources (e.g. Walling and He, 1994, 1999; Rumsby and Macklin, 1994; Collins
et al., 1997; Owens et al., 1999a).
This paper reports results of an investigation which has used floodplain sediment cores collected from
three sites in the Tweed basin, Scotland, to reconstruct: (i) changes in the rate of overbank sedimentation
on the floodplains bordering the main channel system of the River Tweed, and its main tributary, the River
Teviot, over the past c. 100 years; and (ii) changes in the main sources of the overbank sediment deposits,
defined in terms of both source type and spatial location, over the same time period. The evidence of
changing floodplain sedimentation rates and sediment source provided by the cores has been compared with
the available long-term records of precipitation, weather patterns, river flow and land use, in order to decipher
the influence of land use and climate change on sediment sources and sediment delivery in the Tweed
basin.
The Tweed basin has been chosen for the study, because it is one of the most important rivers in the UK:
it is the second largest river basin in Scotland; it ranks as one of the least polluted in the UK, with >99 per
cent of its length classified as Class 1 for chemical water quality; it is amongst the top four salmon and sea
trout rivers in the UK; and it was one of the first riverine aquatic habitats to be designated a Site of Special
Scientific Interest by the Nature Conservancy Council in April 1976 (Clayton, 1997; Currie, 1997; Bronsdon
and Naden, 2000). Although the Tweed basin has a long history of occupation and land use (Tipping, 1992),
and deforestation was probably almost complete by the Medieval period (Quelch, 1993), the last 100 years
or so have seen significant changes in land use and land management. In recent years, there has been much
concern over the detrimental effects of these changes on the flow characteristics and water quality of the
River Tweed (cf. Maitland et al., 1994; Currie, 1997).
STUDY AREA AND METHODS
Study area
The River Tweed is a gravel-bed river, which flows approximately 160 km from its headwaters in the
Southern Uplands to the North Sea. It has a catchment area of 4390 km2 above the Scottish Environment
Protection Agency (SEPA) gauging station, which is located close to the tidal limit at Norham (Figure 1).
Most of the basin, and all of the study area (i.e. the area upstream of the floodplain core sites), lies in the
Borders Region of Scotland, but the southeastern portion of the basin is located in England (i.e. outside
of the Borders Region). The River Teviot is the largest tributary of the River Tweed, and has a catchment
area of 1110 km2 above the SEPA gauging site at Ormiston. The mean annual discharge and mean annual
flood (1959–1995) for the River Tweed at Norham are 78 and 837 m3 s1 , respectively, while equivalent
values for the River Teviot at Ormiston (1960–1995) are 20 and 326 m3 s1 (Fox and Johnson, 1997). The
average annual suspended sediment yields of the River Tweed at Norham and the River Teviot at Ormiston
(1994–1997) are estimated to be c. 11 and 20 t km2 a1 , respectively (Bronsdon and Naden, 2000), although
McManus and Duck (1996) and Owens et al. (1999b) estimated values of c. 15 t km2 a1 for the River
Tweed at Norham for the periods 1975–1983 and 1995–1996, respectively.
Copyright  2002 John Wiley & Sons, Ltd.
Earth Surf. Process. Landforms 27, 403–423 (2002)
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CHANGES IN SEDIMENT SOURCES OVER 100 YEARS
Table I. Contemporary land cover in the Borders Region (based on
Scottish Office, 1994). See Figure 1 for the location of the Borders
Region relative to the Tweed basin
Land cover
Area
2
Woodland and forest (coniferous plantations)
Woodland and forest (mixed and broadleaved)
Arable
Grassland (improved)
Grassland (rough)
Moorland and peatland
Urban
Rural development
Freshwaters and marshes
Other (mosaics)
(km (%)
718Ð9
61Ð8
857Ð8
1230Ð5
743Ð9
483Ð8
36Ð0
10Ð7
24Ð2
562Ð8
15Ð2
1Ð3
18Ð1
26Ð0
15Ð8
10Ð2
0Ð8
0Ð2
0Ð5
11Ð9
geological/topographic zones, namely the Silurian and Ordovician (western and northern uplands) zone, the
igneous (Cheviot Hills) zone, the Devonian (central lowlands) zone and the Carboniferous (eastern lowlands)
zone. Within each of these zones, representative samples were collected from the exposed faces of eroding
channel banks, ditches and gullies (designated channel bank/subsoil material) and from the surface (c. top
2 cm) of woodland, pasture/moorland and cultivated areas (designated topsoil material). Each of the 119
samples was collected from a different field or section of river bank. These source material samples were
subsequently dried at 40 ° C and disaggregated prior to analysis.
Laboratory methods
The core chronologies used to estimate sedimentation rates were established using measurements of the
environmental radionuclides 137 Cs and unsupported 210 Pb. In brief, the depth distribution of 137 Cs in overbank
floodplain deposits can be used to estimate the average sedimentation rate since the peak in atmospheric
fallout of bomb-derived 137 Cs in 1963. In the case of unsupported 210 Pb, it is possible to use measurements
of the excess unsupported 210 Pb inventory (i.e. total inventory minus reference inventory) of floodplain
sediment, the decay constant of 210 Pb, information on the unsupported 210 Pb content of recent overbank
sediment deposits and the CICCS (constant initial concentration of unsupported 210 Pb in deposited sediment
and constant sedimentation rate) model of He and Walling (1996b) to derive a linear depth–age relationship
at a depositional location for the last c. 100 years (i.e. approximately four to five times the 22Ð2 years half-life
of 210 Pb). Because the CICCS model assumes that overbank sedimentation has been quasi-continuous (which
can be tested by examining the unsupported 210 Pb depth profile), it is only possible to derive an average
sedimentation rate over the last c. 100 years (for further details of the technique see He and Walling, 1996b;
Walling and He, 1997). Radionuclide activities were measured simultaneously by -ray spectrometry, using
a low-background, hyperpure n-type germanium coaxial detector (EG&G ORTEC LOAX HPGe) coupled to
a multichannel analyser. Count times were typically in the range 50 000 to 90 000 s, giving a measurement
precision of between š5 and š10 per cent at the 95 per cent confidence level of significance.
Sediment source tracing employed the fingerprinting approach and was based on a comparison of the
geochemical and mineral magnetic properties of the sediment from individual sections of the floodplain cores
with those of potential source materials (for further details see Collins et al., 1997, 1998; Walling et al.,
1999; Owens et al., 1999a, 2000). Magnetic measurements were undertaken at Coventry University using a
Bartington MS2B dual frequency sensor for the susceptibility parameters (), and a Molspin pulse magnetizer
and a Minispin fluxgate magnetometer for saturated isothermal remanent magnetization (isothermal remanent
magnetization at 0Ð8 T). Heavy metal and base cation concentrations (Al, Ca, Cr, Cu, Fe, K, Mg, Mn, Na, Ni,
Pb, Sr and Zn) were measured using a Unicam 939 atomic absorption spectrophotometer after digestion of the
sediment in concentrated hydrochloric and nitric acid (cf. Allen, 1989). Both floodplain sediment and source
Copyright  2002 John Wiley & Sons, Ltd.
