Geomorphology 77 (2006) 320 – 334
www.elsevier.com/locate/geomorph
The effects of seasonal changes to in-stream vegetation cover on
patterns of flow and accumulation of sediment
J.A. Cotton a,⁎, G. Wharton a , J.A.B. Bass b , C.M. Heppell a , R.S. Wotton c
a
UK Department of Geography, Queen Mary, University of London, UK
b
Centre for Ecology and Hydrology, Dorset, UK
c
Department of Biology, University College London, London, UK
Received 5 July 2004; received in revised form 27 June 2005; accepted 6 January 2006
Available online 28 February 2006
Abstract
In-stream macrophytes are typically abundant in nutrient-rich chalk streams during the spring and summer months and modify
the in-stream environment by altering river flows and trapping sediments. We present results from an inter-disciplinary study of two
river reaches in the River Frome catchment, Dorset (UK). The investigation focused on how Ranunculus (water crowfoot), the
dominant submerged macrophyte in the study reaches, modified patterns of flow and sediment deposition. Measurements were
taken on a monthly basis throughout 2003 to determine seasonal patterns in macrophyte cover, associated changes in the
distributions of flow velocities and the character and amount of accumulated fine sediment within stands of Ranunculus.
Maximum in-stream cover of macrophytes exceeded 70% at both sites. Flow velocities were less than 0.1 m s− 1 within the
stands of Ranunculus and accelerated to 0.8 m s− 1 outside the stands. During the early stages of the growth of Ranunculus, fine
sediment mostly accumulated within the upstream section of the plant but the area of fine sediment accumulation extended into the
downstream trailing section of the plant later in the growing season. The fine sediment accumulations were dominated by sand
(63–1000 μm) with silts and clays (0.37–63 μm) comprising b 10% by volume. The content of organic matter in the accumulated
sediments varied within stands, between reaches and through the growing season with values ranging between 9 and 105 mg g− 1
dry weight. At the reach scale the two sites exhibited different growth forms of Ranunculus which created distinctive patterns of
flow and fine sediment deposition.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Chalk streams; Submerged macrophytes; River flows; Sediment; Ranunculus
1. Introduction and background
Fine sediment plays an important role in the storage,
transfer and fate of nutrients through river basins
(Walling et al., 2003). Groundwater-fed rivers throughout lowland England show recent increases in suspended
⁎ Corresponding author.
E-mail address: [email protected]
(J.A. Cotton).
0169-555X/$ - see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.geomorph.2006.01.010
sediment loads and concentrations of macro-nutrients
(e.g. Casey et al., 1993; Wood and Armitage, 1997;
1999). Whilst the causes are not fully understood, the
increases have been attributed to: changes in land use and
agricultural practices resulting in increased terrestrial
inputs of inorganic and organic matter to streams;
increased water abstraction; direct return to the river of
treated sewage effluent; modifications to channel
morphology through river engineering; and changes in
the management of weed growth (Walling and Amos,
J.A. Cotton et al. / Geomorphology 77 (2006) 320–334
1999). Such elevated levels of suspended sediments and
sediment-associated nutrients present a number of
environmental management problems for chalk streams.
These include a general degradation of aquatic habitats
with consequent changes in the invertebrate and
macrophyte populations. Also a decline has occurred in
salmonid stocks, linked to in-channel siltation problems
(MAFF, 2000) and the ingress of sediments to gravels
which prevents interactions between surface water and
groundwater (Sear et al., 1999). Changes to in-stream and
321
floodplain sedimentation processes affect the implementation of measures for local alleviation of floods.
Submerged macrophytes are typically abundant in
lowland permeable streams. For example, Flynn et al.
(2002) report a summer peak of 40% macrophyte cover
in the River Kennet (UK) in August 1999 with
Ranunculus spp. (water crowfoot) accounting for 16%
of the stream channel cover. Temporal and spatial
changes to in-stream vegetation on two reaches of the
River Frome, Dorset (UK) are typical of many lowland
Fig. 1. River Frome at Maiden Newton (left) and Pallington (right): example images of seasonal changes of macrophytes within a chalk stream.
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J.A. Cotton et al. / Geomorphology 77 (2006) 320–334
chalk streams in England (Fig. 1). On the River Frome
Ranunculus penicillatus var. calcareous (R W Butcher)
C D K Cook, hereafter termed Ranunculus, is the
dominant submerged macrophyte and its biomass
increases during the spring and summer (Dawson,
1980). The Ranunculus plants are rooted at the upstream
end with flexible trailing limbs extending up to 4 m
downstream (Dawson, 1978). The species is light
dependant and will not survive where riparian vegetation
or infrastructures (such as bridges) create shaded areas in
the river (Dawson and Kern-Hansen, 1979).
The ability of submerged and emergent macrophytes
to modify the in-channel environment and promote the
trapping of suspended matter and associated nutrients
has been widely noted (e.g. Chambers et al., 1991, 1992;
Sand-Jensen, 1997, 1998). Trapping of suspended
matter is achieved through a combination of processes.
Submerged plants reduce water velocities within their
stands, sometimes by more than 90% relative to the
adjacent open water (Madsen and Warncke, 1983; SandJensen and Pedersen, 1999) and this induces sedimentation and retention of fine material and organic particles
(Kenworthy et al., 1982; Sand-Jensen, 1998). The finely
divided leaves of some macrophyte species (e.g.
