sediment storage in the shallow hyporheic of lowland vegetated river reaches

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HYDROLOGICAL PROCESSES Hydrol. Process. 23, 2239–2251 (2009) Published online 6 March 2009 in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/hyp.7283 Sediment storage in the shallow hyporheic of lowland vegetated river reaches C. M. Heppell, 1 * G. Wharton, 1 J. A. C. Cotton, 1,3 J. A. B. Bass 2 and S. E. Roberts 1 1 Centre for Aquatic and Terrestrial Environments, Department of Geography, Queen Mary University of London, Mile End Road, London, E1 4NS, United Kingdom 2 Centre for Ecology and Hydrology, Dorset, United Kingdom 3 Environment Agency, Rivers House, 21 Park Square South, Leeds, LS1 2QG, United Kingdom Abstract: Excessive fine sediment deposition on the river channel bed together with colmation of finer sediments within the hyporheic are now linked to the degradation of the aquatic habitats of gravel bed rivers in permeable catchments. Previous studies of chalk rivers (associated with outcrops of calcareous rock) have demonstrated the important role of aquatic vegetation in trapping fine sediment on the river channel bed. This research investigated the spatio-temporal patterns and composition of fine sediment stored in two vegetated river reaches, in the Frome and Piddle catchments, Dorset (UK), with contrasting hydrological regimes, in order to establish the importance of aquatic vegetation in controlling the magnitude and timing of sediment storage in chalk rivers. Monthly mapping of macrophyte and sediment cover at the two sites (Maiden Newton and Snatford Bridge, 2003–2004) revealed a cyclical pattern of sediment storage related to the growth and die-back of aquatic vegetation peaking at 66Ð8 kg m 2 in July 2003 at Maiden Newton, and 23Ð5 kg m 2 in October 2003 at Snatford Bridge. Sediment was stored within gravels and beneath vegetation in the margins and mid-channel locations at both sites. Significantly more sediment was stored beneath vegetation than within gravels. The spatio-temporal pattern of sediment storage at the reach scale and the composition of the stored sediments reflected the growth patterns and functional form (flexibility) of the dominant macrophytes Ranunculus penicillatus subsp. pseudofluitans (water crowfoot) and Rorippa nasturtium aquaticum (watercress). Finally, the paper discusses the implications of reach-scale patterns in sediment storage for contaminant storage. Copyright 2009 John Wiley & Sons, Ltd. KEY WORDS sediment storage; reach; permeable river; aquatic vegetation; hyporheic Received 26 August 2008; Accepted 15 January 2009 INTRODUCTION Recently observed increases in the storage of sediment within the hyporheic zone of groundwater-fed rivers have been ascribed to factors such as land use change (Richards et al., 1993), and increased supply from pas- toral and arable agriculture (Walling and Amos, 1999), exacerbated by reduced river flows from over-abstraction (Bickerton et al., 1993). Low flows result in a reduced capacity for rivers to transport sediment (either as bedload or in suspension) and scour the river bed and hyporheos of any accumulated fine material (Wood and Armitage, 1997). Consequently, surficial fine sediment deposits, up to 20 mm in depth, can smother the gravel bed and also ingress into the gravels, reducing hyporheic exchange (Wood and Armitage, 1997; Packman and Mackay, 2003; Rehg et al., 2005). In this paper, fine sediment is defined as inorganic and organic material <2 mm in diameter after (Owens et al., 2005), comprising particulate organic matter, such as seeds (Gurnell et al., 2007), and aggre- gates and/or flocs of organic and inorganic particles * Correspondence to: C. M. Heppell, Centre for Aquatic and Terrestrial Environments, Department of Geography, Queen Mary University of London, Mile End Road, London, E1 4NS, United Kingdom. E-mail: [email protected] (Droppo, 2001; Droppo et al., 1997) including inverte- brate faecal pellets (Joyce et al., 2007; Wotton and War- ren, 2007). The deposition and storage of fine sediment in chalk streams can occur in any zone of reduced flow veloc- ity and/or physical trapping within the river channel such as pools, backwaters, downstream of man-made obstructions, channel margins and within macrophyte beds (Wood and Armitage, 1997). Sediment storage decreases the intrinsic permeability of the bed substrate and reduces the exchange of groundwater and surface waters within the hyporheic. These altered flows lead to changes in oxygen supply and oxygen demand and the organic matter content of the shallow hyporheic. As fine sediment encompasses particulate organic matter and material <63 µm, such bed storage also influences nutrient and contaminant transfers within the hyporheic (McCarthy and Gale, 2001; Kronvang et al., 2003). Eco- logical problems caused by colmation and the smothering of gravel riverbeds by layers of finer sediments include reductions in invertebrate populations, changes to macro- phyte communities (Clarke and Wharton, 2001), and the reduced health and reproductive impairment of game fish (Acornley and Sear, 1999). Copyright 2009 John Wiley & Sons, Ltd.

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HYDROLOGICAL PROCESSESHydrol. Process. 23, 2239–2251 (2009)Published online 6 March 2009 in Wiley InterScience(www.interscience.wiley.com) DOI: 10.1002/hyp.7283

Sediment storage in the shallow hyporheic of lowlandvegetated river reaches

C. M. Heppell,1* G. Wharton,1 J. A. C. Cotton,1,3 J. A. B. Bass2 and S. E. Roberts1

1 Centre for Aquatic and Terrestrial Environments, Department of Geography, Queen Mary University of London, Mile End Road, London, E1 4NS,United Kingdom

2 Centre for Ecology and Hydrology, Dorset, United Kingdom3 Environment Agency, Rivers House, 21 Park Square South, Leeds, LS1 2QG, United Kingdom

Abstract:

Excessive fine sediment deposition on the river channel bed together with colmation of finer sediments within the hyporheic arenow linked to the degradation of the aquatic habitats of gravel bed rivers in permeable catchments. Previous studies of chalkrivers (associated with outcrops of calcareous rock) have demonstrated the important role of aquatic vegetation in trapping finesediment on the river channel bed. This research investigated the spatio-temporal patterns and composition of fine sedimentstored in two vegetated river reaches, in the Frome and Piddle catchments, Dorset (UK), with contrasting hydrological regimes,in order to establish the importance of aquatic vegetation in controlling the magnitude and timing of sediment storage in chalkrivers.

Monthly mapping of macrophyte and sediment cover at the two sites (Maiden Newton and Snatford Bridge, 2003–2004)revealed a cyclical pattern of sediment storage related to the growth and die-back of aquatic vegetation peaking at 66Ð8 kg m�2

in July 2003 at Maiden Newton, and 23Ð5 kg m�2 in October 2003 at Snatford Bridge. Sediment was stored within gravelsand beneath vegetation in the margins and mid-channel locations at both sites. Significantly more sediment was stored beneathvegetation than within gravels. The spatio-temporal pattern of sediment storage at the reach scale and the composition ofthe stored sediments reflected the growth patterns and functional form (flexibility) of the dominant macrophytes Ranunculuspenicillatus subsp. pseudofluitans (water crowfoot) and Rorippa nasturtium aquaticum (watercress). Finally, the paper discussesthe implications of reach-scale patterns in sediment storage for contaminant storage. Copyright 2009 John Wiley & Sons,Ltd.