Earth Surf. Process. Landforms 27, 403–423 (2002)
408
P. N. OWENS AND D. E. WALLING
materials were analysed for each determinand using the same procedures in order to permit direct comparison.
Furthermore, in order to facilitate direct comparison of the tracer property concentrations associated with
both sediment and source materials, all analyses were undertaken on the <63 µm fraction (cf. Horowitz,
1991). Additional correction for differences in particle size composition between the <63 µm fractions of the
floodplain sediment and the source materials was based on the specific surface areas of the samples (cf. Owens
et al., 1999a; Walling et al., 1999). These were estimated from the absolute particle size composition of the
mineral fraction, which was measured using a Coulter LS130 laser diffraction granulometer, after removal of
organic matter with H2 O2 and chemical and ultrasonic dispersion.
Statistical and numerical methods
The statistical and numerical procedures used for source tracing are described in detail in Owens et al.
(1999a, 2000) and Walling et al. (1999). In brief, a two-stage statistical procedure, which uses the Mann–Whitney U-test or Kruskal–Wallis H-test and stepwise multivariate discriminant function analysis, was employed
to select optimum composite fingerprints (cf. Collins et al., 1998). After correcting for differences in property
concentrations between floodplain sediment and source materials due to particle size effects, a numerical
mixing model was used to establish the relative contribution of the different sources to the individual depthincremental samples of floodplain sediment. An assessment of the goodness-of-fit provided by the optimized
mixing model (cf. Collins et al., 1998; Walling et al., 1999) gave mean relative errors for the predicted
concentrations of individual fingerprint properties of the order of š10 per cent.
RESULTS
Changes in overbank sedimentation rates over the last 100 years
Figure 3 shows the depth profiles of 137 Cs and unsupported 210 Pb for the three floodplain cores. For the
137
Cs profiles, there are peaks in 137 Cs content at 7–8 cm (site 1), 6–7 cm (site 2) and 6–7 cm (site 3) depth
and it has been assumed that these equate with 1963 after allowance is made for the slow downward migration
of the 137 Cs peak due to bioturbation and leaching (cf. Owens et al., 1996). The average sedimentation rates
between 1963 and 1994/1995 for the three cores range from 1Ð9 š 0Ð2 to 2Ð2 š 0Ð2 kg m2 a1 . These rates
lie towards the upper end of the range (0Ð16–2Ð2 kg m2 a1 documented by Owens et al. (1999b) who
used 137 Cs measurements to determine average rates of sedimentation for 82 cores collected throughout the
Tweed basin.
The unsupported 210 Pb depth profiles for the three floodplain cores are broadly similar, with concentrations
decreasing approximately exponentially with depth. This reflects the quasi-continuous nature of overbank
sedimentation at these sites (thereby confirming the suitability of using the CICCS model for these cores)
and the radioactive decay of unsupported 210 Pb over time (i.e. with increasing depth). The variability of
unsupported 210 Pb concentrations around this general exponential decline, illustrated in Figure 3, reflects
downcore (and thus temporal) changes in the particle size composition of the deposited sediment (210 Pb is
preferentially associated with finer sediment; cf. He and Walling, 1996c), variations in the unsupported 210 Pb
content of the deposited sediment (due to variations in sediment source) and changes in the magnitude and
frequency of overbank flood events, in addition to the precision of the measurements. Using the CICCS
model, estimates of average rates of overbank deposition over the last 100 years range from 2Ð6 š 0Ð6 (site
2) to 4Ð8 š 0Ð9 kg m2 a1 (site 3).
Table II compares the estimates of average sedimentation rate for the last c. 32 and c. 100 years for the
three floodplain cores based on measurements of 137 Cs and unsupported 210 Pb, respectively. For all three
sites, the average sedimentation rate for the last c. 32 years is less than the average rate over the longer time
period. Table II also lists the average sedimentation rates for the period from c. 1894/95 to 1963, estimated by
subtracting the total deposition since 1963 (estimated from the 137 Cs measurements) from that since 1894/95
(estimated from the unsupported 210 Pb measurements) and then calculating the average deposition rate for the
period 1894/95 to 1963. These values further emphasize the reduction in overbank sedimentation rates since
1963, particularly for the core collected from site 3, where the average value post-1963 is only one-third of
the average rate prior to this date.
Copyright  2002 John Wiley & Sons, Ltd.
Earth Surf. Process. Landforms 27, 403–423 (2002)
409
CHANGES IN SEDIMENT SOURCES OVER 100 YEARS
a) Site 1 (Upper Tweed)
Unsupported 210Pb content (mBq g−1)
Cs content (mBq g−1)
137
0
5
10
15
20
25
0
1963
10
10
15
20
25
30
35
40
10
20
Depth (cm)
20
Depth (cm)
5
0
0
30
40
50
30
40
50
60
60
70
70
80
80
b) Site 2 (Middle Tweed)
Unsupported 210Pb content (mBq g−1)
Cs content (mBq g−1)
137
0
5
10
15
20
25
0
1963
10
10
15
20
25
30
35
40
10
20
Depth (cm)
20
Depth (cm)
5
0
0
30
40
50
30
40
50
60
60
70
70
c) Site 3 (Teviot)
137
0
2
4
Unsupported 210Pb content (mBq g−1)
Cs content (mBq g−1)
6
8
10
12
14
0
16
1963
10
15
20
25
30
35
10
20
20
30
30
Depth (cm)
Depth (cm)
10
40
50
40
50
60
60
70
70
80
80
Figure 3. The depth distributions of
5
0
0
137 Cs
and unsupported
210 Pb
concentrations in the floodplain cores collected from the three sites
Any attempt to interpret the changes in sedimentation rates documented in Table II must take account of the
fact that such rates could be expected to progressively reduce through time if the floodplain surface accretes
relative to the channel bed, and thus floods less frequently. However, if the channel bed also accretes, this
effect will not be important. Unfortunately, there is no information available for the study sites on changes of
the height of the floodplain surface relative to the channel bed over the last c. 100 years that could be used
to examine the effect of this on changes in overbank sedimentation rates.
Copyright  2002 John Wiley & Sons, Ltd.
Earth Surf. Process. Landforms 27, 403–423 (2002)
410
P. N. OWENS AND D. E. WALLING
Table II. Mean annual sedimentation rates for the past c. 32 and c. 100 years based on 137 Cs
and unsupported 210 Pb measurements, respectively, and estimates of the mean annual rate
for the period from c. 1894/95 to 1963
Average sedimentation rate (kg m2 a1 River/site
137
Tweed/1
Tweed/2
Teviot/3
Cs-based (1963 to
1994/95)
Unsupported
Pb-based (c.