Ranunculus) also collect organic particles, and dissolved compounds are adsorbed to biofilms associated
with the plant. Furthermore, submerged macrophytes
provide an important substratum for suspension feeders.
These animals ingest dissolved and particulate organic
matter (DOM and POM), assimilate little of the food
they gather and, thus, egest much larger faecal
aggregates (Wotton and Malmqvist, 2001; Wotton et
al., 2003). Faecal pellets in suspension in chalk streams
are produced primarily by blackfly larvae (Diptera:
Simuliidae) attached to Ranunculus. The number of
faecal pellets in transport in chalk streams has been
recorded at 352–1471 pellets L− 1 for two streams
draining the Chiltern Hills (Wotton and Malmqvist,
unpublished data). The Ranunculus plants are a
substratum for larvae and also act as a trap for
sedimenting faecal pellets and those in transport. It has
been suggested that Ranunculus and the associated
suspension feeders may play a significant role in the
processing of organic matter in fresh waters (Hargrave,
1976; Shepard and Minshall, 1981, 1984; Ladle et al.,
1987; Ward et al., 1994).
Fig. 2. Map of the River Frome and Piddle catchments, including geology and site locations.
J.A. Cotton et al. / Geomorphology 77 (2006) 320–334
Thus, a number of studies on the inter-relationships
between aquatic macrophytes, streamflow and sediments, including work in lowland English rivers (Clarke
and Wharton, 2001) have indicated that macrophytes
could play a highly significant role in the dynamics of
sediments and nutrients in chalk streams. Therefore, to
inform the sensitive and sustainable management of
lowland permeable catchments an urgent need exists for
improved understanding of plant–water–sediment interactions (see review by Madsen et al., 2001). This paper
reports on research undertaken on two contrasting
reaches (Fig. 1) in the River Frome catchment, Dorset,
UK (Fig. 2) during 2003. The effect of seasonal changes
to in-stream macrophyte cover on water velocities and
the quantity and quality of fine sediment (b1 mm)
accumulated beneath individual plant stands at both
sites is described.
The research forms part of a major inter-disciplinary
programme in the UK LOCAR (Lowland Catchment
Research Thematic Programme) funded by the UK
Natural Environment Research Council (NERC). The
main objective of LOCAR is “to undertake detailed,
inter-disciplinary programmes of integrated hydroenvironmental research relating to the input-storagedischarge cycle and in-stream riparian and wetland
habitats within groundwater-dominated systems” (http://
www.nerc.ac.uk/locar).
2. Description of the study area: the Frome
catchment
The River Frome catchment provided the opportunity
to study plant–water–sediment interactions in a permeable catchment with existing high quality ecological
data, including published work on aquatic macrophytes,
river flows and sediments (e.g. Dawson and KernHansen, 1979; Dawson, 1980; Ladle et al., 1987). The
catchment (total area 414 km2) has a complex geology
323
Table 1
Site descriptions
National grid reference
Length of study reach
Drainage basin (km2)
Average Q (m− 3 s− 1)
Peak Q (m− 3 s− 1)
Peak Q: average Q
Average width (m)
Average depth (m)
Width: depth
Bed surface slope (mm− 1)
Maiden Newton
Pallington
3595,0982
25 m
4.3
0.6 a
1.6 a
2.91:1
7.0
0.22
32
0.0071
3739,0900
15 m
206
2.2 a
11.3 b
5.28:1
25.8
0.24
107
0.0032
a
Based on manual data collection during project.
Based on Manning's discharge estimates calculated at the bank full
level.
b
(Fig. 2). River water and sediments may be contributed
from Jurassic limestones, Upper Greensand, Chalk,
sands of the Tertiary deposits, superficial sands or
gravels and clay with flints, with groundwater dominating over surface run-off (LOCAR Task Force Report —
see http://www.nerc.ac.uk/locar). Stream segments
within the catchment provide a diversity of geomorphology with braided sections in parts of the middle and
lower reaches of the Frome and small wetland areas on
the Frome floodplain.
The two sites chosen for study during 2003 are
Maiden Newton and Pallington, located in the
headwater and middle reaches of the catchment,
respectively (Fig. 2). Site characteristics and locations
are provided in Table 1 and the seasonal variation in
discharge is shown in Fig. 3. The reaches exhibit
contrasts in hydrology, geomorphology and macrophyte characteristics. Discharge at both sites declines
steadily through the spring and summer and the lowest
recorded values occurred in October 2003. Summer
flows within the River Frome catchment are maintained by inputs of groundwater from the underlying
chalk geology (Fig. 3).
Fig. 3. Seasonal variations in discharge.
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The study site at Maiden Newton is a wide shallow
reach (Table 1) with a low sinuosity, a flint and gravel
substrate, low banks and occasional patches of marginal
vegetation. Some undercutting of the banks occurs
where marginal vegetation is absent. Shading from alder
trees limits the growth of Ranunculus which dominates
the channel in open areas. The reach is adjacent to a
water meadow which contains a series of narrow (1.5 m)
sinuous spring-fed channels.
