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Influence of basin connectivity on sediment source, transport,and storage within the Mkabela Basin, South Africa

J. R. Miller 1, G. Mackin2, P. Lechler1, M. Lord 1, and S. Lorentz3

1Department of Geosciences and Natural Resources, Western Carolina University, Cullowhee, NC 28723, USA2Department of Mathematics and Statistics, Northern Kentucky University, Highland Heights, KY 41099, USA3School of Bioresources Engineering and Environmental Hydrology, University of KwaZulu–Natal,Pietermaritzburg, South Africa

Correspondence to:J. R. Miller ([email protected])

Received: 8 August 2012 – Published in Hydrol. Earth Syst. Sci. Discuss.: 6 September 2012Revised: 16 January 2013 – Accepted: 24 January 2013 – Published: 21 February 2013

Abstract. The management of sediment and other non-pointsource (NPS) pollution has proven difficult, and requiresa sound understanding of particle movement through thedrainage system. The primary objective of this investigationwas to obtain an understanding of NPS sediment source(s),transport, and storage within the Mkabela Basin, a repre-sentative agricultural catchment within the KwaZulu–NatalMidlands of eastern South Africa, by combining geomor-phic, hydrologic and geochemical fingerprinting analyses.

The Mkabela Basin can be subdivided into three distinctsubcatchments that differ in their ability to transport and storesediment along the axial valley. Headwater (upper catch-ment) areas are characterized by extensive wetlands that actas significant sediment sinks. Mid-catchment areas, charac-terized by higher relief and valley gradients, exhibit few wet-lands, but rather are dominated by a combination of allu-vial and bedrock channels that are conducive to sedimenttransport. The lower catchment exhibits a low-gradient al-luvial channel that is boarded by extensive riparian wet-lands that accumulate large quantities of sediment (and NPSpollutants).

Fingerprinting studies suggest that silt- and clay-rich lay-ers found within wetland and reservoir deposits of the up-per and upper-mid subcatchments are derived from the ero-sion of fine-grained, valley bottom soils frequently utilized asvegetable fields. Coarser-grained deposits within these wet-lands and reservoirs result from the erosion of sandier hill-slope soils extensively utilized for sugar cane, during rela-tively high magnitude runoff events that are capable of trans-porting sand-sized sediment off the slopes. Thus, the source

of sediment to the axial valley varies as a function of sedi-ment size and runoff magnitude. Sediment export from upperto lower catchment areas was limited until the early 1990s, inpart because the upper catchment wetlands were hydrologi-cally disconnected from lower parts of the watershed duringlow to moderate flood events. The construction of a drainageditch through a previously unchanneled wetland altered thehydrologic connectivity of the catchment, allowing sedimentto be transported from the headwaters to the lower basinwhere much of it was deposited within riparian wetlands. Theaxial drainage system is now geomorphically and hydrolog-ically connected during events capable of overflowing damslocated throughout the study basin. The study indicates thatincreased valley connectivity partly negated the positive ben-efits of controlling sediment/nutrient exports from the catch-ment by means of upland based, best management practices.

1 Introduction

Although the impacts of point source pollution on aquaticecosystems have been greatly reduced, the management ofnon-point source (NPS) pollution has proven to be extremelydifficult, and is a leading cause of surface water degrada-tion (USEPA, 2000). Non-point source pollutants are of par-ticular concern in agricultural areas where sediment, nutri-ents, and pesticides may all negatively impact water qual-ity. Attempts to control NPS pollution generally rely on thedevelopment and application of best management practicesthat balance the economic value of land-use activities with

Published by Copernicus Publications on behalf of the European Geosciences Union.

762 J. R. Miller et al.: Influence of basin connectivity on sediment source within the Mkabela Basin

methods to reduce the production and influx of NPS pol-lutants to water resources. To be cost-effective, such man-agement strategies require a sound understanding of the pri-mary NPS pollutant sources, how pollutants are delivered toand transported through aquatic systems, and how variousmanagement scenarios will influence NPS pollutant load-ings. While conceptually simple, development of quantitativetools upon which to base management decisions and strate-gies is complicated by multiple and diffuse sources of pol-lutants, their movement as both solutes and particulates, anddifferences in pathway transport dynamics over varying tem-poral and spatial scales. Fine-grained sediments serve as aparticularly important component of the NPS pollutant prob-lem because of their direct impact on biota, and the sorp-tion of nutrients and other contaminants onto particle sur-faces such that many hydrophobic contaminants are predomi-nantly dispersed through river systems in the particulate form(Miller and Orbock Miller, 2007). It follows, then, that anyattempt to effectively address NPS pollution requires a highlysophisticated understanding of the spatial and causal linkagesamong human activities, fine-grained sediment production,and sediment transport and storage processes over a range oftime scales.

Historically, sediments (and associated pollutants) erodedfrom upland areas of a catchment were assumed to movesemi-systematically through the drainage system to the basinmouth. This classical continuum view has begun to be re-placed in recent years by a segmented, hierarchical perspec-tive of a drainage network in which channel and valley floorenvironments can be subdivided into progressively smallerunits (Frissell et al., 1986; Kishi et al., 1987; Grant et al.,1990, 1995; Montgomery and Buffington, 1993; Brierley andFryirs, 2001, 2005; Miller et al., 2012). Each unit, of a givenscale, is morphologically homogeneous with respect to land-forms, processes, and other controlling factors such as geol-ogy, vegetation, and substrate (Gant et al., 1995). Commonscales of study range from localized channel units (definedon the basis of various river bed features such as pools, rif-fles, bars, etc.), reach-scale units (defined according to thenature of both the channel and valley floor), and larger unitsranging up to and beyond the entire drainage basin. Appli-cation of the hierarchal approach for management purposeshas focused on reach-scale units, often referred to as pro-cess zones (Montgomery and Buffington, 1993; Miller et al.,2012).