KEY WORDS sediment storage; reach; permeable river; aquatic vegetation; hyporheic

Received 26 August 2008; Accepted 15 January 2009

INTRODUCTION

Recently observed increases in the storage of sedimentwithin the hyporheic zone of groundwater-fed rivershave been ascribed to factors such as land use change(Richards et al., 1993), and increased supply from pas-toral and arable agriculture (Walling and Amos, 1999),exacerbated by reduced river flows from over-abstraction(Bickerton et al., 1993). Low flows result in a reducedcapacity for rivers to transport sediment (either as bedloador in suspension) and scour the river bed and hyporheosof any accumulated fine material (Wood and Armitage,1997). Consequently, surficial fine sediment deposits, upto 20 mm in depth, can smother the gravel bed and alsoingress into the gravels, reducing hyporheic exchange(Wood and Armitage, 1997; Packman and Mackay, 2003;Rehg et al., 2005). In this paper, fine sediment is definedas inorganic and organic material <2 mm in diameterafter (Owens et al., 2005), comprising particulate organicmatter, such as seeds (Gurnell et al., 2007), and aggre-gates and/or flocs of organic and inorganic particles

* Correspondence to: C. M. Heppell, Centre for Aquatic and TerrestrialEnvironments, Department of Geography, Queen Mary University ofLondon, Mile End Road, London, E1 4NS, United Kingdom.E-mail: [email protected]

(Droppo, 2001; Droppo et al., 1997) including inverte-brate faecal pellets (Joyce et al., 2007; Wotton and War-ren, 2007).

The deposition and storage of fine sediment in chalkstreams can occur in any zone of reduced flow veloc-ity and/or physical trapping within the river channelsuch as pools, backwaters, downstream of man-madeobstructions, channel margins and within macrophytebeds (Wood and Armitage, 1997). Sediment storagedecreases the intrinsic permeability of the bed substrateand reduces the exchange of groundwater and surfacewaters within the hyporheic. These altered flows leadto changes in oxygen supply and oxygen demand andthe organic matter content of the shallow hyporheic.As fine sediment encompasses particulate organic matterand material <63 µm, such bed storage also influencesnutrient and contaminant transfers within the hyporheic(McCarthy and Gale, 2001; Kronvang et al., 2003). Eco-logical problems caused by colmation and the smotheringof gravel riverbeds by layers of finer sediments includereductions in invertebrate populations, changes to macro-phyte communities (Clarke and Wharton, 2001), and thereduced health and reproductive impairment of game fish(Acornley and Sear, 1999).

Copyright 2009 John Wiley & Sons, Ltd.

2240 C. M. HEPPELL ET AL.

As a consequence of their stable flow regimes, inten-sive abstraction for public and agricultural water supplyand extensive growth of aquatic vegetation, chalk riversare fluvial environments that are particularly vulnerableto problems associated with the storage of fine sedi-ment (Wheater et al., 2007). These rivers support valu-able aquatic and riparian ecosystems, so are recognizedas a priority habitat for protection under the UK Biodi-versity Action Plan (Mainstone and Parr, 2002; Wrightet al., 2002). At peak biomass their characteristic standsof aquatic plants can occupy a significant proportion ofthe river channel providing important habitats for inver-tebrates and fish, and playing a fundamental role in flowand sediment dynamics (Hearne and Armitage, 1993;Sand-Jensen, 1998; Madsen et al., 2001; Clarke, 2002).

Whilst the absolute quantity of sediment stored withina reach is a function of the rate of supply of sediment tothe reaches through the relationship between discharge,suspended sediment concentration and transport by bed-load, Gurnell et al. (2006) suggest that the relative quan-tities of sediment stored within a vegetated reach can becontrolled by plant species arrangement and biomass. Ina detailed review of the literature, Corenblit et al. (2007)emphasize the complex mutual interactions and feedbacksbetween vegetation structure, water flow and sedimentdynamics noting that our ability to predict flows, andconsequently sediment deposition, in vegetated reachesis as yet limited. The flexibility of vegetation is thoughtto be of key importance to flow resistance and sedimenttrapping. Corenblit et al. (2007) distinguish between rigid(or stiff) and flexible vegetation, with the former becom-ing more resistant to flow with increased water depth andvelocity, whilst flexible vegetation is associated with lessfriction when fully submerged and re-alignment of leavesmay further reduce flow resistance (see also Sand-Jensen,2003). Green (2005) similarly relates the functional formof aquatic vegetation to its hydraulic effects and clas-sifies aquatic plants into four groups: emergents, sur-face floating leaves, submergents and free-floating plants.Additionally, the location of plants in the channel (mid-channel or margin) and the spatial arrangement of standsdetermine the composite resistance effect of aquatic veg-etation (Green, 2005) thus controlling the spatio-temporalpatterns of sediment storage in the hyporheic of vegetatedrivers.

Whilst a number of studies have quantified sedimentstorage within the channel of permeable rivers, includ-ing the Frome and Piddle (see for example Walling andAmos, 1999; Collins et al., 2005; Collins and Walling,2007a,b,c), little is known about the significance of stor-age beneath plants. However, recent research has focusedon the composition and volume of fine sediment trappedby submerged plants, specifically Ranunculus spp. (Cot-ton et al., 2006; Wharton et al., 2006) and the reach-scale distribution of fine sediments associated with thechanging spatial patterns of aquatic plants and flow veloc-ities (Gurnell et al., 2006). There is also a need toassess (i) the spatial distribution, timing and durationof sediment storage beneath aquatic vegetation within

the river channel and (ii) the implications of accumu-lation and release of sediment beneath dynamic vege-tation structures for sediment conveyance and contam-inant storage. Therefore, this study aimed to establish(i) the magnitude and timing of bed sediment storage inreaches of two vegetated chalk rivers with contrastingflow regimes, (ii) seasonal and spatial variation in com-position of the bed sediment within the selected reachesand (iii) the consequences of intra-reach variation in sed-iment composition for contaminant sorption to the bedsediments.

METHODS

Site selection and description

The paired, chalk Frome and Piddle catchmentswere instrumented as part of the Natural Environ-ment Research Council (NERC) LOwland CAtchmentResearch (LOCAR) Thematic programme (Wheater andPeach, 2004; http://catchments.nerc.ac.uk/projects/nutrients/) which aimed to improve understanding ofthe hydro-ecological response of groundwater-dominatedlowland catchments. In the context of this research, theprogramme instrumentation provided valuable baselinedata with which to study the interactions between flow,vegetation and sediment storage in a lowland, permeablecatchment and two reaches (henceforth called SnatfordBridge and Maiden Newton) were selected adjacent toLOCAR infrastructure (Figure 1).

Snatford Bridge is a 40 m (6Ð7 m wide) straightreach located on the Bere Stream approximately 1Ð75 kmupstream from the confluence with the River Piddle.The channel has low banks with a plant communitythat is typical of the mid-reaches of a chalk stream interms of species composition and diversity (S. Clarke,personal communication 2004). Ranunculus penicilla-tus subsp. pseudofluitans (water crowfoot) and Ror-ippa nasturtium aquaticum (watercress) are the domi-nant species, with Ranunculus providing the majorityof cover within the channel for much of the year. Ror-ippa is dominant in the margins, and encroaches into theriver channel from mid to late summer. Maiden New-ton on the River Frome (25 m length ð 7 m width) isa shallow, slightly sinuous reach dominated by Ranun-culus penicillatus subsp. pseudofluitans in open areaswith very limited marginal or other vegetation. Thebed of both reaches has a substrate comprising flintand fine to coarse gravels infilled with finer materialcomprising sands, silt and clays. For further detailsof both reaches see Wharton et al. (2006). Thus, thepaired catchment study design provided the opportunityto study the effect of aquatic vegetation on bed sedi-ment storage in two reaches which exhibited the typicallyabundant vegetation of the un-shaded sections of chalkstreams (Mackey et al., 1982; Hearne and Armitage,1993) but with contrasting streamflow regimes. At bothsites, Ranunculus occupied >60% of the channel at peakbiomass.