The reach at Pallington (Fig. 2) was located directly
upstream of a bridge crossing. Twenty metres upstream
of the site, the channel narrows to approximately 15 m
and deepens whilst at Pallington and for 500 m
downstream of the site, the river widens to 25–27 m.
The widening and shallowing of the river encourages
the growth of macrophytes across the river as light
penetrates to the riverbed. Macrophytes dominate the
shallow areas of the reach from March to November.
The substratum at Pallington consists of flints and
gravels. The floodplain adjacent to this reach has a series
of human-made ponds used for recreational fishing and
a commercial watercress farm. Water is abstracted from
the river upstream of the site for the fisheries but water
for the watercress industry is obtained from artesian
groundwater flow. The discharge outlets from both enter
the river downstream of the site. Between the sites at
Maiden Newton and Pallington the town of Dorchester
(with a population of 15,000) has a small sewage
treatment works that discharges treated effluent into the
River Frome at Dorchester. Land use in the River Frome
catchment includes agriculture (pasture and arable),
fisheries, commercial watercress production, light
industry, and some small conifer plantations.
3. Methods
Monthly field measurements of discharge, flow
velocities and sediment accumulations were taken at
the two sites from March 2003 to December 2003.
Additionally, in-stream vegetation was mapped at the
reach scale each month and up until February 2004 to
determine the vegetation coverage over a twelve month
period. Mapping was conducted using fixed reference
points, surveying of individual plant stands using a total
station, and a long profile collage of digital photographs.
Vegetation mapping provided the percentage vegetation
cover of submerged and emergent vegetation for the
entire study reach. At each study site a single stand of
Ranunculus was selected at the start of the growing
season as a representative unit (Sand-Jensen and
Pedersen, 1999) for detailed analyses of the effect of
macrophytes on flow and sediment accumulation. Two
fixed markers were placed in the river bed on each side
of the upstream boundary of the stand of Ranunculus.
These markers were used as temporary supports for a
1.25 m square metal frame, connected to additional
frames surrounding the stand of Ranunculus. The
frames provided a fixed grid for measurements of the
spatial extent of the vegetation and any underlying
deposits of fine sediment. Measurement of the main
study stand for Ranunculus could not be undertaken in
March 2003 at Pallington as water clarity was too low to
confidently demarcate the edges of the vegetation. The
accumulated fine sediment referred to within this study
includes material in the clay (1–4 μm), silt (4–63 μm)
and fine to coarse sand (63–1000 μm) fractions that
accumulate over the gravel-flint riverbed.
Each month depths of water and sediment, macrophyte presence and flow velocity (at 0.6 of the water
depth measured from the water surface, Midgley et al.,
1986), were recorded at 0.5 m intervals across a fixed
transect. These variables were used to investigate the
impact of in-stream vegetation on flow velocities and
sediment accumulation. Transects passed through the
stand of Ranunculus selected for detailed investigation.
Depths of sediment were taken with a measurement
device of fixed diameter to ensure a constant depth of
penetration into the gravel substrate. This was undertaken to reduce errors associated with measuring the
accumulations of fine sediment above an uneven gravel
substrate. Measurements of velocity were recorded
using a Valeport electromagnetic flow meter. The flow
meter has a flat sensor with two electrodes on the upper
side which sense the flow in a volume of water up to 10 mm
above the electrodes. This design enables measurements
of velocity in shallow flow and close to and within
macrophyte stands.
Sediment deposited beneath the selected plants was
sampled manually at six locations using 37 mm
diameter clear Perspex cores. The sampling locations
within the stand included one sediment core from
beneath the trailing fronds of the Ranunculus whilst the
rooted section of the stand was divided linearly into five
zones and a sediment sample was extracted from each
zone. The location of samples within each zone was
chosen by forming x and y co-ordinates from random
number tables. Coring locations changed after every
monthly sampling visit to avoid re-sampling an area
beneath the Ranunculus that had been previously
disturbed by sediment coring. The uppermost 1 cm of
the sediment cores was retained for sediment characterisation. The absolute distributions of particle sizes were
obtained and include the particle size of all mineral
components of the sample (McCave and Syvitski,
J.A. Cotton et al. / Geomorphology 77 (2006) 320–334
1991). For absolute particle size, samples were prepared
following the method set out by Rowell (1994). Organic
matter was removed with 20% hydrogen peroxide
followed by particle dispersal using sodium hexametaphosphate and anhydrous sodium carbonate. The b 1 mm
fractions were passed through a LS100 Beckman
Coulter Counter, which records 100 fractions of
sediment size ranging from 0.375 to 950 μm. The
organic matter of the samples was determined by loss on
ignition. Samples were dried and weighed and then
placed in a muffle oven at 550 °C for 8 h. The weights of
the ashes were recorded to determine the dry weight of
combusted organic matter.
4. Results
4.1. Seasonal changes in the in-stream vegetation cover
and impacts on flow patterns
At both sites the maximum spatial coverage of instream vegetation exceeded 70% with maximum cover
occurring in July 2003 at Maiden Newton (75%) and
June 2003 at Pallington (74%) (Fig. 3). Minimum
coverage occurred in February 2004 at Maiden Newton
(28%) and January 2004 at Pallington (0%). Ranunculus
was the dominant in-stream macrophyte at both sites. At
Pallington grasses also grew, including Phragmites
325
australis (Cav.) Trin. ex Steud and Glyceria fluitans (L.)