1894/95 to 1994/95)
c. 1894/95 to 1963
3Ð0 š 0Ð6†
2Ð6 š 0Ð6
4Ð8 š 0Ð9
3Ð2
2Ð7
5Ð9
210
2Ð0 š 0Ð2Ł
2Ð2 š 0Ð2
1Ð9 š 0Ð2
Ł Uncertainty associated with the location of the 137 Cs peak at the middle of each 1 cm depth increment
(i.e. peak D x š 0Ð5 cm depth). † Uncertainty associated with the measurement of unsupported 210 Pb
activity.
Changes in sediment sources over the last 100 years
Table III presents the results of using the Mann–Whitney (source type) and Kruskal–Wallis (geological/topographic zone) tests to assess the ability of each of the individual geochemical and mineral magnetic
properties to discriminate the various source groups within a particular category. For the purpose of source
type ascription, source materials were originally classified into four groups, namely topsoil from woodland,
topsoil from pasture and moorland, topsoil from uncultivated land, and channel bank/subsoil material. However, the composite fingerprint selected using stepwise discriminant function analysis was able to classify <60
per cent of the source materials into the correct source group. Because of this relatively poor discrimination,
the source type classification was simplified and source materials were designated as either topsoil or channel
Table III. The results of using the Mann–Whitney (source type) and Kruskal–Wallis
(topographic/geological zones) tests to assess the ability of each tracer property to discriminate between source types (topsoil and channel bank/subsoil material) and geological/topographic zones (Carboniferous, Devonian, igneous, and Silurian and Ordovician)
Tracer property
lf 1
fd 2
SIRM3
Al
Ca
Cr
Cu
Fe
K
Mg
Mn
Na
Ni
Pb
Sr
Zn
Source type
Geological/topographic zones
0Ð140
0Ð013Ł
0Ð231
0Ð468
0Ð966
0Ð150
0Ð359
0Ð035Ł
0Ð001Ł
0Ð059
0Ð054
0Ð025Ł
0Ð001Ł
0Ð029Ł
0Ð288
0Ð012Ł
0Ð001Ł
0Ð156
0Ð001Ł
0Ð001Ł
0Ð001Ł
0Ð023Ł
0Ð869
0Ð001Ł
0Ð142
0Ð672
0Ð285
0Ð778
0Ð045Ł
0Ð191
0Ð004Ł
0Ð603
Ł Significant at the 95 per cent confidence level.
1 Low frequency magnetic susceptibility.
2 Frequency dependent magnetic susceptibility.
3 Saturated isothermal remanent magnetization (IRM
Copyright  2002 John Wiley & Sons, Ltd.
at 0Ð8 T).
Earth Surf. Process. Landforms 27, 403–423 (2002)
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CHANGES IN SEDIMENT SOURCES OVER 100 YEARS
bank/subsoil material. Because there is little visual evidence of gullying and other types of subsoil erosion in
the study basin, it is likely that most channel bank/subsoil material is derived from channel banks.
Table IV presents the optimum composite fingerprints (i.e. the sets of tracer properties that afford optimum
discrimination between source groups) selected by stepwise discriminant function analysis. In the case of
source type, the composite fingerprint comprises five properties, which in combination are able to classify 78
per cent of the 119 source material samples into the correct source group (i.e. topsoil or channel bank/subsoil).
In the case of the geological/topographic zones, the composite fingerprint comprises seven properties, and
this was able to classify 72 per cent of the source material samples into the correct source group.
The optimum composite fingerprints were used in a linear mixing model to estimate the relative contributions of the various sources to the sediment represented by the individual depth increments from the floodplain
sediment cores. The chronology for each core was established using the sedimentation rates estimated from
the 137 Cs and unsupported 210 Pb measurements. For the part of the core representing the period between 1963
(the time of peak 137 Cs fallout) and 1994/95 (the time that the cores were collected) the depth increments
were dated using the average sedimentation rate for this period estimated from the 137 Cs measurements.
Prior to 1963, the depth increments were dated using the average sedimentation rate for the period 1894/95
to 1963 (cf. Table II), calculated using the procedure described above. Due to the uncertainties associated
with extrapolating sedimentation rate estimates beyond the time period for which they were derived, detailed
changes in sediment sources have only been investigated for the last c. 100 years. However, to place these
results within a slightly longer timeframe (in order to identify trends), sediment sources have been determined
for a limited number of sediment increments below that dated at 1900, although no dates have been assigned
to pre-1900 sediment.
There are three additional points which need to be taken into account when interpreting the evidence
for downcore changes in sediment source. Firstly, because the two optimum composite fingerprints selected
by discriminant function analysis were unable to discriminate 100 per cent of the source materials in each
category, some degree of error is likely to be associated with the estimates of the relative contributions
from the various sources provided by the mixing model. Secondly, some downcore variations in the various
geochemical and mineral magnetic properties could reflect post-depositional transformation and migration,
rather than changes in sediment source, and this would compromise their effectiveness as source fingerprints
(cf. Collins et al., 1997; Foster et al., 1998; Hudson-Edwards et al., 1998; Owens et al., 1999a). However,
it is unlikely that all the sediment properties included in a composite fingerprint would be affected by postdepositional transformation, and the use of composite fingerprints incorporating a combination of different
geochemical and mineral magnetic properties should reduce this potential problem. Thirdly, the results of the
sediment source ascription are expressed in terms of the percentage contribution from the individual sources.
Changes in the contribution from a particular source may not reflect changes in the amount of sediment
Table IV. The results of using stepwise multivariate discriminant function analysis to identify
which combination of tracer properties provides the best composite fingerprint for discriminating source materials on the basis of source type (topsoil and channel bank/subsoil) and
geological/topographic zones (Carboniferous, Devonian, igneous, and Silurian and Ordovician)
Source type
Tracer
property
Cumulative % samples
discriminated correctly
Ni
K
fd
Zn
Na
Copyright  2002 John Wiley & Sons, Ltd.
72Ð8
77Ð2
77Ð2
76Ð3
78Ð1
Geological/topographic zones
Tracer
property
SIRM
Al
lf
Ni
Sr
Cr
Fe
Cumulative % samples
discriminated correctly
55Ð2
56Ð3
64Ð8
68Ð0
68Ð0
69Ð0
72Ð1
Earth Surf. Process. Landforms 27, 403–423 (2002)
412
P. N. OWENS AND D. E. WALLING
mobilized from that source. They could result from either an increase or a decrease in the importance of
another source, causing the relative contribution from the source in question to change without any change
in the amount of sediment mobilized from that source.
Figure 4 presents the source type ascription results for the three floodplain sediment cores for the last c.