Copyright 2009 John Wiley & Sons, Ltd. Hydrol. Process. 23, 2239–2251 (2009)DOI: 10.1002/hyp

SEDIMENT STORAGE IN THE SHALLOW HYPORHEIC OF LOWLAND VEGETATED RIVER REACHES 2241

Figure 1. Location of study sites and geology of the Frome and Piddle catchments, Dorset (UK)

Discharge and suspended sediment concentrationmeasurements

The two reaches selected for study were located nextto LOCAR infrastructure sites where 15-min measure-ments of discharge were taken using calibrated StarflowUltrasonic Doppler system (Unisense) throughout thestudy. Mean daily discharge was calculated for each reachusing the 15-min data. Self-cleaning McVan InstrumentsAnalite 195/4/30 turbidity sensors linked to dataloggerswere also installed at each site. Due to instrument driftcontinuous records of turbidity for these sites are onlyavailable from January 2004. Sensor lenses were auto-matically wiped every 6 h to remove biofilm growth.Voltage output from the probes was converted to Neph-elometric Turbidity Units (NTU) using a site-specificrelationship derived by field calibration using polymerstandards. Suspended sediment concentrations were cal-culated using ambient water samples collected manuallyduring site visits (n D 21 at each site).

Calculating percentage of vegetation and sediment cover

Vegetation cover was mapped at the reach scaleeach month from March 2003 to November 2004 tocalculate percentage of vegetation cover of submergedand emergent vegetation for the reach. The aerial extentof sediment deposited beneath the vegetation was clearlyvisible, and mapped at each reach to calculate the

percentage of vegetation underlain by surficial sediment.Mapping was conducted using fixed reference points,surveying individual plant stands using a total station anda long profile collage of digital photographs.

Estimating bed sediment storage overlying vegetatedgravels

In order to quantify the magnitude and timing of bedsediment storage beneath vegetation, sediment depthsand macrophyte presence or absence were recordedon a monthly basis at 0Ð5 m intervals across a fixedtransect in each reach (March until December 2003). Thetransects passed through a Ranunculus stand selected fordetailed investigation. Six sediment depth measurementswere established within the selected Ranunculus standcomprising one from beneath the trailing fronds ofRanunculus and five from the rooted section. The locationof the sediment depth measurement within the standwas selected using random number tables. A singlesediment sample was also collected from each locationusing a small Perspex cylinder (3Ð7 cm diameter) andthe top 0–1 cm of sediment was collected for sedimentcharacterization (organic matter content and particle sizeanalysis).

The transect and main stand measurements of sedimentdepth (n D 10 to 17 in total) were then combined tocalculate the average sediment depth beneath vegetationin the reach on the sampling date. Average sediment

Copyright 2009 John Wiley & Sons, Ltd. Hydrol. Process. 23, 2239–2251 (2009)DOI: 10.1002/hyp

2242 C. M. HEPPELL ET AL.

depth was then multiplied by the percentage of vegetationcover and the percentage of vegetation underlain bysediment to calculate the total volume of sedimentbeneath vegetation in the reach. Total volume was thenconverted to weight of sediment by multiplying by theaverage sediment bulk density. Finally, the total weightof sediment beneath vegetation in the reach was dividedby the total area of the reach in order to report the weightof sediment stored beneath vegetation per metre squareof reach.

During the second year of fieldwork, from April toNovember 2004, a grid of ten transects was set up acrossthe entire reach so that both mid-channel and marginalsediment could be monitored separately. The transectswere 2 m apart at Snatford Bridge (Bere Stream) and1Ð5 m apart at Maiden Newton (Rive Frome). Waterdepth and fine sediment depth above the gravel bedwere recorded at 0Ð5 m intervals along each transect. Thespecies of vegetation and its depth within the water col-umn were also recorded at each point along the transect.The transect data were input into the three-dimensionalmapping software ‘Surfer’, and a triangulation algorithmwas used to extrapolate between points within the transectgrid, and calculate the volume of sediment in the reach.Total volume was then converted to weight of sedimentstored beneath vegetation per metre square of reach asdescribed above.

Five sediment samples were collected from beneathsubmerged Ranunculus cover at each site, with the exactlocation of the samples selected using a random num-ber table. At Snatford Bridge three additional marginalsediment samples were collected from beneath stands ofRorippa, which had been observed to dominate vege-tation cover at the site by the end of the first summerof fieldwork. Again, the exact location of these sampleswithin the marginal zone was determined using a randomnumber table. As before, samples were collected for sedi-ment characterization using a small Perspex core (3Ð7 cmdiameter) and the uppermost 0–1 cm of sediment wasretained for analysis.

Sediment bulk density measurement

Thirty-eight cores of known volume were taken frombeneath vegetated sediment patches at Maiden Newtonand Bere Stream throughout 2004 and the sediment wasdried and weighed in order to calculate average sedi-ment bulk density. The bulk density of river sedimentsis highly variable reflecting water content and degreeof compaction (Jepsen et al., 1997). There was no sig-nificant difference between the average bulk density atBere Stream or Maiden Newton so the total averagebulk density (kg m�3) of vegetated sediment during2004 was used in all calculations; 1091 kg m�3 (S.E.C/� 99). The relatively low bulk densities reported herereflect high water and organic matter content with littlecompaction.

Estimating bed sediment storage within and overlyingun-vegetated gravels

From April till November 2004 the re-suspensiontechnique developed by Lambert and Walling (1988)was used to determine the amount of sediment storedin un-vegetated gravels. A large metal cylinder (1 mheight with surface area of 0Ð16 m2) was pushed intothe gravel to 5 cm depth to create a seal. The water,gravel and finer sediment within the cylinder werethen stirred vigorously with a metal rod to remobilizematerial into the water column. Representative samplesof re-mobilized sediment were collected immediatelyfollowing agitation in 1-l bottles. On return to thelaboratory the sediment was determined gravimetricallyby vacuum filtration using 0Ð45 µm pore Whatman GF/Cglass fibre filter papers. Particle size analysis of selectedsamples indicated that material up to 2 mm was collectedusing this re-suspension technique. Sediment storageat the time of sampling was then calculated usingEquation (1) in Collins and Walling (2007c). On thebasis of evidence from the literature, Collins and Walling(2007c) suggest that whilst ingress depth is highlyspatially variable, sediment infiltrates gravel beds to anaverage depth of 10 cm. So, in order to estimate thesediment storage in the gravels the weight of sedimentreleased per unit surface area of the channel bed to 5 cmdepth (g cm�2) was doubled. This assumes even storageof sediment in the gravels with depth.