R. Br., at the channel margins but these were not
included in the values of in-stream vegetation cover.
The planform diagrams in Fig. 4 show how the
spatial patterns of in-channel macrophyte growth and
die-back gave rise to changes in flow patterns through
the vegetated reaches. In April 2003 at Maiden Newton
flow was confined to the right side of the channel
because the stands of Ranunculus established and grew
on the left side (Fig. 4a). As the vegetation extended
across the channel in July, flow was confined to
increasingly narrow channels around the stands of
Ranunculus. By November, the stands of Ranunculus
that were established in spring had died back resulting in
a new dominant flow path along the left side of the
channel. At Pallington the extensive stands of Ranunculus were established by late spring and at maximum
extent in July 2003 forced flow into two narrow flow
paths close to the margins of the channel (Fig. 4b). The
effect of this flow diversion was the creation of an area
of slow flow through the centre of the channel. The dieback of Ranunculus commenced in August and by
September emergent vegetation had colonised and
shaded out the surviving stands of Ranunculus. This
left a small number of patches of in-stream vegetation
and flow was no longer confined to narrow channels.
Whilst the two sites had similar peak values of in-stream
Fig. 4. Planform representation of the distribution of in-stream macrophytes and changing flow patterns over the growing season: (a) Maiden Newton
(b) Pallington.
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macrophyte cover in 2003, marked differences occurred
in the growth patterns (Fig. 4). The growth was
characterised by discrete patches of Ranunculus at
Maiden Newton whereas at Pallington the growth of
Ranunculus was so extensive from May to August 2003
that individual stands could not be discerned.
4.2. Patterns of flow velocity across macrophytedominated reaches
Velocity, depth of water and sediment and macrophyte presence were recorded at monthly intervals along
a fixed transect at both reaches. Fig. 5 shows seasonal
Fig. 5. Modification of velocity profiles and accumulation of sediments by in-stream vegetation. River Frome at Pallington and Maiden Newton.
J.A. Cotton et al. / Geomorphology 77 (2006) 320–334
changes in these physical parameters on the River
Frome at Pallington and Maiden Newton which
demonstrate the seasonal influence of in-stream macrophytes on patterns of flow. At both sites, the recorded
transects passed through upstream rooted sections of
stands of Ranunculus and downstream trailing sections
of other stands. Because the stems of Ranunculus are
hollow (Dawson, 1980) the trailing limbs of stands of
Ranunculus comprise floating stems and leaves with an
area of free water beneath.
Data recorded across the transect at Pallington in
May 2003 illustrate the reduction of velocities
through sections of rooted and trailing stands of
Ranunculus (Fig. 5). Flows accelerated to a maximum
velocity of 0.8 m s− 1 around the stands and were as low
as 0.05 m s− 1 within the stands. Overall, the flow
velocities within the stands of Ranunculus were 48%
lower than flow around the stands. The average depth of
fine sediment (b1 mm), deposited above the gravel bed
across the transect in May, was 0.06 m and these
accumulations of sediment were confined to the areas of
lower velocities of flow beneath the trailing and rooted
sections of the stands of Ranunculus. In July, when
Ranunculus was close to its maximum spatial coverage
(Fig. 6), the flow within the stands had fallen to 27% of
that outside the stands and the same pattern of
accumulation of sediment within areas of low flow
velocity was also evident. In September 2003 the three
main sections of rooted vegetation recorded along the
transect at Pallington solely comprised emergent vegetation with a dense root and stem network that created a
barrier to flow. The velocities within the emergent
Fig. 6. Percentage and timing of maximum and minimum extents of the
spatial coverage of in-stream vegetation over the length of reach
investigated during the spring 2003 to winter 2003–2004 growing
season.
327
vegetation were 12% of the average flows around stands
of macrophytes. By November only a single patch of instream vegetation was recorded across the transect at
Pallington (Fig. 5). With the reduced in-stream cover of
macrophytes, flow was no longer constricted within the
channel and the average velocity across the transect
decreased from 0.23 m s− 1 in September to 0.17 m s− 1 in
November. This occurred even though the discharge
increased from 1.4 to 2.2 m3 s− 1 (Fig. 3).
At Maiden Newton, stands of Ranunculus also
resulted in lower velocities of flow and patches of
deposition of fine sediments throughout the growing
season (Fig. 5). In May 2003, velocities of flow within
the in-stream vegetation were 20% of those recorded
around the stands of plants, and up to 0.16 m of fine
sediment was deposited within the low flow areas of
the stands of Ranunculus. Similar patterns were also
recorded in July and September. Velocities of flows
around the stands reached 0.7 m s− 1 in both months
despite falling discharges (0.28 m3 s− 1 in July to
0.19 m3 s− 1 in September). In November, although
some die-back had occurred resulting in the thinning
of stands of Ranunculus, velocities of flows within
the stands were 22% of the average flow around the
stands. At the same time, the accumulation of fine
sediment beneath the in-stream macrophytes reached
0.08 m in depth.