100 years and provides evidence of downcore changes in the relative contributions from topsoil and channel
bank/subsoil sources at all three sites. In the case of the core collected from the River Tweed at site 1,
which is located about 40 km downstream of the headwater source of the river, channel bank/subsoil sources
dominate below a depth of c. 28 cm. Over the last 100 years, there has been a marked increase in the relative
contribution from topsoil sources, with contributions of c. 70–80 per cent evident in the 1890s (24–26 cm
depth), 1940s to early 1960s (c. 14–8 cm depth), and the 1980s to mid-1990s (upper 4 cm). Over the past c.
100 years there were also periods with increased contributions from channel bank/subsoil sources and these
can be dated to the early 1900s (22–24 cm depth), the 1930s (14–16 cm depth) and the late 1960s to early
1980s (8–4 cm depth). Overall, however, there has been a trend of increasing relative contributions from
topsoil sources from the late 19th century to the 1990s.
For the core collected from the River Tweed at site 2, which is located c. 50 km downstream of site 1,
topsoil sources represent the dominant source over most of the period represented by the core. Over the last
c. 100 years there were two major periods when contributions from topsoil sources exceeded 60 per cent,
and these can be approximately dated to the periods extending to the early 1930s (30–14 cm depth), and
from the late 1970s to mid-1990s (upper 4 cm). Periods with increased relative contributions from channel
bank/subsoil sources occurred during the 1940s (12–14 cm depth) and 1960s (6–8 cm depth).
The core collected from the River Teviot at site 3 also shows evidence of marked changes in the relative
contributions from topsoil and channel bank/subsoil sources over the last c. 100 years. Below 38 cm depth,
channel bank/subsoil sources were again dominant. Between 42 cm and 32 cm depth, there was an increase
in the relative contribution from topsoil sources from <20 per cent to c. 70 per cent. Since the turn of the
20th century, periods with increased contributions from channel bank/subsoil sources occurred in the 1910s
(26–28 cm depth), the 1920s, 1930s and early 1940s (24–16 cm depth), and the late 1940s to mid-1950s
(14–10 cm depth), whereas increased contributions from topsoil sources are evident for the late 1910s to
early 1920s (24–26 cm), the 1950s (14–16 cm) and the late 1960s to mid 1990s (upper 6 cm).
Site 1 (Upper Tweed)
Source contribution (%)
0
20
40
60
80
Site 3 (Teviot)
Source contribution (%)
Site 2 (Middle Tweed)
Source contribution (%)
0
100
20
40
60
80
100
0
5
5
20
40
60
80
100
0
0
0
5
1963
1963
10
1963
10
15
20
1900
25
15
1900
20
Depth (cm)
15
Depth (cm)
Depth (cm)
10
20
25
30
35
1900
40
30
35
25
45
50
30
Topsoil
Channel bank/subsoil
Figure 4. Downcore changes in the relative contributions of topsoil and channel bank/subsoil sources documented for the three floodplain
cores
Copyright  2002 John Wiley & Sons, Ltd.
Earth Surf. Process. Landforms 27, 403–423 (2002)
413
CHANGES IN SEDIMENT SOURCES OVER 100 YEARS
Figure 5 presents equivalent evidence for downcore changes in the contributions from the main geological/topographic zones for the cores collected from the three sites. For sites 2 and 3, there are only two main
geological/topographic zones (Devonian, and Silurian and Ordovician) in the catchments upstream of the core
sites, and the relative contributions from areas underlain by Carboniferous and igneous rocks (which only
occupy a small area of the upstream catchments) are negligible or so low that they are not recognized by the
mixing model. In the case of the core collected from site 1, the contributing catchment was classified simply
as either Silurian and Ordovician or ‘other’, where ‘other’ represents the small area in the northwest of the
basin underlain by a complex mixture of Carboniferous, Devonian and igneous rocks. From Figure 5, it can
be seen that for the core collected from site 1, contributions from areas underlain by Silurian and Ordovician
rocks dominate, and this is consistent with the dominance of this zone in the upstream catchment. Below
34 cm depth, the overbank sediment deposited at site 1 was derived almost entirely from the areas underlain
by Silurian and Ordovician rocks. Above 34 cm depth, the relative contribution from the headwater area in
the northwest of the basin underlain by ‘other’ rocks increased, reaching a maximum of c. 30 per cent in the
1920s (16–18 cm depth), after which time this contribution steadily decreased until the late 1980s (2 cm).
Since the late 1980s, the relative contribution from the areas underlain by ‘other’ rocks has increased to
c. 40 per cent.
For the core collected from site 2, which is located downstream of site 1, contributions from areas underlain
by Silurian and Ordovician rocks dominate. This is again consistent with the dominance of this geological/topographic zone in the catchment upstream of site 2 (cf. Figure 2). Between 30 cm depth and the early
1940s (12 cm depth), there was a small increase in the relative contribution from areas underlain by Devonian rocks, after which time this contribution decreased slightly. Since the mid-1970s (upper 4 cm), the
relative contribution from areas underlain by Devonian rocks has again increased to c. 40 per cent in 1995.
At 26–28 cm depth there is a short-lived increase in the contribution from Devonian rocks.
For the floodplain core collected from the River Teviot at site 3, the relative contribution from areas
underlain by Devonian rocks is more important than for the core collected from nearby site 2. This reflects the
greater proportion of the upstream catchment underlain by Devonian rocks compared to site 2 (cf. Figure 2a).
However, the general downcore trend in the relative contributions from Silurian and Ordovician, and Devonian
zones for the core collected from site 3 is similar to that for site 2. Between 44 cm depth and the late 1930s
Site 1 (Upper Tweed)
Source contribution (%)
0
20
40
60
80
Site 2 (Middle Tweed)
Source contribution (%)
100
0
20
40
60
80
Site 3 (Teviot)
Source contribution (%)
100
0
20
40
60
80
100
0
0
0
5
5
5
1963
1963
10
1963
10
15
20
1900
25
15
1900
20
Depth (cm)
15
Depth (cm)
Depth (cm)
10
20
25
30
1900
35
40
30
25
35
45
50
30
Silurian & Ordovician
Other
Silurian & Ordovician
Devonian
Silurian & Ordovician
Devonian
Figure 5. Downcore changes in the relative contributions of the main geological/topographic zones in the upstream catchments documented for the three floodplain cores
Copyright  2002 John Wiley & Sons, Ltd.
Earth Surf. Process. Landforms 27, 403–423 (2002)
414
P. N. OWENS AND D. E. WALLING
(18 cm depth), there was a steady increase in the relative contribution from areas underlain by Devonian
rocks, which increased from c. 40 per cent to c. 70 per cent. Between the late 1930s and the late 1980s (18 to
2 cm) the relative contribution from areas underlain by Devonian rocks decreased from 70 to 40 per cent, but
this contribution subsequently increased to c. 60 per cent. As with site 2, there is also a short-lived increase
in the contribution from areas underlain by Devonian rocks between 42 and 44 cm depth.