Inherent within the hierarchal systems approach is the per-ception that process zones (as well as units defined at otherscales) differ in their ability to produce, transport, and storesediment. A process zone therefore represents a fundamentalunit of watershed management that allows distinct strategiesto be developed for specific parts of the drainage network.

A closely related concept to the hierarchal view of a riversystem is connectivity. Connectivity, as used here, refers tothe degree to which water and sediment can be transferredfrom one process zone to the next downstream zone (Hooke,

2003). The geomorphic and hydrologic connectivity of thesystem is highly dependent on the time scale under consider-ation. For example, drainage systems located in areas char-acterized by seasonal rainfall may be hydrologically con-nected during the wet period, but disconnected during thedry months when the channel possesses both perennial andephemeral reaches (Miller et al., 2012). Over longer timescales (years to decades), sections of the drainage networkmay become incised, thereby increasing the surface connec-tivity between the zones, or become filled, creating a dis-continuous drainage system with decreased connectivity be-tween process zones.

In light of the above, the movement of NPS pollutantsthrough a drainage system will not only depend on exist-ing hillslope conditions and management practices, but onthe hierarchal structure (morphometry) of the watershed andthe connectivity between process zones and other hierarchalunits. The primary objective of this paper is to describe spa-tial variations in the source(s), transport, and storage of sed-iment over annual to decadal time scales within the Mka-bela drainage basin, a representative catchment within theKwaZulu–Natal Midlands of eastern South Africa. The uti-lized analysis integrates field and cartographic data of fluviallandforms and processes with detailed geochemical analy-ses of sediment provenance. The latter analyses allow forthe quantification of long-term (decadal-scale) changes insedimentation rates and basin connectivity along the axialdrainage system.

2 Study area: geologic, geographic and climaticcharacteristics

The Mkabela Catchment is located within the KwaZulu–Natal Midlands of eastern South Africa, approximately25 km from Pietermaritzburg (Fig. 1). The Mkabela Riverbasin was selected for study because (1) it is representativeof catchments in the region in terms of size, relief, underly-ing geology, and land use, and (2) is a tributary to the muchlarger Mgeni River that drains most of the Midlands, and forwhich a decline in water quality, by means of eutrophication,has been an increasing concern. Important nutrient sourcesinclude direct waste water inputs, broken sewer lines, animalwastes, and non-point source inputs, particularly sediment-associated nutrients from agricultural lands.

Climatically, the Mkabela Basin is characterized by semi-arid conditions, receiving on average 890 mm of precipitationper year, greater than 80 % of which falls during the sum-mer months of October to March. Temperatures, measuredbetween 2005 and 2009, ranged between 4 and 44◦C. Eco-logically, the basin and surrounding area are characterized bysavannah type vegetation (referred to as the Savannah Ecore-gion; WRC, 2002), although land cover has been dramati-cally altered during the past century. Historically, land usewithin the basin was utilized for forestry (pine, wattles, gum),

Hydrol. Earth Syst. Sci., 17, 761–781, 2013 www.hydrol-earth-syst-sci.net/17/761/2013/

J. R. Miller et al.: Influence of basin connectivity on sediment source within the Mkabela Basin 763

Fig. 1. Map showing the location of the Mkabela Catchment inSouth Africa and the position of the defined subcatchments withinthe basin.

pasture and dairy operations, and maize. Significant areas ofthe catchment were converted to sugar cane in 1968 to 1970.Current land use varies between three morphologically dis-tinct catchment areas, referred to as the upper, middle (ormid), and lower subcatchments (Fig. 2). Land use withinthe upper subcatchments is dominated by sugar cane on hill-slopes, and pasture (grasslands), maize and other vegetableson valley floors. Forested areas of wattle and pine are alsopresent on upland areas locally. The middle subcatchment isdominated by sugar cane on hillslopes and pastures (grass-lands) on the valley bottom (Fig. 2). Valley bottom wetlandsare common within the upper catchments, and are replaced inmid-catchment areas by riparian wetlands and channelizedalluvial valleys periodically interrupted by dams and reser-voirs, most of which were constructed between 1970 and1980 (the downstream most reservoir within the study area,Fig. 1 was build in 1950). The lower subcatchment is char-acterized by forested riparian wetlands on the valley floorand hillslopes that are predominantly covered by sugar canevegetation.

The basin as a whole is underlain primarily by shales, silt-stones, and red sandstones of the Natal Sandstone and, toa lesser degree, the Dwyka Groups. Nine soils types havebeen described in sediments overlying the bedrock includingAvalon, Cartref, Clovelly, Glencoe, Glenrosa, Hutton, Kat-spruit, Longlands, and Westleigh (Fig. 3).

Fig. 2.Map showing distribution of observed land-use types withinthe Mkabela Catchment (modified from Lorentz et al., 2011).

3 Methods

3.1 Process zone mapping and characterization

Field and cartographic observations indicated that the catch-ment could be subdivided into distinct process zone and sub-catchment areas (Figs. 1, 4 and 5). Subcatchment areas weredefined according to changes in hillslope and valley mor-phology (e.g., gradient, width), whereas the processes zonesrepresent stream reaches defined on the basis of their posi-tion on the landscape, their dimensions and cross-sectionalform, the composition and nature of the bounding materials(bedrock vs. sediment; sediment size, stratification, etc.), andthe relief/gradient of the channel and surrounding terrain. It isimportant to recognize that while zone types were defined ge-omorphically, each type exhibits specific traits with regard togeomorphic processes (including erosion and deposition) op-erating within the channel, and hydrologic sources and sinks(Table 1).