4.3. Patterns and volume of accumulation of sediment
beneath stands of Ranunculus
Monthly measurements of the dimensions of an
individual stand of Ranunculus, selected for detailed
study, and associated deposition of sediment were used
to analyse the changes in accumulations of sediment at
the individual plant scale throughout the growing
season at the two study sites (Fig. 7). The data from
both sites indicate that a high percentage of the aerial
extent of the stands is underlain by trapped sediment
from April 2003 onwards. At Pallington, monthly
increases from April to July in the spatial coverage of
fine sediment corresponded with the growth of the
main stand of Ranunculus. When the maximum size of
the stand was reached in July, sediment deposits
covered over 90% of the riverbed beneath the stand of
Ranunculus. The die-back of Ranunculus from July to
September caused an associated reduction in the spatial
area of sediment beneath the plant. At Maiden Newton
little change occurred in the aerial coverage of the main
stand of Ranunculus and the underlying spatial coverage
of fine sediment deposits from April to August. When
the stand began to die-back in September, a large
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J.A. Cotton et al. / Geomorphology 77 (2006) 320–334
Fig. 7. Changes in the spatial coverage of the main study stand and underlying fine sediment (b1 mm). River Frome at Maiden Newton and
Pallington: April to September 2003.
proportion of it was still underlain by deposits of fine
sediment.
Data on the extent and depths of fine sediment
beneath the two stands of Ranunculus, selected for
detailed study, allowed the calculation of the volume of
accumulated sediment per month at each site (Fig. 8).
The data are cubic metres of sediment per area of stand
of Ranunculus and, therefore, can be compared between
months irrespective of changes to the size of the stand of
Ranunculus studied. At Maiden Newton relative
increases in the accumulated sediment occurred between
April and May and between June and July 2003. The
volume of accumulated sediment then declined through
to October with a further small rise in November. At
Pallington, the volume of accumulated sediment per
square metre of Ranunculus increased from May to a
peak in July and then declined through to September.
The volumes of accumulated sediment at Pallington
were much lower than those recorded at Maiden Newton
throughout the whole growing season. The maximum
areal extents of the stands of Ranunculus occurred in
June at Maiden Newton and in July at Pallington.
4.4. Characteristics of accumulated sediments beneath
Ranunculus
Fig. 8. Changes in volume of fine sediment per metre square of
Ranunculus during the lifespan of the main stand.
Seasonal changes (through 2003) in the absolute
particle sizes of fine sediment deposited beneath the
J.A. Cotton et al. / Geomorphology 77 (2006) 320–334
329
Fig. 9. Seasonal changes to the absolute particle size of sediment trapped beneath the main stand of Ranunculus.
main stands of Ranunculus at Maiden Newton and
Pallington are shown in Fig. 9. At both sites the fine
sediment was dominated by the sand fraction (N 63 μm);
silts and clays were less than 10% of the volume. At
Maiden Newton a greater proportion of fine sand
(125–250 μm) occurred whilst at Pallington the
trapped sediments were dominated by medium sand
(250–500 μm). At Maiden Newton a very slight
coarsening of the sediments occurred from March to
September followed by increases in the very fine sand
and silt (4–125 μm) fractions in October and
November 2003. At Pallington, little change in the
distribution of particle sizes occurred from March to
July after which increases in the medium sand fraction
were recorded.
The values for organic matter (mg g− 1 dry weight) in
Fig. 10, represent the monthly average of six samples
extracted from sediment deposited beneath the same
stand of Ranunculus at the two study sites. At Maiden
Newton, the content of organic matter of the accumulated sediments varied from 56.3 mg g− 1 sediment in
May to 13.6 mg g− 1 sediment in December. At
Pallington a greater variability in organic matter content
was recorded; a maximum value was recorded in April
(105.8 mg g− 1 sediment) whereas the minimum value
occurred in September (9.1 mg g− 1 sediment) following
a steady decline from April onwards.
5. Discussion
5.1. Plant–water interactions at the reach scale
The high in-stream coverage by macrophytes of over
70% recorded at both sites resulted from the extensive
Fig. 10. Contents of organic matter of sediment deposited beneath a single Ranunculus plant at sites in the River Frome catchment during the growing
season.
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growth of Ranunculus and greatly exceeded the summer
peak recorded by Flynn et al. (2002). Dawson (1980)
has observed that the flowering of Ranunculus precedes
the maximum biomass of the plant and flowering
occurred in late May and early June at both sites.
Maximum coverage of in-stream vegetation was
recorded in June at Pallington and July at Maiden
Newton. Although both sites have similar peak values of
cover of Ranunculus, the growth patterns at the reach
scale differed. At Maiden Newton, Ranunculus maintained discrete stands throughout the 2003 growing
season whereas, at Pallington, stands of Ranunculus
merged to create a continuous cover of growth that
extended across the reach.