Although the technique has been proven to be reliableand reproducible for a range of un-vegetated gravelenvironments (Walling et al., 1998; Collins et al., 2005),vigorous stirring within the cylinder disturbs and cutsthrough the roots and stems of vegetation. This cylindertechnique is not suitable for repeated measurementswithin vegetated patches. So, to determine the quantityof sediment stored in vegetated zones small diameterPerspex cores were used (as described previously) orderto ensure minimal disturbance to the vegetated sedimentoverlying the gravels.

Composition of bed sediment

Effective particle size distribution of suspended sed-iment is best measured in situ using laser techniquesto avoid changes to flocs and/or aggregate sizes dur-ing measurement or storage (Phillips and Walling, 1995).Measurement of effective particle size of bed sedimentby such in situ techniques is more challenging, how-ever, because requiring sediment to be in suspensionnecessitates analysis during deposition or immediatelyon re-suspension. Due to the operationally defined natureof particle size, Phillips and Walling (1999) recommendthat the same technique be used for both absolute andeffective particle size distribution. Our approach was totransport intact cores of bed sediment samples (togetherwith pore water) back to the laboratory to minimizeany disturbance. Once in the laboratory, measurement ofeffective particle size distribution could be undertaken bysub-sampling the top 0–1 cm of the intact core, and tak-ing measurements immediately on re-suspension of the

Copyright 2009 John Wiley & Sons, Ltd. Hydrol. Process. 23, 2239–2251 (2009)DOI: 10.1002/hyp

SEDIMENT STORAGE IN THE SHALLOW HYPORHEIC OF LOWLAND VEGETATED RIVER REACHES 2243

sediment within the particle size analyser. Accordingly,wet sediment samples were brought back to the labo-ratory in their pore water to minimize any disturbanceto settled flocs. The samples were kept cool overnight(4 °C), and analysed within 24 h for effective particlesize distribution using a Beckman Coulter LS 13 320Laser Diffraction Particle Size Analyser (with polarisa-tion intensity differential scattering).

Samples (0–1 cm depth) for organic matter and abso-lute particle size analysis were dried at 105 °C until a con-stant weight was reached. The dried sediment was thendivided into two sub-samples. Organic matter content wasdetermined on the first sub-sample by loss-on-ignition(LOI). The second sub-sample was oxidized with hydro-gen peroxide, sieved and then dis-aggregated in Calgonprior to determination of the absolute particle size ofthe <2 mm fraction using the Laser Diffraction ParticleSize Analyser. The volume of size fractions from 2 mmto 0Ð375 µm was recorded. Thus, analyses of effectiveparticle size include fine particulate organic matter andaggregates and/or flocs of inorganic and organic material,whereas for determination of absolute particle size onlydis-aggregated mineral material was analysed. Particlesize is reported in terms of median absolute and effectivegrain size (d50) of particles with diameter <2 mm, andthe percentage mineral material of a size fraction <63 µmin diameter is referred to as the silt-clay fraction.

Quantification of herbicide sorption to bed sediments

Herbicide sediment-water partition coefficients werequantified using sediment from marginal and mid-channellocations in the two reaches in order to assess whetherintra-reach variation in sediment composition might haveany implications for reach-scale contaminant storage.Mid-channel sediment was collected from beneath standsof Ranunculus. Marginal sediment was collected fromun-vegetated channel margins at Maiden Newton andmarginal vegetated sediments at Snatford Bridge. Ineach case composite samples of the upper 1 cm of bedsediment were collected using a Perspex corer.

The investigation focused on the s-triazine herbicides,atrazine and simazine and the phenylurea herbicide, iso-proturon, as model organic contaminants whose bindingto sediment is chiefly controlled by partitioning and sorp-tion mechanisms (Chefetz et al., 2004). The sediment-water partition coefficient (Kd) is the measure of thedistribution of a contaminant between sediment and water(Katagi, 2006). As the organic matter content of sedi-ment is an important control on sorptive properties, thesediment-water partition coefficient is often normalized toKoc (Koc D Kd/foc where foc is the fraction of organiccarbon in the sediment) to describe the extent of sorption.All sorption experiments to establish herbicide sediment-water partition coefficients were carried out according tothe methodology described in OECD (2000). Sedimentswere shaken for 24 h with water spiked with the appro-priate herbicide, and then centrifuged. Sample extractswere analysed by direct injection of 90 µl of the aqueous

supernatant with 10 µl of deuterated atrazine-d5 inter-nal standard into an Agilent LC-MSn with atmosphericpressure chemical ionization (APCI) source. Liquid chro-matographic (LC) separation of the study pesticides wascarried out using a Supelcosil ABZC 25 cm ð 4Ð6 mmi.d. LC column (Supelco, Belfonte, USA). Isocratic elu-tion was performed using acetonitrile (solvent A) andwater (solvent B) at 35 : 65 A : B, modified with 0Ð1%formic acid. Detection was achieved by MSn operatingin full scan from 100 to 300 m/z. The ion trap was oper-ated in positive ionization mode, isolating the pseudo-molecular [M C H] ion with multiple reaction monitoring(MRM) between the precursor and product ion. Preci-sion, reported as relative standard deviation, for eachcompound based upon repeat injections of a 100 ng l�1

standard was 2Ð4% for atrazine, 1Ð4% for simazine and2Ð5% for isoproturon.

RESULTS AND DISCUSSIONTemporal variations in the magnitude of sedimentstorage

The streamflow response of the second site at MaidenNewton on the River Frome reflects the influence ofsuperficial Jurassic clay deposits with poor permeabilityin the headwaters of the catchment (Figure 1) such thatthe groundwater regime is superimposed by individualstormflow hydrographs (Figure 2a). Median and maxi-mum suspended sediment concentrations recorded duringthe monthly fieldwork were 3 and 57 mg l�1 respectivelywith marked elevation of suspended sediment concentra-tions associated with stormflow. This pattern of changingsuspended sediment concentration indicates that sedimentgeneration in the River Frome reflects the catchmentresponse to rainfall with sediment delivery from a com-bination of surrounding catchment slopes via overlandprocesses combined with entrainment of material storedwithin the channel. Collins and Walling (2006) reportthat suspended sediment from tributaries dominated thesuspended sediment flux at the Frome catchment outletbetween 1st October 2003 and 28th January 2004, whilstbed sediment re-mobilization and eroded channel banksediment comprised approximately 25–28% and 5–23%of the suspended sediment flux respectively depending onthe sampling period. Storm-period suspended sedimentconcentrations in the River Frome are typical of thosemeasured in other UK chalk stream systems, such as theHampshire Avon being an order of magnitude lower thanmany other UK lowland river systems (Heywood andWalling, 2003).

Snatford Bridge on the Bere Stream, a tributary of theRiver Piddle (Figure 1), is predominantly groundwater-fed from the underlying Tertiary chalk aquifer, witha stable streamflow response characteristic of perme-able catchments with minimal stormflow component(Figure 2b). The median suspended sediment concentra-tion of 6 mg l�1 recorded during the study was higherthan that at Maiden Newton, and concentrations fluc-tuated on a daily basis, but the maximum suspended

Copyright 2009 John Wiley & Sons, Ltd. Hydrol. Process. 23, 2239–2251 (2009)DOI: 10.1002/hyp

2244 C. M. HEPPELL ET AL.

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sediment concentration was lower (13 mg l�1). Maxi-mum concentrations occurred during spring and earlysummer corresponding with algal growth (Marker et al.,1986) and so the observed patterns in suspended sed-iment concentration may reflect the importance of theautochthonous production of organic matter.