Delineation and mapping of process zones utilized aniterative approach where distinct reaches of the drainagenetwork were classified and mapped on 2007 georectifiedSPOT images, with the aid of stereoscopic viewing of 2004,1 : 10 000 aerial photographs. Once mapped, the georectifiedand locally field-checked data provided spatial information

www.hydrol-earth-syst-sci.net/17/761/2013/ Hydrol. Earth Syst. Sci., 17, 761–781, 2013


Table 1.Summary of process zones and their general characteristics.

Process Morphology Sediment storage Dominant process(es)zone

Process Waterway Wide, shallow, often Minimal sediment Minor sedimentzones in u-shaped man-made storage in bed of production; dominatedlow order channels; channel waterway by sediment transportvalleys oriented parallel to slope; over “rough” bed;

bed and banks typically Hydrologically, a zonecovered in vegetation of recharge

Upland Man-made, trapezoidal Minimal sediment Dominated by sedimentditch channel; deeper than storage as bars on transport through low

waterways, with less channel bed gradient, but efficientvegetation on channel channels;bed; local depositional hydrologicallybars present; channel is dominated by rechargesemi-parallel to slope

Upland Natural, single-thread Moderate storage Dominated by sedimentchannel channel bound by within channel as bars transport, but local

alluvium; channel may and on floodplain overbank depositionlocally be modified by occurs during floods;human activities channel may exhibit

both influent andeffluent conditions,depending on season

Process Bedrock Well-defined, often Very little, if any, Dominated by bedrockzones channel rectangular channel sediment storage erosion and thewithin bound by bedrock; transport of sedimentbedrock locally, banks may delivered to the channelvalleys consist of alluvial from upstream reaches

sediments; channel and adjacent hillslopesgradients are relativelysteep

Process Axial Narrow, deep trapezoidal Minimal sediment Dominated by sedimentzones ditch channel excavated into storage transportwithin alluvial valley fill andalluvial wetlands; often lowvalleys gradient

Alluvial Natural, meandering Moderate quantities of Dominated by achannel channel developed in sediment are stored combination of

alluvial sediments along within channel bars sediment transport andrelatively wide valleys and as overbank deposition, depending

deposits on floodplain on flow conditions

Alluvial Single thread, Significant sediment Dominated by sedimentchannel meandering channel storage as bars in deposition, althoughwith developed in alluvial channel and overbank some sediment iswetlands sediment; found in wide deposits on floodplain transported through

valleys with riparian and within riparian channel; localizedwetlands adjacent to wetlands zones of groundwaterstream discharge

Wetlands Wide, flat valley area High quantities of Dominated by sedimentwith water table at or near sediment stored across deposition andthe ground surface; the wetland groundwater dischargeshallow channel(s) arelocally present

Reservoir Perennial bodies of open Significant sites of Dominated bywater formed by sediment storage deposition; finedownstream dam sediments may be

transported throughreach during highflow events



Av

Av

Av

Ka

Lo

LoAv

Cv

Ka

Av

Av

Lo

We

We

WeGs

Gs

.

Av

Av

Gc

Gc

Cf

We

Lo

Cf

Cf

Hu

CfHu

Gc

Av - AvalonCf -CartrefCv - ClovellyGc - GlencoeGs - GlenrosaHu - HuttonKa - KatspruitLo - LonglandsWe - Westleigh

We

Soil Form

Variations in Slope (%)

0-1

4-7

2-4

1-2

7-15

15-20

20-100

Fig. 3. Six soil types found on Mkabela Catchment (after Le Rouxet al., 2006).

on the type and distribution of the process zones and the abil-ity of the drainage network to transfer water, sediment, andany nutrients that they carry down catchment.

3.2 Geochemical tracing

Two types of geochemical tracing studies were applied to aseries of sediment cores extracted from wetland and reser-voir deposits within the Mkabela Catchment. First, a geo-chemical fingerprinting and mixing model approach was uti-lized to determine the relative percentage of sediment withina wetland of the upper subcatchment and an upstream sec-tion of the middle subcatchment that was derived from theprimarily soil and land-use types within the basin. This anal-ysis provides insights into the provenance of sediment de-livered to depositional areas of the valley floor. Second, Cuand Zn were used as tracers to gain insights into the geomor-phic connectivity, and changes in connectivity, with the entiredrainage system through time. Cu and Zn could be utilized astracers within the Mkabela Catchment because they are pri-marily associated with soil amendments used on vegetable(cabbage, maize) fields predominately located in headwa-ter areas. Thus, variations in their concentration within sedi-ment extracted from cores located along the drainage system

Fig. 4. Sampling and coring locations within the MkabelaCatchment.

reflected the downstream movement of particulates from theupper subcatchment, through the middle subcatchment, andultimately to a riparian wetland cored within the lower sub-catchment. When combined, results from the two methodsyield insights into both sediment provenance from upland ar-eas to the valley floor and the geomorphic connectivity of thedrainage network over a period of decades.

3.2.1 Collection, sedimentology and analysis ofsediment cores

The two geochemical tracing methods described above wereapplied to four cores collected in 2008 from the upper andmiddle subcatchments, including one core from the marginof the upstream-most reservoir (R1-C1), and three cores fromthe wetland (WT-C1, WT-C2, and WT-C3) (Fig. 4). A fifthcore (B2WTC1) was collected in 2009 from a riparian wet-land located along the channel in the uppermost portion ofthe lower catchment (Fig. 5). All of the cores were shippedto the Nevada Bureau of Mines and Geology and subse-quently described, photographed, and sampled for geochem-ical analyses.