The different growth patterns of Ranunculus at the
two sites produced different flow patterns at the reach
scale. At Maiden Newton, the mosaic of discrete stands
of Ranunculus created zones of low flow velocities and
deposition of fine sediments within the vegetation
(Sand-Jensen and Pedersen, 1999; Green, 2005) and
channels of accelerated flows in the constricted zones
between the plants. At Pallington, the extensive growth
of Ranunculus created a continuous zone of reduced
flow velocities and deposition of sediment across the
reach. Flow velocity has been identified as a key factor
controlling the growth and die-back of stands of
Ranunculus (Dawson and Newman, 1998; Cranston
and Darby, 2002) and high velocity flows appear to be
necessary to allow Ranunculus to flourish (Holmes,
1999). At Maiden Newton the high velocity flows
around the discrete stands of Ranunculus continued to
promote the healthy growth of existing stands and the
establishment of new stands in clean gravels. We
hypothesise that the operation of this feedback mechanism at Maiden Newton can partly explain the presence
of Ranunculus throughout the year with a minimum
cover value of 28% in February 2004. The overwintering of Ranunculus at Maiden Newton provides a
template for the spatial distribution of stands of
Ranunculus in the subsequent growing season (SandJensen and Mebus, 1996). At Pallington, the Ranunculus plants began to die-back from July 2003 onwards
but, in contrast to Maiden Newton, new Ranunculus
plants did not establish. We speculate that the absence of
Ranunculus regeneration resulted from the extended
area of low flow velocities and lack of clean gravel
substrate. By September the stands of Ranunculus at
Pallington were colonised and shaded out by emergent
vegetation. This emergent vegetation was washed out
during the higher discharges at the end of 2003 and by
January 2004 no in-stream vegetation remained at this
site.
5.2. The impact of in-stream macrophytes on flow
velocities and accumulation of sediment
In-stream macrophytes create physical barriers to
flow and retard flow velocities which induce sediment
deposition (Ladle and Casey, 1971; Sand-Jensen, 1998;
Haury and Aidara, 1999; Madsen et al., 2001; Madsen
and Cedergreen, 2002). Data from Maiden Newton and
Pallington show the effect of in-stream macrophytes on
flow velocities and patterns of deposition of fine
sediment whilst seasonal data provide further insight
into the modification of flows (and associated sediment
accumulation) by macrophytes during a period of
changing vegetation cover. At Maiden Newton the
continual presence of Ranunculus resulted in the
accumulation of fine sediments throughout the growing
season and during the winter months. This contrasts
with the findings from previous research where the
impacts of in-stream macrophytes on patterns of
sediment deposition were constrained within a welldefined spring–summer growing season (Sand-Jensen,
1998; Schulz et al., 2003).
At Pallington the accumulation of fine sediments
occurred when the spatial coverage of Ranunculus was
high. With the die-back of vegetation across the reach
from July onwards, however, constriction to flow was
reduced within the channel. This lowered the average
flow velocities recorded through the un-vegetated
sections of the reach. The die-back of Ranunculus also
exposed deposits of fine sediments that had accumulated
beneath the plants. These exposed deposits, however,
were still present in November because of the lower
flow velocities created by the plant die-back. This is in
contrast to the complete erosion of exposed deposits of
fine sediments following the die-back of macrophytes
observed by Schulz et al. (2003). Any subsequent
erosion and transport of fine sediment retained in the
reach at Pallington would, therefore, be dependent upon
the winter flow regime.
The data from the two sites within the River Frome
catchment demonstrate that spatial and temporal
patterns of macrophyte growth and die-back and
resulting hydrological impacts affect the extent of
retention of fine sediments. Sand-Jensen (1998) has
calculated that macrophytes in Danish eutrophic streams
may retain up to 80% of the sediment transported
downstream. Therefore, the dynamics of in-stream
vegetation and associated hydrological processes at the
reach scale will impact upon the catchment scale
transfer of sediments and nutrients throughout the year.
The differences in flow velocities across the reaches
at Maiden Newton and Pallington, resulting from the
J.A. Cotton et al. / Geomorphology 77 (2006) 320–334
growth and die-back of submerged and emergent
vegetation, create a mosaic of zones of low flow
velocity with associated accumulation of fine sediments
and zones of high flow velocity with associated clean
gravels. This heterogeneous distribution of flows and
fine sediments in time and space leads to increased
diversity of habitats as a result of variations in hydraulic
roughness at the reach scale, as noted by Sand-Jensen
and Pedersen (1999) and Morin et al. (2000). The
individual patches and associated physical processes
created by in-stream macrophytes have been termed
medium scale habitats or mesohabitats (sensu Armitage
and Cannan, 2000; Tickner et al., 2000). These
mesohabitats form through hydrological, geomorphological and ecological interactions and may comprise
distinct biotic communities (Dole-Olivier and Marmonier, 1992; Armitage et al., 1995). The concept of
mesohabitats provides a tool for analysing the complexity of these sub-reach-scale hydrological and
ecological elements. Data from Maiden Newton and
Pallington indicate that such complexity develops
within these vegetated reaches and results in a shifting
mosaic of mesohabitats throughout the growing season.