At both sites, average discharge was slightly elevatedduring the second year of study compared to the first(0Ð46 m3 s�1 compared to 0Ð42 m3 s�1 at Maiden New-ton, and 0Ð37 m3 s�1 compared to 0Ð35 m3 s�1 at Snat-ford Bridge for March to October 2004 compared toMarch to October 2003 respectively). At Maiden New-ton the number of days on which mean daily dischargeexceeded 1 m3 s�1 increased from 6 days during the firstyear of study (March to October 2003) to 12 days duringthe second year (March to October 2004). Mean dailydischarge at Snatford Bridge did not exceed 0Ð9 m3 s�1

during the entire study period.During the study, greater quantities of sediment (in

terms of kg m�2) were stored at Maiden Newton com-pared to Snatford Bridge (Figures 3a and 3b). Sedimentstorage at Maiden Newton ranged from 11Ð6 kg m�2 inApril 2003 to a maximum of 66Ð8 kg m�2 in July 2003,whilst storage at Snatford Bridge ranged from a mini-mum of 0Ð9 kg m�2 in February 2004 to 23Ð5 kg m�2 inOctober 2003. Higher quantities of stored sediment were

recorded at both sites during the first year of study (2003)in comparison to the second (2004). This was despitethere being little difference in total vegetation cover overthe two growth cycles at each site, and a higher aver-age discharge and storminess in the second year of studyin comparison to the first. The higher sediment storagevalues observed at Maiden Newton may be explainedby the higher flow discharges which increased the sup-ply of suspended sediments (as reflected in the highersuspended sediment concentrations associated with stormevents) and bed material load which in turn were trappedand retained by aquatic vegetation. But the differences insediment storage between 2003 and 2004 at both sitespoint to complexities in the relationship between dis-charge and sediment storage in vegetated river reaches.Higher flows may be associated with elevated sedimentstorage (through increased supply) if plants can with-stand the increased discharges and are not washed out.However, if thresholds are exceeded, which may arisefrom increases in absolute flow magnitudes or particu-lar sequences of high flow events, scouring of vegetationand/or sediment deposits will occur causing reduced stor-age at the reach scale.

The pattern of sediment storage within the reachesreflects the seasonality of the growth and die-back ofvegetation with maximum storage of sediment duringperiods of greatest vegetation cover (Figures 3a and 3b).

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At Maiden Newton, the peak in total vegetation covercorresponds to maximum cover by the submerged in-channel macrophyte Ranunculus in May/June and July,whereas at Snatford Bridge peak vegetation cover, arisingfrom a combination of Ranunculus and the emergentmacrophyte Rorripa, occurs later in the summer fromAugust through to October.

Accumulation of fine sediments within aquatic plantsoccurs due to the reduced flow velocities causing particlesto fall out of suspension and the incorporation of bedmaterial into stands due to the physical barrier effectof the plant. Cotton et al. (2006) described how finesediments accumulated in the upstream rooted sectionof Ranunculus plants during the early stages of growthbut then extended into the downstream trailing section ofthe plant later in the growing season. The authors alsoobserved how the structural form of Ranunculus appearedto control the spatial pattern of fine sediment deposits.

Significantly, in our study, measurable sediment stor-age occurred beneath vegetation throughout the year at

both Maiden Newton and Snatford Bridge (Figures 3aand 3b). Although there was some loss of stored sedi-ment in the autumn as the percentage of vegetation coverdeclined, the results clearly show that mid-channel andmarginal vegetation persisted in both reaches and retainedsignificant quantities of sediment through the period ofsenescence and die-back over the autumn and wintermonths. The protection afforded by aquatic vegetationto fine sediment deposits through the autumn and wintermonths when flows increase has important implicationsfor the storage of sediments beyond the annual time-scale.

Data from Maiden Newton also illustrate the influenceof storm events on sediment storage within a vegetatedreach. There are three sampling periods during which netsediment loss occurred from the reach during periods ofactive vegetation growth, and these can all be attributed tostorm events. An event on 3rd May 2003 with maximumdaily discharge of 1Ð53 m3 s�1 resulted in a net lossof sediment between sampling on 30th April and 4thJune 2003 (marked (A) on Figure 2a) during a period

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2246 C. M. HEPPELL ET AL.

of Ranunculus growth. Three storm events at the end ofJuly 2003 (marked (B) on Figure 2a) may account for thenet loss of sediment observed between 2nd July and 13thAugust 2003.

An event with a max daily discharge of 0Ð71 m3 s�1 on24th June 2004 contributed to net sediment loss between10th June and 5th July 2004 (marked (C) on Figure 2a).Data from the second year of fieldwork (2004) enablea distinction to be made between sediment stored andlost from (i) marginal zones, (ii) from beneath mid-channel vegetation and (iii) from gravels within eachreach (Figure 3a). Between 10th June and 5th July 2004,53% of the total sediment previously stored within thereach was washed downstream with twice as muchsediment lost from the un-vegetated marginal zones asremoved from the vegetated channel. Over the sameperiod there was a doubling of the quantity of sedimentstored within the gravels. Sediment loss from withinthe channel was largely due to removal of protectiveRanunculus cover. During the first year of fieldwork,characterized by low flows with few storm events, therewas a significant linear correlation between vegetationcover and sediment storage (r D 0Ð835, p < 0Ð001, n D10). Expansion and contraction of vegetation coverexplains up to 70% of the changes in sediment storageat Maiden Newton during this period (R2 D 0Ð698, p <0Ð003, F D 18Ð45). During the second year, althoughpeak vegetation cover is comparable to the first yearof study, sediment storage is not significantly correlatedwith vegetation (r D 0Ð297, p < 0Ð238, n D 8) whichcould be related to the higher discharge and increasedstorminess from March until October 2004.

At Snatford Bridge, where the stream is character-ized by a more stable flow regime, there is a significantcorrelation between variations in vegetation cover andsediment storage (r D 0Ð616, p < 0Ð05 for the first sea-son and r D 0Ð859, p < 0Ð003 for the second season).Here, however, the relationship is complicated by theencroachment of Rorippa into the main channel. Fromlate winter through to spring 2004 the reach at SnatfordBridge was dominated by Ranunculus. In May/June Ror-ippa growth began to accelerate and encroach into themid-channel from the margins (Figures 4a and 4b; andFigure 4c for a comparison with Maiden Newton) reach-ing a peak in September at a time when Ranunculus isdeclining. This peak in Rorippa cover corresponds to themaximum sediment storage values recorded for the reacharising from increases in sediment stored both within themargins and mid-channel areas (Figure 3b). The increasein sediment accumulated within the reach from July istherefore related to the growth of Rorippa into the mid-channel from the margins, and highlights the importanceof macrophyte species composition in controlling sedi-ment storage at the reach scale.

Spatial variations in sediment storage at the reach scale

At Maiden Newton the relative proportions of sedi-ment in the different storage zones, with the exception of

June 2004, did not vary greatly throughout the year. Mid-channel storage by Ranunculus accounted for 60–70%of total sediment storage in the reach whilst un-vegetatedmarginal areas stored 20–30% of total sediment. Sed-iment storage within the gravels generally comprised<10% of the total weight of sediment stored within thereach throughout the study period (April to November2004). However, no measurements were taken during thewinter months when storage within un-vegetated gravelsis likely to increase substantially in relative amount andimportance.