The samples were analyzed using a Micromass PlatformICP-HEX-MS for major elements (e.g., Si, Al, Fe, Ca, Mg,Mn, Na, K, Ti, and P), total acid-soluble trace metals andmetalloids (e.g., Pb, Zn, Cd, Cu, Au, Ag, Se, As), selectedrare earth elements (e.g., Ga, Nb, La, Lu, Hf), and selectedisotopes (e.g.,204Pb,206Pb,207Pb,208Pb). Analysis involved



Fig. 5.Delineation of process zones and subcatchment areas withinthe upper and middle subcatchments.

the digestion of 200 mg of dried and homogenized sediment,< 2 mm in size, in 125 mL polypropylene screw-top bottlescontaining 4 mL of aqua regia. These were sealed and heldin a 100◦C oven for 60 min. The leachates were then trans-ferred to 200 mL volumetric flasks, brought up to volumewith ultra-pure water and stored until analyzed by ICP-MS.With respect to total elemental concentrations, the Platformwas calibrated using USGS, NIST, and in-house standardreference materials (SRMs). Reagent blanks and the ana-lyte concentrations for the SRMs were plotted against blank-subtracted integrated peak areas. A regression line was fittedto this array of calibration points and the equation of the linewas used to quantify unknown sample concentrations. Devi-ation of standards from the regression line was used to esti-mate analytical accuracy, which was generally± 3 to 5 % ofthe amount present when determining total concentrations.Replicate analyses were used to determine analytical preci-sion, which was generally< ± 5 % for most elements. Withrespect to Pb isotopic analyses, precision when comparingdata from individual digestions was 0.2 to 0.3 % relative de-viation (one sigma) for206Pb,207Pb, and208Pb. Instrumen-tal precision was better. Accuracy of isotopic measurementswas assessed with the NIST 981 lead isotope standard. Ac-curacy was typically better than± 0.5 %, and systematic in-strumental bias was corrected. Given the limited abundanceof 204Pb, precision and accurracy values were much higher;thus,204Pb was not used as a potential tracer.

Selected cores were dated by means of210Pb (137Cs lev-els were too low to yield useful results). The analyses werecarried out by Flett Research Ltd. located in Winnipeg,Canada. Flett Research was also contracted to use the raw210Pb data to model the age-depth relationships within thecores. As is typical of210Pb analyses, background values of210Pb in the cores was determined by comparison with226Ra

Table 2.Summary of samples collected in May 2008. Samples fromroads were not included in the statistical analysis (see text).

Land-use # Samples Soil type # Samplescategory

Pasture 10 Avalon 10Pine Forest 2 Cartref 12Roads 10 Clovelly 3Sugar Cane 35 Glencoe 18Vegetables 10 Katspruit 6Wattles 6 Longlands 5

Westleigh 9

Total 73 Total 63

measured near the bottom of the core. The results were thenused to determine sedimentation rates at the site. Depth-agecurves were modeled using the constant rate of supply (CRS)method (Appleby and Oldfield, 1978).

3.2.2 Upland sediment sampling and analysis

Geochemical fingerprinting and mixing model analyses areused to assess the relative contribution of sediment from spe-cific sediment sources to wetland and reservoir deposits lo-cated along the axial channel. In previous studies, sedimentsources within a catchment have been defined in differentways (e.g., by land-use category, geological unit, or tribu-tary basin) depending on the primary intent of the analysis.For this investigation, a total of 73 samples were collectedfrom upland areas in May 2008 in order to characterize sed-iment sources (Fig. 4). The utilized sampling strategy wasdeveloped to allow the data to be stratified (categorized) intwo different ways to determine the relative contribution ofsediment from six land-use categories and seven soil types.The sampling scheme, then, allows an assessment of howboth land use and soil type influence sediment productionand availability (Table 2). The number of randomly collectedsamples in each land-use category roughly corresponds to thearea that it covers within the catchment. All of the sampledsoils were obtained from approximately the upper 2 cm ofthe ground surface and categorized according to land-coverand soil type, the latter by means of existing soils maps. Thesediments therefore represent the material most likely to beeroded during a runoff event from shallow rills or interill ar-eas. Erosion of sediment from hillslopes or valley depositsfrom gullies was not observed to be significant within thecatchment, and the occurrence of unsupported210Pb withina wetland core suggests that the majority of the sediment isderived from surface materials that have been subjected tothe atmospheric deposition of210Pb. In order to reduce fieldvariance in elemental concentrations, subsamples were col-lected from about 5 to 10 locations within a 5 m radius of thesampling site and composited to create a single sample.



All of the upland samples were loaded into pre-cleanedpolypropylene sampling containers, which were subse-quently placed in plastic sampling bags, and shipped to theNevada Bureau of Mines and Geology in the USA where theywere analyzed by ICP-MS for the same elements as the coresamples (as described above).

3.2.3 Source modeling procedure

Determination of the relative contribution of sediments fromthe source areas relies on the use of a sediment mixing model.The basic premise underlying the use of geochemical finger-printing of NPS pollutants is that the processes involved inthe erosion, transport, and deposition of sediment ultimatelyresult in a deposit that represents a mixture of material de-rived from multiple source areas within the catchment. It isthen possible to characterize the sediments within the sourceareas and the downstream alluvial/lacustrine deposits for asuite of parameters and statistically compare their parametercharacteristics to unravel the relative proportion of sedimentthat was derived from each source type (Miller and OrbockMiller, 2007). During this investigation, the original modelused by Miller et al. (2005) was modified using the approachprovided by Rowen et al. (2000) to estimate sediment sourcecontributions from the hillslopes to the cored deposits.