5.3. Patterns of the accumulation of sediments beneath
submerged macrophytes
During the early stages of Ranunculus growth at
Pallington, sediment accumulated predominantly within
the upstream rooted section of the plant. A similar
pattern was also observed by Sand-Jensen (1998) in his
analysis of sedimentation beneath stands of Ranunculus
in Danish streams. Sand-Jensen attributed the pattern to
turbulence associated with the trailing limbs of the plant
which resulted in erosion of sediments from beneath the
trailing section. At Pallington, however, by May 2003
fine sediment extended beyond the rooted section of the
plant into the trailing section. At Maiden Newton,
accumulated fine sediment was recorded beneath rooted
and trailing sections of the stand of Ranunculus
throughout the growing season. Measurements of
velocity within stands of Ranunculus have suggested
that water deflects around the outside of the plants and
does not enter the canopy further downstream (SandJensen, 1998). If this process occurred around stands of
Ranunculus within the reaches of the River Frome then
the sediment beneath the trailing limbs may not derive
from material dropping out of suspension but may
reflect a slow migration of deposited sediment through
the stand of Ranunculus. More detailed investigations
into the modifications of flow within and around stands
at different stages throughout the growing season are
331
required to elucidate patterns of flow, shear stresses and
patterns of sediment deposition, movement and residence times, through the rooted and trailing limbs of
Ranunculus (Sand-Jensen and Pedersen, 1999).
The volume of accumulated sediment per metre
square of the stands of Ranunculus represents the ability
of each unit area of plant to trap and retain fine sediment.
The volume is, therefore, a function of the trapping
efficiency of the individual plants and the quantity of
sediment passing through each reach. At Maiden
Newton and Pallington, a general trend was observed
of higher volumes of sediment being trapped by a larger
area of plant, suggesting that a stand of Ranunculus of
large spatial area will have a greater ability to trap and
retain fine sediment. Similarly, Horvath (2004) concluded that the retention of sediment was a function of
plant biomass. At Maiden Newton, the sharp rise and
fall in the volume of fine sediment trapped from April to
June, however, did not correspond to changes in the size
of the stand of Ranunculus but instead related to two
high discharge events during May that altered the supply
of sediment. A sharp rise and fall in the volume of
accumulated sediment beneath Ranunculus at Maiden
Newton was also recorded between October and
December 2003 that may relate to high discharge events
at the end of October.
The peak in the volume of accumulated sediment at
Pallington was recorded in July 2003 and corresponded
to the largest extent of the stand of Ranunculus.
Interestingly, the variability in accumulated sediment
observed between April and June 2003 at Maiden
Newton was not recorded downstream at Pallington
despite the site also experiencing high discharges in May
2003. This difference results from the complex dynamics
of fine sediment movement through river systems (see
Walling et al., 2001; Slaymaker, 2003). From research
undertaken in the River Piddle catchment (adjacent to the
River Frome), Walling and Amos (1999) recorded the
storage of fine sediments in the winter and the
subsequent movement of fine sediment as ‘slugs’ during
baseflow conditions. Limited information is available,
however, on the upstream–downstream connectivity
within chalk river systems despite the movement of
sediment at the catchment scale having significant
biochemical implications (Clarke, 2002). An important
point to be considered in future studies of sediment
routing through chalk catchments is the widespread
presence of multi-thread channels in the middle reaches
of chalk rivers (Mainstone, 1999). The middle reaches of
the River Frome divide into a maximum of five
geomorphologically distinct channels (Fig. 2) that are
widely distributed across the width of the floodplain and
332
J.A. Cotton et al. / Geomorphology 77 (2006) 320–334
may only be connected laterally during overbank floods.
The division of channels effects a corresponding division
in stream power, with resultant impacts on the potential
for transport and storage of sediments and nutrients
within the system. This discontinuity, inherent in chalk
river systems, requires further analysis to understand its
geomorphological and ecological implications (c.f.
Poole, 2002) and establish the characteristics of
hydrological connectivity between the headwaters and
middle to lower reaches of the catchments. In the context
of the catchment of the River Frome, the discontinuity
provided in the middle reaches will affect the movement
of sediment slugs from the headwater reach at Maiden
Newton to the middle reach at Pallington.
5.4. Characteristics of accumulated sediments
Mainstone (1999) describes a healthy chalk river as
one characterised by low loads of sediment with fine
organic-rich suspended sediments dominating during
the summer months. But at Maiden Newton and
Pallington, the distribution of particle sizes in the
accumulated fine sediments beneath the stands of
Ranunculus revealed a dominance of particles in the
sand-sized fraction (63–1000 μm). Sand-Jensen (1998)
also reported median particle sizes of 230 to 448 μm for
sediment trapped beneath in-stream macrophytes in
Danish streams. In the catchment of the River Frome,
the relatively small areas of sandstone geology give rise
to sandy soils that may be currently eroding in the
headwaters (Fig. 2). This may contribute to the supply of
sediment, although high intensity agriculture and winter
wheat cultivation, which exposes the thin soils that
typify chalk-dominated catchments, may also be a
contributory factor.
The presence of sand implies that the majority of
sediment trapped by Ranunculus was derived from
saltating fine-grained bed material (Phillips and Walling,
1999). The monthly changes in the volume of fine
sediment that accumulated beneath the stands of
Ranunculus at Pallington and Maiden Newton suggests
a continual movement of sediment into and through the
plants. The high velocities of flow through the reaches,
resulting from accelerated flows around the stands, may
also promote the transport of sand-sized particles. Thus,
the trapping of sand transported as saltation load by instream macrophytes may be a significant process in these
river systems.