DISCUSSION
The changes in both overbank sedimentation rates and the relative contribution from the main sediment sources
(both source type and geological/topographic zones) over the last c. 100 years are likely to reflect changes
in both land use (e.g. afforestation and the conversion of grassland to arable) and climate (e.g. precipitation
amounts and weather patterns). Disentangling the contributions of land use and climate change to the changes
in sedimentation rates and sediment source is, however, problematic (cf. Foster et al., 2000). The downcore
trends identified above (i.e. Table II and Figures 4 and 5) can, nevertheless, be compared with the available
long-term records of precipitation, weather patterns, river flow and land use change for the study area, in an
attempt to identify the likely causes of changes in overbank sedimentation and sediment source in the Tweed
basin over the last c. 100 years.
Comparison of changes in overbank sedimentation rates and sediment source with long-term records of river
flow, precipitation and weather patterns
Continuous records of river flow for the study rivers extend back only to the early 1960s and are illustrated in
Figure 6 for the Rivers Tweed and Teviot. At all four sites there are common trends, with below-average flows
in the early and mid-1970s. Several studies have, however, reconstructed river flows and flood chronologies
a) River Tweed at Peebles
b) River Tweed at Boleside
50
20
15
10
5
0
1960
Annual mean discharge
Average annual discharge (1960−1995)
1970
1980
Discharge (m3 s−1)
Discharge (m3 s−1)
25
40
30
20
10
0
1960
1990
Annual mean discharge
Average annual discharge (1961−1995)
1970
Year
12
30
10
25
8
6
4
0
1960
Annual mean discharge
Average annual discharge (1961−1995)
1970
1990
d) River Teviot at Ormiston
1980
Year
1990
Discharge (m3 s−1)
Discharge (m3 s−1)
c) River Teviot at Hawick
2
1980
Year
20
15
10
5
0
1960
Annual mean discharge
Average annual discharge (1961−1995)
1970
1980
1990
Year
Figure 6. Long-term records of annual mean discharge for the River Tweed at (a) Peebles and (b) Boleside, and for the River Teviot at
(c) Hawick and (d) Ormiston
Copyright  2002 John Wiley & Sons, Ltd.
Earth Surf. Process. Landforms 27, 403–423 (2002)
415
CHANGES IN SEDIMENT SOURCES OVER 100 YEARS
for the study rivers, extending back over longer periods. Steel (1999) used daily rainfall records to reconstruct
flow records since 1914 and found that, although there had been an increase in the frequency of (simulated)
peak over threshold flow events in the period 1981–1995 compared to 1914–1981, there was no significant
difference in flood magnitudes between the two periods. McEwan (1990) used river flow data and historical
information to reconstruct historical flood chronologies for the study rivers and Table V lists the major floods
that occurred on the River Tweed at Kelso and the River Teviot at Hawick (see Figure 1 for locations) between
1900 and 1983. The highest flood estimated for the upper and middle reaches of the River Tweed over the
last c. 250 years occurred on 12–13 August 1948 and the highest gauged flow for the River Tweed upstream
of site 1 was recorded on 7 January 1949. For the River Teviot, the largest flood in living memory occurred
on 13 November 1938.
The reconstructed records of river flow and flood chronologies for the study rivers suggest that, although
flood frequency may have increased in recent decades, there is no evidence to suggest that flood magnitude has
changed significantly over the past c. 100 years (Steel, 1999), and may even have decreased over this period
(cf. Table V). Previous research by the authors in the catchment of the River Ouse, Yorkshire (cf. Walling et al.,
1998; Owens et al., 1999a) has demonstrated that large-magnitude flood events generally deposit considerably
more sediment (often by an order of magnitude) than the long-term average for a particular sampling location.
The apparent decrease in overbank sedimentation rates for the study rivers since 1963 is not inconsistent with
the reconstructed river flow and flood magnitude records, although it is important to recognize the equivocal
nature of the evidence provided by the reconstructed river flow data. Similarly, Walling et al. (1999) and
Owens et al. (2000) have reported that the relative contribution of channel bank/subsoil material to the
suspended sediment loads transported by the rivers Ouse and Tweed during large-magnitude flows (and thus
likely to be deposited on floodplains) is significantly greater than that transported during lower flows. Thus
the peak in the contribution from channel bank/subsoil sources at site 2 dated to the 1940s (c. 12–14 cm)
coincides with the largest flood event recorded for the middle reaches of the River Tweed in 1948. However,
a similar channel bank/subsoil peak at this site at 6–8 cm depth (dated to the 1960s) does not coincide
with a similar high-magnitude flood, although it does coincide with lower magnitude floods in 1962, 1963
and 1967. The lack of evidence of a clear link between the river flow records and downcore variations in
Table V. A reconstructed record of major flood events between 1900 and 1983
for the Rivers Tweed and Teviot (from McEwan, 1990). See also Figure 6 for
flow records for the period 1960–1995
River Tweed at Kelso
Year
Estimated return
period (years)
24/06/1911
14/12/1914
18/08/1920
05/11/1926
13/01/1938
12/08/1948
07/01/1949Ł
06/11/1951
28/08/1956
19/01/1962
21/11/1963
27/02/1967
31/10/1977
04/01/1982
Ł River
33
58
>230
>100Ł
23
23
23
33
River Teviot at Hawick
Year
Estimated return
period (years)
10/08/1901
27/08/1905
17/01/1909
24/06/1911
18/08/1920
27/12/1924
05/11/1926
05/01/1933
13/11/1938
13/08/1948
28/08/1956
09/01/1962
07/10/1964
27/02/1967
30/10/1977
19/12/1982
39
47
>230
116
Tweed at Peebles.
Copyright  2002 John Wiley & Sons, Ltd.
Earth Surf. Process. Landforms 27, 403–423 (2002)
416
P. N. OWENS AND D. E. WALLING
sediment source (both type and geological/topographic zones) is further demonstrated by the fact that there
is no consistent change in sediment source during the period of low flow recorded for both study rivers in
the 1970s (Figure 6).
Figure 7 presents long-term annual precipitation data for three sites in the Tweed basin (see Figure 1 for
locations). These evidence similar trends and have common periods of higher (i.e. late 1920s, 1980s) and
a) Marchmont House
Annual precipitation (mm)
1600
1400
1200
1000
800
600
400
Annual rainfall series
5 year running mean
200
0
1860
1880
1900
1920
1940
1960
1980
Year
b) Hawick
Annual precipitation (mm)
1600
1400
1200
1000
800
600
400
Annual rainfall series
5 year running mean
200
0
1860
1880
1900
1920
1940
1960
1980
2000
Year
c) Eskdalemuir
Annual precipitation (mm)
2500
1928
1954
2000
1500
1000
Annual rainfall series
5 year running mean
1911−1999 average
500
0
1910 1920 1930 1940 1950 1960 1970 1980 1990 2000
Year
Figure 7. Long-term records of annual precipitation for (a) Marchmont House, (b) Hawick and (c) Eskdalemuir (based partly on McEwan,
1989, reproduced by permission of Blackwell Science Ltd.)