Constraints on the mixing model require that (1) eachsource type contributes some sediment to the mixture, andthus the proportions (xj , j = 1, 2, ...n), derived fromn in-dividual source areas must be non-negative (0≤ xj ≤ 1), and(2) the contributions from all of the source areas must equalunity, i.e.,

n∑j=1

xj = 1. (1)

In addition, some differences (error) between the values ofthem measured parameters, in the source area,aij (i= 1 ...m,j = 1 ...n) and the mixture,bi (i = 1 ...m) must be allowed.The residual error corresponding to thei-th parameter can bedetermined as follows:

εi = bi −

n∑j=1

aij xj (2)

for i = 1, 2, ...m, whereaij (i = 1, 2, 3 ...m, j = 1, 2, ...n)are the measurement on the correspondingi-th parameterwithin the j -th source area andxj is the proportion of thej -th source component in the sediment mixture. When thenumber of measured parameters is greater than the numberof source areas (m ≥ n), the system of equations is over-determined, and a “solution” is typically obtained using aniterative computational method that minimizes an objectivefunction using a gradient search, thereby obtaining a best fitsolution to the entire data set (Yu and Oldfield, 1989). Thereare several ways to obtain a best fit, but in previous studies,

the objective function,f , has taken the form of the sum ofthe relative errors (Yu and Oldfield, 1989) where

f (x1, ..., xm) =

m∑i=1

|εi/bi | (3)

or (Collins et al., 1997a)

f (x1, ..., xm) =

m∑i=1

(εi/bi)2 . (4)

However, in the case wheref is relatively “flat”, the gradientnear zero may halt an iterative search method prematurely.

We take an alternative route, following Rowan et al. (2000)and Nash and Sutcliffe (1970), whereby we create the effi-ciency function

E(x1, ..., xm) = 1 −

m∑i=1

(εi)2

m∑i=1

(bi − di)2, (5)

wheredi (i = 1, 2, 3 ...m) is the mean of thei-th param-eter over all source regions. An ideal solution would re-sult in E = 1 or 100 % efficiency. We then create a parti-tion of all possible combinations of non-negativen-tuples(x1, ..., xn) satisfying the unity constraint (Eq. 1), by in-crements of1x = 0.05. By evaluatingE at each of then-tuples, we are able to determine the specific combination ofthe source contributionŝx = (x1, ..., xn) yielding the maxi-mum efficiency on the partition.

As Rowan et al. (2000) pointed out, the efficiency func-tion E has a maximum value atx̂, but there may be a rangeof n-tuples having an efficiency within a specified toleranceof the maximum efficiency. That is, there are a number ofsolutions that are statistically equivalent. For example, us-ing the data from sample WT-C1-1, the optimal efficiencyvalue was 0.9963 when 50 % of the contribution was fromcane, 25 % from Maize and vegetable, and 25 % from wattlegroves. Yet we see that there is a small range of proportionsfor each source that yields efficiency levels at the 0.95 levelor above.

Attempts to model complex processes generally requiresimplifying assumptions, and the use of the sediment mix-ing model described above to assess sediment provenance isno exception. An important assumption inherent in the uti-lized approach is that it documents the ultimate source ofsediment and not its proximal one. It is possible that sedi-ment was eroded from a defined sediment source, transporteddownvalley and temporally deposited within the channel (orsome other site) before being remobilized and transported toits current resting point where it was sampled. We believethat deposition and remobilization along the studied drainagesystem was minimal because the catchment areas upstreamof the sampled sites are relatively small (< 1000 ha). The



limited catchment areas increase the probability that erodedsand-sized and smaller sediment will be transported from thesource to the point of deposition within the timeframe repre-sented by a sample extracted from the analyzed cores.

It order for the model to describe the relative quantity ofsediment eroded from each of the source areas, it must be as-sumed that the sediment leaves all sources and is transporteddownstream at an equal rate so that it arrives at the samplingpoint simultaneously. This assumption is often violated bydifferences in the proximity of a source to the sampled depo-sitional area, or differences in the rate at which particles ofdiffering size are transported downstream. It must be recog-nized, then, that the estimated contributions with regards toupland erosion rates are likely to be biased. We believe, how-ever, that the bias is minimal because with the exception ofthe vegetable plots (which includes areas of maize), the de-fined sources upstream of the sampled wetland and reservoirare distributed throughout much of the catchment (Fig. 2).More importantly, the mixing model will accurately describethe source of the sediment delivered to, and found within, thewetland and reservoir deposits within the limitation of theutilized statistical methods. In this study, our interests pri-marily are focused on the relative contribution of sedimentfrom the various source types found within the wetland andreservoir deposits (rather than the amount of sediment erodedfrom various source types) as these differences are reflectiveof downstream sediment delivery and connectivity.

4 Results and discussion

4.1 Drainage network characteristics

Field and cartographic data show that the drainage networkwithin the Mkabela Basin can be subdivided into nine distinctprocess zones on the basis of their position on the landscape,valley dimensions and form, the underlying geological de-posits in which they are developed, and the degree to whichthey have been affected by human activities (Lorentz et al.,2011). Each of these defined process zones is dominated by asuite of geomorphic and hydrologic processes as described inTable 1, and occurs with different frequencies within distinctsegments of the catchment referred to as the upper, middle,and lower subcatchments (Fig. 5). The general size and reliefof the subcatchments are provided in Table 3.