The content of organic matter of the accumulated fine
sediments ranged between 9–106 mg g− 1. Sand-Jensen
(1998) recorded values of organic matter of sediment
deposited beneath Ranunculus peltatus Schrank. of 8.5–
102 mg g− 1. Sand-Jensen also found variability in the
contents of organic matter at intra- and inter-stand levels.
This was attributed to the morphology of the stand and
extent of the rooted section of the plant, which
determined the near-bed velocity, the depositional
environment and, hence, the trapping efficiency of
individual plants. Data from both sites within the
Frome catchment also show inter-stand variability in
the content of the organic matter which may relate to the
source and composition of the sediment supply in
addition to the trapping efficiency of the plants.
The content of the organic matter in accumulated
sediments varied seasonally at Pallington with values for
March and April 2003 c. 50% higher than the seasonal
average. The organic component contains autochthonous and allochthonous material. The allochthonous
component of the organic matter is likely to derive from
the suspended sediment load (Sand-Jensen, 1998) and
consists of coarse and fine particulate organic matter and
aggregates, for example faecal pellets (Wotton and
Malmqvist, 2001). The low velocity environments
within the stands of Ranunculus encourage the deposition of suspended sediment and also provide habitat for
benthic algae (Marker et al., 1984) and sites for microbial
activity (Sobczak and Findlay, 2002) which will
contribute to the autochthonous material. The variable
quantities of organic matter in the accumulated sediments, thus, reflect the interactions between physicallybased controls on allochthonous material and the
seasonal changes in algal productivity, microbial activity
and production of faecal pellets. In his review of chalk
river systems, Mainstone (1999) suggested that summer
flows are dominated by particulates from within the
channel (autochthonous material) whereas winter flows
are dominated by inputs from run-off (allochthonous)
and, hence, the latter have a reduced organic component.
Our data suggest that patterns in the Frome catchment are
more complex. The observed inter-stand and seasonal
variability in the organic matter content of the accumulated sediments highlights the need for further research
on the composition and sources of organic matter within
catchments (c.f. Kendall et al., 2001).
6. Conclusions
The findings of this study show how the growth and
die-back of in-stream macrophytes at the reach scale
have a fundamental effect on the dynamics of flow and
sediments of chalk streams. Seasonal changes in
vegetation cover and associated patterns of flow velocity
and accumulation of fine sediments were studied on two
reaches of the River Frome, Dorset (UK) throughout
J.A. Cotton et al. / Geomorphology 77 (2006) 320–334
2003. The reaches at Maiden Newton and Pallington had
extensive growth of in-stream macrophytes dominated
by Ranunculus, with peak cover values exceeding 70%
in July and June, respectively. At both sites lower flow
velocities were recorded within the stands of Ranunculus, typically below 0.1 m s− 1 and higher flow velocities
between the plants (up to 0.8 m s− 1) because of the
constriction of flow. The low flow areas promoted the
deposition of fine sediment within the stands of
Ranunculus with a maximum accumulation of sediment
of 20 cm recorded at Pallington in May, June and July
2003. A high percentage of the aerial extent of
Ranunculus was underlain with fine sediment throughout the growing season and the volume of sediment
accumulated per m2 of Ranunculus increased with the
size of the stand. The quantity of accumulated sediment
was also controlled by changes in the supply of sediment
in addition to the trapping efficiency of the plant.
At Maiden Newton, the growth pattern of Ranunculus
at the reach scale was in the form of discrete stands. The
high flow velocities around these discrete stands
appeared to create conditions for the survival of existing
and the establishment of new stands beyond the main
summer growth period. As a result, the minimum spatial
cover value for Ranunculus was 28% in February 2004.
At Pallington, Ranunculus formed a complete cover in
the centre of the channel because of the merging of
individual stands. This created an extensive area of low
velocity and an absence of clean gravel substrate so that,
in contrast to Maiden Newton, when die-back of
Ranunculus began in July 2003 the conditions may not
have been conducive to the establishment of new plants.
The fine sediments accumulated beneath Ranunculus
at Maiden Newton and Pallington were dominated by the
sand (63 to 1000 μm) implying that the majority of the
accumulated sediment derived from saltating finegrained bed material. The organic matter content of the
accumulated sediments varied seasonally at both sites
and ranged between 9 to 106 mg g− 1. The accumulated
fine sediments were present throughout the year at
Maiden Newton within the low flow areas of the stands
of Ranunculus. Whereas at Pallington, fine sediment was
retained after the die-back of Ranunculus because of the
reduced average velocities of flow across the reach.
Acknowledgements
The authors gratefully acknowledge the award of a
NERC LOCAR Grant (NER/T/S/2001/00932) and the
support of the Frome/Piddle Catchment Service Team
and the NERC LOCAR Data Centre. We would like to
thank Henry Bell, Ángel Ferrero Serrano, Judith
333
Hilhorst, Alison Kemp and Anna Sowter for assistance
with the fieldwork and analysis of samples. We would
also like to thank Mr. J. Aplin, Mr. A. Baggs, Mr. J.
Bowerman, Dr. R. Pearson, Mr. C. Tottle and the
Freshwater Biology Association for allowing access to
the sites. We are also grateful to K.J. Gregory, A.
Sukhodolov and one anonymous reviewer for their
helpful comments on an earlier version of this paper.
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