Copyright  2002 John Wiley & Sons, Ltd.
Earth Surf. Process. Landforms 27, 403–423 (2002)
CHANGES IN SEDIMENT SOURCES OVER 100 YEARS
417
lower (i.e. early 1920s, 1970s) precipitation totals. There is also close agreement between the sites in the
timing of maximum values. The similarity in the precipitation records from the three sites suggests that the
temporal trends in annual precipitation illustrated in Figure 7 are likely to be representative of the Tweed
basin as a whole. The records for Eskdalemuir and Hawick (which include the period from 1980 to 1995),
show no evidence of significant temporal trends in annual precipitation totals over the last c. 100 years. For
example, at Eskdalemuir the average annual precipitation between 1911 and 1962 was 1541 mm, whereas
that for the period 1963–1995 was 1567 mm (an increase of <2 per cent). Because annual precipitation has
remained approximately constant over the last c. 100 years, it cannot explain the apparent decrease in overbank
sedimentation rates over this period. There has, however, been a marked shift in the seasonal distribution of
precipitation, with a c. 8 per cent increase in winter (October–March) precipitation totals between 1911–1962
and 1963–1995. As most flooding in the Tweed basin occurs in the winter period (cf. McEwan, 1990; Steel,
1998, 1999), such an increase in winter precipitation since 1963 might be expected to increase overbank
sedimentation rates, but Table II indicates that sedimentation rates have decreased since 1963.
It is also possible to compare the downcore changes in sediment source over the last c. 100 years documented
for the three floodplain sites with the long-term records of precipitation. For source type (Figure 4), there
appear to be some periods (i.e. last decades of the 19th century) when above-average annual precipitation
coincided with relatively high (sites 1 and 2) or increasing (site 3) contributions from topsoil sources, but
there are also other periods when changes in annual precipitation are not reflected by a consistent change
in sediment source. In the case of the contributions from the main geological/topographic zones, again the
trend in annual precipitation does not explain the changes in the contribution from the main zones. This is
probably because such changes will primarily reflect changes in the spatial location of sediment sources (i.e.
Silurian and Ordovician rocks mainly occur in the upland and hilly areas, while Devonian rocks occur in the
lowland areas), whereas changes in precipitation amounts are more likely to affect the whole basin. There
are no statistically significant relationships between temporal variations in annual precipitation (based on the
five-year running mean for Eskdalemuir) and downcore changes in sediment source (both source type and
geological/topographic zones).
The Lamb (1972) catalogue of daily weather types (LWTs) has been used extensively in studies of British
climate. Foster et al. (1997) found that there was a link between LWTs and river flow for Scottish rivers. For
the River Tweed and other rivers in the eastern part of Scotland, the magnitude and frequency of flooding is
primarily controlled by cyclonic, or C-type, weather patterns. Furthermore, several studies (e.g. Wilby et al.,
1997; Foster and Lees, 1999) have found that an increase in the frequency of C-type weather coincides with
periods of increased soil erosion, sediment transport in rivers, and lake sedimentation. Figure 8 shows the
C-type frequency (days/year)
60
58
56
54
52
50
48
46
44
42
40
38
1840 1860 1880 1900 1920 1940 1960 1980 2000
Decade
Figure 8. Decadal variations in the mean annual LWT C-type frequency (based on Wilby et al., 1997, reproduced by permission of John
Wiley and Sons, Ltd.)
Copyright  2002 John Wiley & Sons, Ltd.
Earth Surf. Process. Landforms 27, 403–423 (2002)
418
P. N. OWENS AND D. E. WALLING
frequency of C-type weather in the UK for the period 1861–1990 and demonstrates that there has been an
increase in the frequency of C-type weather over the last 100 years, particularly during the 1960s, 1970s and
1980s. This increase in the frequency of C-type weather since the 1960s might be expected to result in an
increase in flooding and an increase in river sediment loads, but instead appears to coincide with a period of
decreasing overbank sedimentation rates.
If the downcore trends in the relative contribution of sediment sources are compared to the frequency of
C-type weather, then for some time periods the two types of record match, while in other cases they do not.
Thus, for example, in the case of source type (Figure 4), the increase in the frequency of C-type weather
in the period after 1960, and in particular the 1980s, coincides with an increase in the relative contribution
of topsoil sources. However, there are marked peaks in C-type weather in the 1870s and during the period
extending from the 1910s to the 1930s which do not coincide with an equivalent increase in contributions
from topsoil sources. Similarly in the case of geological/topographic zones, the 1980s (and 1990s) and the
1940s are decades when contributions from areas underlain by Silurian and Ordovician rocks are relatively
low for all three sites. However, although the frequency of C-type weather was at a post-1900 maximum in
the 1980s, it was at a minimum in the 1940s.
Comparison of changes in sedimentation rates and sediment source with long-term records of land use and
management
Figure 9 presents information on changes in land cover in the Borders Region of Scotland over the last
100 years. Land cover data for the Borders Region are presented, as opposed to data for the Tweed basin,
because the southeastern portion of the basin is located in England and the available data for that portion are
not directly comparable with those for the Borders Region (cf. Figure 1). Figure 9 indicates that the total area
of agricultural land has shown a progressive decline since 1900. The areas under rough grassland, moorland
and mire were all characterized by an appreciable decline during this period, whereas the areas under arable
land and grassland (total) showed a more gradual decline. These changes coincided with a marked increase
5000
Total agricultural land
Woodland (including forestry plantations)
Moorland
Mire
Grassland (total)
Grassland (rough)
Arable
Land cover (km2)
4000
3000
2000
1000
0
1900
1920
1940
1960
1980
2000
Year
Figure 9. Changes in land use/cover in the Borders Region since 1900 (based on data in Scottish Agricultural College, 1991; Borders
Regional Council, 1991, 1994; McPhillimy, 1993; Mackey et al., 1998)
Copyright  2002 John Wiley & Sons, Ltd.
Earth Surf. Process. Landforms 27, 403–423 (2002)
CHANGES IN SEDIMENT SOURCES OVER 100 YEARS
419
in the area under woodland and forest. There has been a dramatic increase in the area of forest in the Borders
Region from c. 1 per cent at the turn of the century to c. 16 per cent in the 1990s, most of which has occurred
since the 1940s. Figure 9 also shows that during the Second World War and the immediate post-war years,
a significant area of grassland was ploughed up for the production of cereals and other crops, in a drive for
self-sufficiency within the UK. Since the 1970s, arable land has been converted back to grassland, and more
recently to set-aside. In addition to the changes in land cover shown in Figure 9, an extensive programme of
land drainage was instigated in the study area during the mid to late 19th century (McEwan, 1990).