Upper subcatchments comprise the upstream-most (head-water) areas of both axial and large tributary drainage sys-tems (Figs. 1 and 5). Within these upper subcatchments, hill-slope drainage has often been significantly modified by cul-tivation, particularly in areas of sugar cane, in part to reducethe removal of sediment from the cultivated slopes. However,hillslopes are locally traversed by a relatively high density(approximately 2.5 km km−2) of man-made, low-gradient, u-shaped “waterways” that are oriented perpendicular to topo-graphic contours (Fig. 6a, Table 1). Hillslope runoff and flow

f. Alluviated Valley & Riparian Wetlandse. Bedrock Channel

a. Waterway

c. Axial Ditch

b. Upland Ditch Ditch

d. Wetland

Fig. 6. Photographs of selected process zones types defined withinthe Mkabela Catchment.

along contoured features primarily deliver sediment to thesewaterways that is then feed into larger, incised channels, re-ferred to as “upland ditches” (Figs. 5 and 6b). (Note thatdue to their small scale, lines of drainage parallel to contourswere not considered here as a type of process zone.) The up-land ditches represent heavily modified or relocated uplandchannels, or entirely man-made features. In either case, up-land ditches are more incised and v-shaped than the u-shapedwaterways. Further downstream, upland ditches and water-ways deliver water and sediment to wetlands, or more fre-quently, an “axial ditch” (Figs. 5 and 6c). The largest axialditch within the upper subcatchment was cut into unconsoli-dated valley fill deposits that comprise the axial valley. Untilrecently (∼ 1990), this portion of the axial valley was dom-inated by a natural wetland devoid of a well-defined chan-nel, but after the creation of the axial ditch upstream areas ofthe wetland were drained and converted to pasture (Fig. 5).Farther downstream, however, the axial ditch decreases dra-matically in size and depth as it enters a less disturbed area ofwetland formed upstream of a bedrock constriction in the val-ley floor (Fig. 5). This wetland defines the downstream limitof this upper subcatchment. The mouth of a tributary enter-ing the axial valley from the north-west (Fig. 5) also exhibits



Table 3. Summary of subcatchment characteristics. See Figs. 1and 4 for subcatchment locations.

Subcatchment Area Relief(ha) (m)

Upper subcatchment 1 1300 80(headwaters)

Upper subcatchment 2 720 150

Upper subcatchment 3 175 90

Middle subcatchment 700 91

Lower subcatchment 4205 175(upstream of coringsite)

Lower subcatchment 19 700 626(total area)

an extensive wetland devoid of an integrated network of sur-face channels. Field observations indicate that the water ta-ble within the wetland is below the ground surface duringthe dry season, but is at or near the ground surface duringwetter months. The extensive wetlands found within the ax-ial valley system of the upper catchment are dominated byrelatively low channel and valley gradients and depositionalprocesses.

The middle subcatchment begins downstream of thebedrock constricted wetland (Fig. 5). Hillslopes within thisportion of the basin are also dominated by sugar cane fieldsthat possess numerous waterways (Fig. 2). However, manyof the waterways along the western side of the catchmentare short, drain relatively small areas, and are disconnectedgeomorphically from the axial valley, limiting their abilityto directly deliver sediment to the axial channel. Perhaps ofmore importance with respect to sediment transport, the axialdrainage system within the middle subcatchment is charac-terized by a continuous, relatively shallow, but high gradient(∼ 0.008m m−1) alluvial channel that is locally boarded bydisturbed riparian wetlands. The channel is disrupted by fourseparate farm dams and their associated reservoirs (Fig. 5).Water flows over the top of the dams throughout most of theyear, but during wet season storms the movement of waterthrough the reservoir and over the dam can be particularlyintense. Immediately below the fourth dam, the stream con-sists of a bedrock channel locally characterized by a seriesof knickpoints (Fig. 6e). The river then enters the lower sub-catchment.

The lower subcatchment is dominated by a low gradient,alluvial channel boarded by extensive and forested riparianwetlands (Figs. 2 and 6f). The wetlands are intermingled withareas of sugar cane that also occur locally on hillslopes. Inmarked contrast to the upstream subcatchments, upland areaswithin the lower subcatchment exhibit very few waterways,axial ditches, or upland channels, in spite of the fact that val-ley floors are incised well below the surrounding terrain. The

Table 4. Discriminate analysis classification matrix.(A) Soil typeand(B) land use.

(A) Number of samples classified per soil type %

Soil type Av Cf Cv Ka Gc Lo We Correct

Av 9 0 0 1 0 0 0 90Cf 0 9 3 0 0 0 0 75Cv 0 0 3 0 0 0 0 100Ka 0 0 1 5 0 0 0 83Gc 3 0 1 0 11 1 2 61Lo 0 0 0 0 0 5 0 100We 0 0 0 0 1 0 8 89Totals 12 9 8 6 12 6 10 79

(B) Number of samples classified per soil type %

Land use Sc Veg Wt Pine Rds Past Correct

Sc 22 3 9 0 1 0 63Veg 0 8 2 0 0 0 80Wt 0 0 5 1 0 0 83Pine 1 0 0 1 0 0 50Rds 0 0 2 0 6 2 60Past 0 0 2 0 1 7 70Totals 23 11 20 2 8 9 67

Av – Avalon; Cf – Cartfer; Cv – Clovelly; Ka – Katspruit; Gc – Glencoe; Lo –Longlands; We - Westleigh; Sc – Sugar Cane; Veg. – Vegetables; Wt – Wattles;Pine – Pine Grove; Rds – Roads; Past – Pasture

low gradient nature of the axial channel, and the broad allu-vial valley consisting of extensive riparian wetlands, forms ahighly depositional environment that allows for the storageof large volumes of sediment (as described below).