The distribution of sediment between the margins andmid-channel varied during the summer at Snatford Bridgedue to the growth patterns of Ranunculus and Rorippadescribed above. The channel margins were the mainareas of sediment storage in the spring accounting for50–65% of total storage in the reach. Once Rorippagrew into the main channel from July, then sedimentstorage in the mid-channel rose rapidly accounting for60% of total storage from August until the end offieldwork in November 2004. Storage in un-vegetatedgravels steadily declined throughout spring accountingfor 20% of total sediment storage in April but only 7%by July 2004.

Gurnell et al. (2006) describe how the interactionof submerged and emergent macrophytes at the reachscale controls flow patterns and the retention of finesediment. At locations in the River Frome catchment,they observed how threads of higher flow velocities aredeflected towards the margins as aquatic plants growin the main channel. Their findings indicate that theseflows supply fine sediments to the margins and explainthe higher average fine sediment deposition and higherplant propagule counts (see Gurnell et al., 2007) recordedaround emergents such as Sparganium erectum comparedto submerged plants such as Ranunculus penicillatussubsp. pseudofluitans. On the Bere Stream at SnatfordBridge we also observed that extensive growth of Ranun-culus plants created high velocity flow paths betweenstands including towards the margins where Rorippa waspresent (Wharton et al., 2006). Interestingly, however,Rorippa also grew from the margins into the middleof the channel from late summer, sometimes colonizingRanunculus plants. In this mid-channel position Rorippais exposed to faster flows and increased supply of finesediment and its more rigid structure gives it a hydraulicadvantage over the more flexible Ranunculus plants intrapping and retaining fine sediment. This explains howthe middle of the channel at Snatford Bridge is trans-formed into a zone of increased sediment storage fromlate summer to autumn.

Mean fine bed sediment storage in un-vegetated gravelsat Maiden Newton (River Frome) over the study periodwas 1500 g m�2. This is comparable to the mean finebed sediment storage for the River Frome of 918 g m�2

(for the period February 2003 to July 2004) reported byCollins and Walling (2007a), and measured using thesame re-suspension technique developed by Lambert andWalling (1988). Similarly, Collins and Walling (2007a)

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report a mean fine bed sediment storage in the RiverPiddle of 1580 g m�2. Mean fine sediment storage inun-vegetated gravels at Snatford Bridge (Bere Stream)during this study was 1136 g m�2. These results pro-vide additional confirmation of the usefulness of themanual re-suspension technique for investigating chan-nel bed sediment deposition and storage, whilst high-lighting the need to improve estimates of sediment stor-age beneath vegetation which can account for signifi-cant storage in vegetated reaches. Chalk streams typi-cally support abundant stands of aquatic vegetation inthe un-shaded and semi-shaded reaches that account forup to ca 60% of their total length in the UK (Environ-ment Agency, 2004). Cover by vegetation is extensivefor at least 6 mo of the year from May to October

(see for example Flynn et al., 2002) and our resultssuggest that there could be conveyance loss at theannual time-scale during years when vegetation does notfully die-back and is not washed out by higher winterflows, reducing the downstream transport and supply ofsediment.

Temporal and spatial variation in composition of bedsediments

In terms of absolute particle size, sands dominatedthe vegetated mid-channel at both sites, with the veg-etated mid-channel sediments classified as fine sandsand medium sands on the Wentworth Scale at MaidenNewton and Snatford Bridge respectively (Figure 5a).The sediment in the marginal zones was finer grained;

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2248 C. M. HEPPELL ET AL.

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very fine sands dominated the absolute size range atMaiden Newton and silt was found in the margins atSnatford Bridge (Figure 5b). When explaining the pro-cesses resulting in the transport and deposition of dif-ferent absolute size ranges of particles it is importantto consider the size and composition of flocs (or aggre-gates) within which individual mineral particles are beingtransported. The median absolute particle size of the sed-iments beneath mid-channel vegetation at both sites wasgreater than the median effective particle size (Figure 5a).At mid-channel sites the latter is influenced by an abun-dance of fine particulate organic matter and/or flocs andaggregates, which were oxidized during the analysis ofabsolute particle size. In the margins at Maiden New-ton the median absolute and effective particle sizes ofsediment are comparable, however, at Snatford Bridgethe median effective particle size of marginal sedimentexceeds median absolute particle size (Figure 5b) sug-gesting a different aggregate composition of transportedand trapped material.

The proportion of organic matter in the sedimentswas greater in the margins in comparison to themid-channel locations at both sites with the highest

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organic matter content (mean organic matter content D22%) in the sediment beneath the Rorippa at SnatfordBridge (Figures 6a and 6b). The silt-clay fraction of themarginal sediments was also higher at both sites withmuch finer sediments recorded in the margins at Snat-ford Bridge than Maiden Newton (mean percentage ofsilt-clay D ca 75% and 33% respectively).

The large range in median effective and absoluteparticle sizes beneath mid-channel vegetation at Snat-ford Bridge can be explained by the change in vege-tation from Ranunculus to Rorippa over the summerand autumn months. Figure 7a illustrates the tempo-ral variation in silt and clay size particles and organicmatter over the study period. The periods of elevatedSC and organic matter content are associated withencroachment of Rorippa into the mid-channel. Theinset box demonstrates the changing relationship betweeneffective and absolute particle size of sediment duringperiods of vegetation cover by Ranunculus and Ror-ippa respectively. During periods of cover by Rorippathe sediment composition in the mid-channel locationsapproaches that of the margins (Figure 5b and inset boxin Figure 7a)

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In comparison to Snatford Bridge the sediment beneathmid-channel vegetation at Maiden Newton exhibited sim-ilar characteristics throughout the year (Figure 7b). Therewas little seasonal variation in organic matter content ofthe sediment but the silt and clay content was elevated inspring and summer in 2004 in comparison with the pre-vious year, potentially due to increased storminess. Thereis often a strong positive correlation between organicmatter content and the amount of fine-grained particu-lates in sediments so the relationship between organicmatter and the silt-clay fraction of marginal and mid-channel sediment was explored on a month-by-monthbasis. Significant positive correlation between organicmatter and percentage of silt-clay occurred in the un-vegetated marginal zones (r D 0Ð957, p < 0Ð01, n D 12).This relationship was not observed in sediment beneathRanunculus, with the exception of samples taken in July,following a storm event, when a strong positive corre-lation was observed (r D 0Ð961, p < 0Ð01, n D 15). Astrong correlation between organic matter and the silt-clay fraction within the un-vegetated margins suggests

that fine material being trapped within these marginalsediments may be aggregates and flocs comprisingfine-grained material intimately associated and coatedwith organics, typical of the suspended sediment trans-ported during storm events. Sanders et al. (2007) reportedstrong 15N enrichment of N2 in the sediment pore waterbeneath a Ranunculus stand at the same reach over thesame time period. Such a pattern is consistent with theprocessing of organic matter associated with suspendedsediment of terrestrial origin in the sediments. Trappingof suspended sediment from storm events has implica-tions for contaminant storage and biogeochemical cyclingwithin the sediment beneath aquatic plants.