The changes in land use and management illustrated in Figure 9 are likely to have resulted in changes in
the frequency and magnitude of flooding and in soil erosion and sediment yield. For example, the extensive
programme of land drainage in the mid and late 19th century is likely to have increased the magnitude
and duration of flooding in the latter half of the 19th century and early 20th century (cf. McEwan, 1990).
Learmonth (1950) states that land drainage in the upper parts of the basin resulted in increased runoff rates
on steep slopes and contributed to the August 1948 flood event (cf. Table V). The greatest change in land use
over the past 100 years is the increase in the area of forest land from c. 1 to 16 per cent. Numerous studies
(cf. Beschta, 1978; Soutar, 1989) have demonstrated that afforestation in upland areas (and the associated loss
of areas of moorland and rough grazing) generally result in an increase in soil erosion and sediment transport
in rivers, particularly during road construction, drainage and harvesting operations. In addition, Acreman
(1985) found that drainage improvement prior to afforestation increased rates of runoff in the headwaters of
the Ettrick Water, which lies adjacent to the Teviot catchment, and is therefore likely to have increased the
magnitude of flood events. Thus, the programme of afforestation which commenced in the period after 1940
is likely to have resulted in an increase in sediment loads and suspended sediment concentrations in rivers
and an increase in the magnitude of flooding, and may explain the high sedimentation rates in the period prior
to 1963. The drainage operations prior to tree planting, however, are likely to result in short-lived changes,
and over longer periods of time afforestation is likely to result in reduced runoff and a reduction in the
magnitude and frequency of flooding and thus overbank deposition. There would have been an increase in
soil erosion and sediment transport in rivers during harvesting operations, but these effects would have been
short-lived and local.
During the 1940s and 1950s, there was a period of conversion of grassland to arable land. The present-day
contribution of sediment from arable land to rivers in the Tweed basin is proportionally greater than that from
grassland (Owens et al., 2000), and this conversion of grassland to arable is, therefore, likely to have resulted
in an increase in the suspended sediment load of the rivers and in the suspended sediment concentrations
associated with overbank flows. Since the 1970s, arable land has been converted back to grassland, and more
recently to set-aside, and both are likely to have resulted in a decrease in suspended sediment loads and thus
reduced rates of overbank sedimentation.
Another possible cause of the reduction in overbank sedimentation rates since 1963 is the construction of
eight major reservoirs in the headwaters of the catchment, which are likely to have reduced the frequency
and magnitude of downstream flooding (Fox and Johnson, 1997). However, many of these reservoirs were
constructed at the turn of the century, prior to 1963 (D. McCraw, pers. comm.).
The changes in source type at sites 1 and 3 (Figure 4) indicate that there was a period immediately prior
to 1900 which was characterized by increasing contributions from topsoil sources and which coincided with
extensive land drainage in the basin (cf. McEwan, 1990). Land drainage operations are likely to have increased
topsoil erosion by encouraging cultivation and by increasing grazing pressure on pasture land. At all three
sites, the period between the 1900s and 1920s was marked by a decrease in the relative contribution of
topsoil sources. Since then, there has been a general trend of increasing relative contributions from topsoil
sources, which coincides with the rapid expansion of afforestation. This is likely to have increased topsoil
erosion compared to the pre-existing moorland and rough grazing land cover. A study undertaken by Stott
(1997), in the Balquhidder catchments, Scotland, found that bank erosion rates were significantly lower for
an afforested stream than an upland moorland stream. The two short-lived peaks in topsoil contributions in
the 1920s and 1950s at site 3 can be linked to the high precipitation totals in 1928 and 1954 recorded at
Eskdalemuir (Figure 7), and these peaks are superimposed upon the general trends resulting from land use
change. At sites 1 and 2, there is also a period of increased contributions from topsoil sources in the late
Copyright  2002 John Wiley & Sons, Ltd.
Earth Surf. Process. Landforms 27, 403–423 (2002)
420
P. N. OWENS AND D. E. WALLING
1940s and 1950s (and early 1960s at site 1) that coincided with the conversion of grassland to arable land in
the post-war years, which is likely to have increased topsoil erosion. At all three sites, there was an increase
in the relative contribution from topsoil sources in the 1970s and 1980s. This coincided with a period of
increased cereal production and a shift from spring to winter sowing, which has been reported as being partly
responsible for increased soil erosion in the Tweed basin (Speirs and Frost, 1985). Cultivated land is most at
risk from soil erosion during the interval between seed-bed preparation and the development of a sufficiently
protective crop cover. With winter cereals the period of greatest risk is October–November, which is usually
wetter than the equivalent period for spring cereals (March–April) (Speirs and Frost, 1985). Since the late
1980s, contributions from topsoil sources have either remained constant or have decreased slightly. This
may reflect a trend away from cereals and an increase in the importance of set-aside land in the last decade
(Scottish Agricultural College, 1991).
In the case of the geological/topographic zones, the temporal changes in their source contributions are
similar for the three sites. The cores collected from sites 2 and 3 provide evidence of a trend of increasing
relative contributions from the areas of the catchment underlain by Devonian rocks from the late 19th century
to the 1930s/1940s. This period coincides with one of land drainage and the conversion of grassland to arable
land. Most of this change is likely to have occurred in the flatter parts of the catchment, which occur in the
middle and lowland parts of the Tweed basin (cf. Figure 2). For the catchment areas contributing to sites 2
and 3, much of the flatter land which is likely to have been cultivated is underlain by Devonian rocks. The
greater relative contribution from areas underlain by Devonian rocks evident for site 3, compared to site 2,
reflects the greater proportion of both this rock type and cultivated land within the catchment upstream of
site 3. At both sites 2 and 3, there are short-lived peaks in the contribution from areas underlain by Devonian
rocks, which lie at similar positions in the depth profiles and have been tentatively dated to the 1870s. The
fact that this peak occurred at both sites 2 and 3, but not at site 1 (which has no major areas of cultivated land
at present), further suggests that increases in the sediment contribution from areas underlain by Devonian
rocks reflect the conversion of grassland to arable in the lower parts of the Tweed and Teviot catchments.
At sites 2 and 3 there is also a trend of increasing relative contributions from areas underlain by Silurian
and Ordovician rocks between the 1930s/1940s and the 1970s/1980s. This period broadly coincides with
the expansion of afforestation. The main areas of afforestation are in headwater areas, which are primarily
underlain by Silurian and Ordovician rocks. The headwaters of the River Teviot are one of the main areas of
afforestation (cf. Figure 2) and this undoubtedly explains the rapid increase in the relative contribution from
areas underlain by Silurian and Ordovician rocks from <30 per cent in the 1940s to c. 60 per cent in the
1980s. Afforestation only occupies a small proportion of land in the catchment of the main stem of the River
Tweed, and this explains the smaller increase in contributions from areas underlain by Silurian and Ordovician
rocks at sites 1 and 2. The period extending from the 1930s/1940s to the 1970s/1980s also coincides with the
dramatic increase in the conversion of grassland to arable during the immediate post-war years. Because the
best land, which was located in the lowlands, was already under cultivation, the ploughing-up of grassland
and moorland is most likely to have affected more marginal areas of land in the upper parts of the basin,
which are underlain by Silurian and Ordovician rocks.