4.2 Sediment provenance analyses

4.2.1 Delineation of geochemical fingerprints

The use of geochemical fingerprinting methods to elucidatesediment provenance from non-point sources has increasedsignificantly during the past two decades (Slattery et al.,1995; Collins, 1995; Collins et al., 1997a,b, 1998; Wallinget al., 1999; Bottrill et al., 2000; Russell et al., 2001; Dou-glas et al., 2003, 2005, 2010; Miller et al., 2005; Foster etal., 2007, 2012). Geochemical fingerprinting methods wereused here to determine the predominant source of sedimentsfound in three wetland and one reservoir sediment cores col-lected from along the drainage network. Two separate anal-yses were carried out, one in which sediment sources weredefined on the basis of land use (Fig. 2), and the other forwhich sources were defined on the basis of soil type (Fig. 3).

With respect to the soil type, eight elements were iden-tified as fingerprints, including Ti, Cr, Ga, Nb, La, Ce, Lu,and Hf using a discriminant function analysis as describedin the methods section. All of these elements are known tobe highly immobile in freshwater systems with normal Ehand pH conditions. The discriminant function correctly clas-sified 79 % of the samples (Table 4a). The most incorrectlyclassified samples were obtained from Cartref and Glencoesoils, possibly because both are found in similar locations



Fig. 7. Relative percent of sediment derived from specific soil types within the catchment. Av – Avalon, Cf – Cartref, Cv – Clovelly, Gc –Glencoe, Ka – Katspruit, Lo – Longlands, We - Westleigh.

on the landscape (steep slopes) and possess sandy-texturedhorizons.

A stepwise discriminate analysis was also carried out forsediment sources defined by land use. The selected param-eters were the same as those used to differentiate soil types(Ti, Cr, Ga, Nb, La, Ce, Lu, and Hf) (Table 4b). Source mate-rial samples collected from specific land-use categories wereincorrectly classified about a third of the time. The difficultyof correctly identifying a particular land cover may be relatedto two factors. First, crop rotations may potentially producea mixed geochemical signal with regards to land use. Themapping of sediment provenance onto land use when landuse has changed through time is a recognized problem asso-ciated with the use of geochemical fingerprinting techniques.Primarily at issue is whether alterations in land use will leadto changes in the concentration of the elements used to definethe geochemical fingerprint such that the geochemistry of thesediment source samples reflects both its current and pastland-cover history, or, perhaps more accurately, inhibits theidentification of a geochemical fingerprint capable of defin-ing a specific land-use type. While potentially problematic,we believe that shifting land use did not significantly affectour results. Land areas covered in wattle and pine forests as

well as pasture have presumably changed little over at leastthe past four decades. Areas planted in sugar cane also havebeen relatively stable since its introduction into the catch-ment in 1968. Most maize fields were present prior to 1968,although some areas of maize were converted to sugar caneas it was introduced into the catchment. The most recenttransition in land use is associated with the conversion of adairy operation in the headwaters of the catchment to cab-bage fields around 2001. Difficulties in creating a fingerprintas a result of these land-use alterations that occurred are mostlikely to be associated with elements applied to agriculturalfields as a soil amendment (e.g., fertilizers). The geochemicalfingerprints developed for the various land-use types definedin this study were based on highly immobile rare earth el-ements which are less likely of exhibiting rapid changes inconcentration within the sampled materials.

The second potential cause for the observed misclassifica-tion of land-use type is that a given land-use category may beunderlain by several soil types, complicating its geochemicalsignature. In fact, when soil type is added to the discriminateanalysis as a numeric value the ability to correctly classifyland cover increases significantly (to 86 %).



Fig. 8.Relative percent of sediment derived from specific land-use types within the catchment.

4.2.2 Modeling results

The location and general characteristics of cores collected,geochemically analyzed, and modeled to assess sedimentprovenance within the Mkabela Basin are provided in Fig. 5and Table 5, respectively. The soil source modeling (in whichprovenance was defined on the basis of soil type) showsthat three distinct intervals are present in WT1-C1 (Fig. 7).Samples 10–16 are composed exclusively of Clovelly (CV)and Katspruit (Ka) soil types. The relative contributions ofClovelly range from 10 to 70 %, and average 40 %; Katspruitranges from 40 to 90 %.

Samples 7–9 are dominated by Katspruit (> 60 %), withminor contributions of Westleigh (We), Avalon (Av), Catref(Cf), and Longlands (Lo), in three of the samples. The upperpart of the core (samples 1–4) primarily consist of Avalon(30–60 %), Katspruit (10–70 %), and Longlands (10–40 %)soil types, with minor contributions of Glencoe (Gc) andWestleigh.

Boundaries between two of the major source type inter-vals within the core roughly correspond to stratigraphic unitboundaries. The boundary between the mid-interval (sam-ples 7–9) and the lower interval (samples 10–16) impreciselycorrelate with a gradational stratigraphic boundary within thecore.

Core WT-C1 can also be subdivided into three distinct in-tervals with respect to modeled land-use sediment sources(Fig. 8). The three intervals correlate with the intervals de-noted for soil type. Samples 10–16 are composed predom-inantly of sediment from cane (15–75 %) and vegetable(30–80 %) fields. The samples also contain minor amountsof sediment from pastures (< 10 %). The intermediate in-terval (samples 7–9) is dominated by sediment from veg-etable fields (generally> 70 %). However, in comparison tothe lower unit, the interval exhibits a notable increase insediment from pastures (∼ 10–25 %), and localized, minoramounts of material from roads and wattle-covered terrain.With the exception of sample 5, the upper 6 samples containa wider range of source inputs. The dominant sources forthese samples include sediment from vegetable fields, pas-tures, and cane fields, with lesser amounts of sediment fromwattle-covered terrain (Fig. 8).