At Snatford Bridge there was no significant correla-tion between organic matter content and silt-clay frac-tion beneath Ranunculus, and only a weak correlationin the marginal sediment beneath Rorippa (r D 0Ð544,p < 0Ð05, n D 14). Nevertheless, data from Collins andWalling (2007a) suggest that the silt-clay fraction is pre-dominantly of agricultural origin with cultivated land andpasture together contributing ca 80% of silt-clay to the

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2250 C. M. HEPPELL ET AL.

channel bed of Bere Stream (at Tanpits Bridge). How-ever, the source of the sand-sized mineral fraction withinthe bed sediment has not been investigated.

Consequences of intra-reach variation in sedimentcomposition for contaminant sorption

The sorption of organic contaminants to organic mat-ter and the silt-clay fraction of sediments is one of thekey processes controlling their transport and fate in thefluvial environment (Karickhoff et al., 1979). The vari-ations in sediment characteristics that we have observedbeneath the different vegetation types in these rivers mayresult in bed sediments with spatially distinct sorptionpotentials for organic contaminants, such as pesticides.The sediment-water partition coefficient (Kd) and theorganic carbon normalized sediment-water partition coef-ficient (Koc) are used to describe the extent of pesticidesorption to sediment. Figure 8 illustrates the Koc of threeherbicides, atrazine, simazine and isoproturon that wereused extensively in the Frome and Piddle catchments dur-ing the study for the control of maize, field beans andwheat/barley respectively. The extent of herbicide sorp-tion to sediment was significantly higher for the marginalsediments at Snatford Bridge in comparison to the mid-channel locations, even when the influence of organicmatter content was removed. These differences may bedue to the influence of contaminant sorption to the siltand clay size material (Figures 6a and 6b) and/or varia-tion in the composition of organic matter in the differ-ent zones. For example, algal content, snail mucus andblackfly silks are all biogenic components of the sedi-ment which have a demonstrable effect on the strengthof contaminant sorption (Brereton et al., 1999; Katagi,2006). In either case, the data indicate that the marginalareas at Snatford Bridge have a greater propensity tosorb these types of moderately polar contaminants. Addi-tionally, the higher proportion of silt-clay sized materialstored in these zones, sourced from cultivated areas ofthe catchment which directly receive herbicide applica-tions, suggests that at Snatford Bridge the stream margins,

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beneath Rorippa, could be zones of elevated contaminantstorage.

CONCLUSIONS

1. This paper highlights the importance of aquatic veg-etation for sediment storage at the reach scale. Themagnitude of sediment stored within the two reachesstudied exceeds that commonly reported in the litera-ture for lowland permeable rivers.

2. The patterns of sediment storage and subsequentrelease were cyclical, reflecting the growth and senes-cence of the vegetation, and a significant quantity ofsediment was retained in the monitored reaches overthe autumn and winter months. Our study, therefore,indicates that the storage of sediments within the shal-low hyporheic of vegetated rivers can occur on time-scales lasting over 1 year.

3. The relative proportions of sediment stored in marginsand mid-channel was controlled by the growth patternand structural characteristics of the dominant macro-phytes. At Snatford Bridge the main area of sedimentstorage changed from the margins to mid-channel fromlate summer due to the colonization of the latter by theemergent macrophyte Rorippa nasturtium aquaticum,which has a more rigid structure than the more flexible,submerged Ranunculus spp. plants.

4. The composition of stored sediments at the reach scaleis also linked to the spatial and temporal patternsof aquatic vegetation. Channel margins, with densegrowth of emergents, store finer sediments with higherorganic matter content than mid-channel vegetatedareas and as a consequence could be zones of elevatedcontaminant storage.

ACKNOWLEDGEMENTS

The research was funded by the UK NERC as part ofthe LOCAR Thematic Programme (grant NER/T/S/2001/00932). The herbicide work was undertaken through astudentship awarded to S. E. Roberts, tied to the NERCLOCAR grant NER/T/S/2001/00936. We are extremelygrateful to: colleagues Roger Wotton, Mark Trimmer,Luke Warren and Ian Sanders for their contributions tothe project; Gareth Old and Ned Hewitt for help withthe acquisition and quality control of the discharge andsuspended sediment concentration data; Ed Oliver forassistance with the figures; and to local landowners forpermission to access field-sampling sites.

REFERENCES

Acornley RM, Sear DA. 1999. Sediment transport and siltation of browntrout (Salmo trutta L.) spawning gravels in chalk streams. HydrologicalProcesses 13: 447–458.

Bickerton M, Petts G, Armitage P, Castella A. 1993. Assessing theecological effects of groundwater abstractions on chalk streams: threeexamples from eastern England. Regulated Rivers 8: 121–134.

Copyright 2009 John Wiley & Sons, Ltd. Hydrol. Process. 23, 2239–2251 (2009)DOI: 10.1002/hyp

SEDIMENT STORAGE IN THE SHALLOW HYPORHEIC OF LOWLAND VEGETATED RIVER REACHES 2251

Brereton C, House W, Armitage P, Wotton R. 1999. Sorption ofpesticides to novel materials: snail pedal mucus and blackfly silk.Environmental Pollution 105: 55–65.

Chefetz B, Bilkis Y, Polubesiva T. 2004. Sorption-desorption behaviourof triazine and phenylurea herbicides in Kishon river sediments. WaterResearch 38: 4383–4394.

Clarke SJ. 2002. Vegetation growth in rivers: influences upon sedimentand nutrient dynamics. Progress in Physical Geography 26: 159–172.

Clarke SJ, Wharton G. 2001. Sediment nutrient characteristics andaquatic macrophytes in lowland English rivers. Science of the TotalEnvironment 266: 103–112.

Collins AL, Walling DE. 2006. Investigation of the remobilization of finesediment stored on the channel bed of lowland permeable catchmentsin the UK. In Sediment Dynamics and the Hydromorphology of FluvialSystems, (Proceedings of a symposium held in Dundee, UK, July 2006).International Association of Hydrological Sciences Publication No.306, Wallingford, UK, 471–479.

Collins AL, Walling DE. 2007a. Fine-grained bed sediment storagewithin the main channel systems of the Frome and Piddle catchments,Dorset, UK. Hydrological Processes 21: 1448–1459.

Collins AL, Walling DE. 2007b. Sources of fine sediment recovered fromthe channel bed of lowland groundwater-fed catchments in the UK.Geomorphology 88: 120–138.

Collins AL, Walling DE. 2007c. The storage and provenance of finesediment on the channel bed of two contrasting lowland permeablecatchments, UK. River Research and Applications 23: 429–450.

Collins AL, Walling DE, Leeks GJL. 2005. Storage of fine-grainedsediment and associated contaminants within the channels oflowland permeable catchments in the UK. In Sediment Budgets 1 ,Walling DE, Horowitz A (eds). IAHS Publication No. 291, IAHSPress: Wallingford, 259–268.

Corenblit A, Tabacchi E, Steiger J, Gurnell AM. 2007. Reciprocalinteractions and adjustments between fluvial landforms and vegetationdynamics in river corridors: a review of complementary approaches.Earth Science Reviews 84: 56–86.

Cotton JA, Wharton G, Bass JAB, Heppell CM, Wotton RS. 2006. Theeffects of seasonal changes to in-stream vegetation cover on patternsof flow and accumulation of sediment. Geomorphology 77: 320–334.