Synthesis
The records of annual precipitation for the study area and the frequency of C-type weather patterns in
the UK provide no evidence to suggest that the decrease in rates of overbank sedimentation or changes
in sediment source evidenced by the three floodplain sites can be explained by climate change alone. It
is, nevertheless, important to recognize that the apparent lack of a clear link between changes in overbank
sedimentation rates and sediment source, and climate change, could reflect buffering associated with sediment
delivery and the storage interposed between upstream soil erosion and sediment transport in rivers. The
reconstructed records of land use and management do, however, appear to be linked to many of the changes
in overbank sedimentation rates and sediment sources documented in this study. In particular, land drainage
during the end of the 19th century, the increase in afforestation since the 1940s, the conversion of grassland
to arable land in the post-war period and the subsequent return to grassland, and the recent increase in
set-aside land, all appear to have coincided with changes in overbank sedimentation rates and sediment
sources.
Copyright  2002 John Wiley & Sons, Ltd.
Earth Surf. Process. Landforms 27, 403–423 (2002)
CHANGES IN SEDIMENT SOURCES OVER 100 YEARS
421
Limitations and uncertainties
There are several limitations and uncertainties associated with the results and interpretation presented above
which need to be identified. Firstly, the results and interpretations of changes in sediment source and overbank
deposition rates are based on only three cores collected in a basin of 4390 km2 . The limited number of cores
collected and analysed was primarily determined by the objectives of this investigation within the larger
Land–Ocean Interaction Study (cf. Walling et al., 1998, 1999; Owens et al., 1999a,b, 2000). Furthermore, as
long as a core is collected from a location that is undisturbed, with a continuous record of sedimentation, and
the location is representative of the area of the floodplain experiencing overbank deposition of fine sediment,
then the results obtained should be representative of the aggregated changes in sediment source and rates of
overbank sedimentation in the upstream catchment. However, the collection and analysis of additional cores
would clearly have increased the confidence that can be ascribed to the results obtained and interpretations
made for this relatively large basin. In large basins (i.e. >103 km2 , several cores should ideally be collected
both from each site (in order to examine within-site variability) and along the length of the river (in order to
investigate downstream changes in sediment response). In smaller basins (c. 100 –102 km2 , a few replicate
cores from one or two downstream locations should prove sufficient to determine the sediment response of
the upstream catchment to land use and climate change. In the present study, additional cores collected from
downstream reaches of the River Tweed near to the tidal limit at Norham (cf. Figure 1) would probably have
provided useful information on changing sediment dynamics in the lower parts of the basin and, in particular,
changes in the contribution from sources in downstream reaches, such as cultivated land, and areas underlain
by Carboniferous and igneous rocks. However, most areas of floodplain in these reaches have been ploughed
over the last 100 years, making them unsuitable.
A second limitation is related to the comparison of the inferred changes in sedimentation rates and sediment
source with historical information and reconstructed records of climate, river flow and land use. Although
137
Cs and unsupported 210 Pb measurements have enabled a chronology to be established, the chronology is
relatively crude, as it has been necessary to assume that sedimentation rates have been constant within each
time period (i.e. 1894/95–1963 and 1963–1994/95). Furthermore, it has been necessary to assume that 1963
equates with the depth increment with the highest 137 Cs concentration, once account is taken of the slow
downward migration of the 137 Cs peak, and from Figure 3 it can be seen that in all cores there are several
smaller peaks on either side of the main peak which could be ascribed to 1963. These problems add uncertainty
to the precise timing of changes in sediment source, and thus make comparison with climate and land use
data problematic. In addition, the climate and land use data are relatively limited (i.e. river flow and land use
data) and partially ambiguous (reconstructed river flow and flood chronologies). In consequence, although
the interpretations presented appear to be supported by the available evidence, there are clearly problems and
uncertainties associated with these interpretations. A more accurate chronology of overbank sedimentation
and more detailed climate and land use information would have greatly improved this attempt to unravel
the relative importance of climate change and land use change in influencing fine sediment dynamics in the
Tweed basin over the last c. 100 years.
CONCLUSION
Cores collected from overbank floodplain deposits at three sites in the middle reaches of the Tweed basin
have been used to reconstruct downcore, and thus temporal, changes in sedimentation rates and sediment
source over the last c. 100 years. Average rates of overbank sedimentation for the period 1963–1994/95
are lower than for the period 1894/95–1963. There is also evidence of significant changes in the source of
the overbank sediment, both in terms of source type (topsoil and channel bank/subsoil) and spatial location
(main geological/topographic zones). When the temporal changes in sedimentation rate and sediment source
are compared with the available long-term records of precipitation, weather patterns, river flow and land use,
the changes recorded by the floodplain deposits appear to have coincided with changes in land use and land
management. This study suggests that changes in land use appear to be the main cause of the changes in
sedimentation rates and sediment source in the Tweed basin. However, due to uncertainties associated with
the accuracy and resolution of the core chronologies and with the limited and, in places, ambiguous climate
Copyright  2002 John Wiley & Sons, Ltd.
Earth Surf. Process. Landforms 27, 403–423 (2002)
422
P. N. OWENS AND D. E. WALLING
and land use data for the study basin for the last c. 100 years, disentangling the roles of climate and land
use change in causing changes in sediment source and overbank sedimentation rates has proved difficult.
Given the current emphasis placed on studies that attempt to predict the impact of future climate change on
geomorphic activity (cf. Boardman and Favis-Mortlock, in press), there is clearly a need to consider also
the likely impact of future changes in land use and management on geomorphic activity, in general (cf.
Slaymaker, 2001), and catchment sediment dynamics and river response, specifically.
ACKNOWLEDGEMENTS
Financial support for the work reported in this paper was provided by research grant GST/02/774, within
the framework of the UK NERC Land–Ocean Interaction Study (LOIS). Special thanks are extended to Ian
Foster and Joan Lees (both Coventry University) for mineral magnetic analysis of floodplain sediment and
source material samples, to Michael Shewry (Scottish Natural Heritage) and John Reville (Scottish Office)
for providing land cover data, to Mike Steel (University of Dundee) for providing unpublished synthetic river
flow data, and to Drew McCraw (SEPA, Galashiels) for providing precipitation and river flow data. Thanks
are also due to Helen Jones for producing the diagrams.
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