Droppo IG. 2001. Rethinking what constitutes suspended sediment.Hydrological Processes 15: 1551–1564.

Droppo IG, Leppard GG, Flannigan DT, Liss SN. 1997. The freshwaterfloc: a functional relationship of water and organic and inorganic flocconstituents affecting suspended sediment properties. Water Air andSoil Pollution 99: 43–54.

Environment Agency. 2004. The State of England’s Chalk Rivers .Environment Agency: Bristol.

Flynn NJ, Snook DL, Wade AJ, Jarvie HP. 2002. Macropyte andperiphyton dynamics in a UK Cretaceous chalk stream: the RiverKennet, a tributary of the Thames. Science of the Total Environment282–283: 143–157.

Green JC. 2005. Modelling flow resistance in vegetated streams:review and development of new theory. Hydrological Processes 19:1245–1259.

Gurnell A, Goodson J, Thompson K, Clifford N, Armitage P. 2007.The river-bed: a dynamic store for plant propagules? Earth SurfaceProcesses and Landforms 32: 1257–1272.

Gurnell AM, Van Oosterhout MP, De Vlieger B, Goodson JM. 2006.Reach-scale interactions between aquatic plants and physical habitat:River Frome, Dorset. River Research and Applications 22: 667–680.

Hearne JW, Armitage PD. 1993. Implications of the annual macrophytegrowth cycle on habitat in rivers. Regulated Rivers-Research &Management 8: 313–322.

Heywood MJT, Walling DE. 2003. Suspended sediment fluxes in chalkstreams in the Hampshire Avon catchment, UK. Hydrobiologia494(1–3): 111–117.

Jepsen R, Roberts J, Lick W. 1997. Effects of bulk density on sedimenterosion rates. Water Air and Soil Pollution 99: 21–31.

Joyce P, Warren LL, Wotton RS. 2007. Faecal pellets in streams:their binding, breakdown and utilization. Freshwater Biology 52:1868–1880.

Karickhoff S, Brown D, Scott T. 1979. Sorption of hydrophobicpollutants on natural sediments. Water Research 13: 241–248.

Katagi T. 2006. Behavior of pesticides in water-sediment systems.Reviews of Environmental Contamination and Toxicology 187:133–251.

Kronvang B, Laubel A, Larsen SE, Friberg N. 2003. Pesticides andheavy metals in Danish streambed sediment. Hydrobiologia 494:93–101.

Lambert CP, Walling DE. 1988. Measurement of channel storage ofsuspended sediment in a gravel-bed river. Catena 15: 65–80.

Mackey AP, Ham SF, Cooling DA, Berrie AD. 1982. An ecologicalsurvey of a limestone stream, the river Coln, Gloucestershire, England,in comparison with some chalk streams. Archiv Fur Hydrobiologie 64:307–340.

Madsen JD, Chambers PA, James WF, Koch EW, Westlake DF. 2001.The interaction between water movement, sediment dynamics andsubmersed macrophytes. Hydrobiologia 444: 71–84.

Mainstone CP, Parr W. 2002. Phosphorus in rivers—ecology andmanagement. Science of the Total Environment 282: 25–47.

Marker A, Hopgood H, Randall C. 1986. Studies on epilithic andepiphytic diatoms in a chalkstream—comparative estimates ofchlorophyll-a and its derivatives. British Phycological Journal 21:171–182.

McCarthy KA, Gale RW. 2001. Evaluation of persistent organiccompounds in the Columbia river basin using semi-permeablemembrane devices. Hydrological Processes 15: 1271–1283.

OECD. 2000. Guideline for the Testing of Chemicals: adsorption-desorption using a batch equilibrium method, 106.

Owens PN, Batall RJ, Collins AJ, Gomez B, Hicks DM, Horowitz AJ,Kondolf GM, Marden M, Page MJ, Peacock DH, Petticrew EL,Salomons W, Trustrum NA. 2005. Fine-grained sediment in riversystems: environmental significance and management issues. RiverResearch and Applications 21: 693–717.

Packman AI, Mackay JS. 2003. Interplay of stream-subsurface exchangeclay deposition and stream bed evolution. Water Resources Research39(Y): Article No. 1097, 4-1–4-9. DOI: 10.1029/2002WR001432.

Phillips JM, Walling DE. 1995. An assessment of the effects of samplecollection, storage and resuspension on the representativeness ofmeasurements of the effective particle size distribution of fluvialsuspended sediment. Water Research 29: 2498–2508.

Phillips JM, Walling DE. 1999. The particle size characteristics offine-grained channel deposits in the River Exe Basin, Devon, UK.Hydrological Processes 13: 1–19.

Rehg KJ, Packman AI, Ren J. 2005. Effects of suspended sedimentcharacteristics and bed sediment transport on streambed clogging.Hydrological Processes 19: 413–427.

Richards C, Host GH, Arthur JW. 1993. Identification of predominantenvironmental factors structuring stream macroinvertebrate commu-nities within a large agricultural catchment. Freshwater Biology 29:285–294.

Sanders IA, Heppell CM, Cotton JA, Wharton G, Hildrew AG, Flow-ers EJ, Trimmer M. 2007. Emission of methane from chalk streamshas potential implications for agricultural practice. Freshwater Biology52: 1176–1186.

Sand-Jensen K. 1998. Influence of submerged macrophytes on sedimentcomposition and near-bed flow in lowland streams. Freshwater Biology39: 663–679.

Sand-Jensen K. 2003. Drag and reconfiguration of freshwater macro-phytes. Freshwater Biology 48: 271–283.

Walling DE, Amos CM. 1999. Source, storage and mobilisation of finesediment in a chalk stream. Hydrological Processes 13: 323–340.

Walling DE, Owens PN, Leeks GJL. 1998. The role of channel andfloodplain storage in the suspended sediment budget of the River Ouse,Yorkshire, UK. Geomorphology 22: 225–242.

Wharton G, Cotton JA, Wotton RS, Bass JAB, Heppell CM, Trimmer M,Sanders IA, Warren LL. 2006. Macrophytes and suspension-feedinginvertebrates modify flows and fine sediments in the Frome and Piddlecatchments, Dorset (UK). Journal of Hydrology 330: 171–184.

Wheater HS, Peach D. 2004. Developing interdisciplinary science forintegrated catchment management: the UK Lowland CatchmentResearch (LOCAR) Programme. Water Resources Development 20:369–385.

Wheater HS, Peach D, Binley A. 2007. Characterising groundwater-dominated lowland catchments: the UK Lowland Catchment ResearchProgramme (LOCAR). Hydrology And Earth System Sciences 11:108–124.

Wood PJ, Armitage PD. 1997. Biological effects of fine sediment in thelotic environment. Environmental Management 21: 203–217.

Wotton RS, Warren LL. 2007. Impacts of suspension feeders on themodification and transport of stream seston. Fundamental and AppliedLimnology 169/3: 231–236.

Wright JF, Gunn RJM, Winder JM, Wiggers R, Vowles K, Clarke RT,Harris I. 2002. A comparison of the macrophyte cover andmacroinvertebrate fauna at three sites on the River Kennet in themid 1970s and late 1990s. Science of the Total Environment 282–283:121–142.

Copyright 2009 John Wiley & Sons, Ltd. Hydrol. Process. 23, 2239–2251 (2009)DOI: 10.1002/hyp