Core WT-C2 (also from the upper catchment wetland) canbe subdivided into three intervals on the basis of sedimentprovenance with respect to soil types (Fig. 7). The lowermostinterval including samples 13–14 consists of a medium sandyloam, and possesses sediment from a variety of soil typesincluding Longlands (10–60 %), Glencoe (20–40 %), Cartref(0–30 %), Katspruit (0–30 %), and Clovelly (0–10 %). Thislowermost interval is overlain by a 16 cm unit ranging from



Table 5.Summary of collected cores.

Core Location Characteristics

Core WT-C1 Located within an extensive wetland Total length is 122 cm. Stratigraphic relationships,approximately 40 m from its including the presents of a buried paleosol consisting ofdownstream terminus. The area is a light brown, very sticky, gravelly clay loam, suggestsdominated by a relatively flat that historic sediment is 107 cm thick. A total of 16surface about 1 m above a small samples, collected at 7 cm increments were obtainedchannel that traverses the wetland. from historic deposits. No sample was collected acrossAt the time of coring, the water a stratigraphic unit boundarytable was about 20 cm below thesurface, but based on the vegetation,periodically rises to ground level.

Core WT-C2 Obtained from a flat surface about 1 m Total length is 112.5 cm. Sediments below 90 cm inabove the wetland channel, about depth have been interpreted to pre-date historic250 m upstream of the wetland’s deposition; they are heavily weathered, and exhibitdownstream terminus. The location significant accumulations of clay. A total of 16 sampleswas located about 15 m from the were collected at 7 cm increments. As was the case forchannel edge. The water table was the other cores, samples did not cross stratigraphicabout 20 cm below the surface at boundaries.the time of coring.

Core WT-C3 Obtained from a flat surface about 1 m Total length is 130 cm. The upper 69.5 cm of sedimentabove the wetland channel, about is thought to be of historic age based on stratigraphic and250 m upstream of the wetland’s geochemical data. A total of 10 samples were collecteddownstream terminus. from historic sediments at 7 cm increments. None of the

samples cross stratigraphic boundaries.

Core R1-C1 Collected from the edge of the first Total length is 140 cm. The core was sampled at 7 cmreservoir along the main drainage in increments for210Pb dating, generating a total ofa low-lying area that is inundated 20 samples. A total of 21 samples were collected forduring flood events. geochemical analyses. Two samples of similar thickness

were collected for units more than 5 cm thick. All of thesediment appears to be of historic age.

BW2TC1 Riparian wetland within lower Total length is 179 cm. All sediment appears to be ofcatchment historic age. A total of 21 samples were collected at

8 cm intervals.

sample 11 to 12 that is dominated by sediment from Clovelly(30–90 %) and Katspruit (10–60 %) soils, with minor contri-butions from Avalon soils (0–10 %). The majority of the core,ranging from sample 1 to 11, is dominated by sediment fromKatspruit (80–90 %), with< 20 % coming from Catref soils,except in the lowermost sample. This latter sample containssediment from Glencoe rather than Catref soils. Changesin sediment source contributions correspond to stratigraphicunit boundaries.

With regards to land use, sediment provenance within CoreWT-C2 can be subdivided into two intervals which closely,but not precisely, match the boundaries denoted for soils.The lowermost deposits (sample 10) contain relatively largepercentages of sediment from cane fields (Fig. 8), whereasthe overlying sediments are predominantly derived from veg-etable fields (> 80 %) with lesser contributions from roads.Sample 10, located along the boundary between the two

intervals appears transitional in terms of source, consistingof large amounts of sediment from vegetable fields (as is thecase for the overlying deposits), as well as minor amounts ofsediment from cane fields and pastures (as is the case for thelower deposits).

The sediments in Core WT-C3 (from the upper catchmentwetland) can be subdivided into two predominate intervalsin terms of the soil types from which the sediments werederived (Fig. 7). The lowermost sediments (samples 7–10)are composed primarily of Avalon, Westleigh, and Katspruitsoils, with minor amounts (10 %) of Catref and Longlands insample 7. The uppermost part of the core (samples 1–5) con-sists primarily of sediment from Longlands soils, with lesser(∼ 10 %) from Catref soils. Sample 6, which separates thetwo intervals and which is found at the top of a stratigraphicunit, is highly anomalous, consisting exclusively of sedimentfrom Clovelly soils.



Changes in sediment provenance modeled with respect toland use closely parallel noted changes in provenance as-sessed by soil type. The lowermost sediments (samples 7–10) are composed primarily of sediment from pastures (15–70 %), and in decreasing order, vegetable fields (5–70 %),wattles groves (5–15 %), pine groves (0–15 %), cane (5–20 %) and roads (0–10 %). The uppermost part of the core(from 0–30 cm, samples 1–5) consists primarily of sedimentfrom cane fields (10–100 %) and wattles (45–85 %), withsmall amounts (5 %) from vegetable fields and pastures insample 5. Sample 6, which separates the two intervals andwhich is found at the top of a stratigraphic unit, consists ex-clusively of sediment from cane fields.

Core R1-C1 was obtained from the margin of reservoir lo-cated within the middle catchment. The majority of sedimentwithin the reservoir was derived from Longlands soil, withthe exception of five notable, but thin, horizons. Samplingintervals 18 and 21 at the bottom of the core are composedpredominantly of sediment from Katspruit type soils. Sam-ples 13–14 contains 30–50 % Covelly soil material, in ad-dition to Longlands. The loamy fine sand to loam texturedmaterials associated with samples 6 and 7 contain no de-finable sediment from Longlands soils, but are dominatedby Katspruit (sample 6) or a mixture of Katspruit, Glencoe,and Clovelly (sampling interval 7). The uppermost sedimentsalso contain significant amounts of Clovelly type materialsas well as Glencoe and Katspruit in the case of samplinginterval 1.

With regards to land use, the majority of the sand domi-nated sediment within Core R1-C1 appears to have been de-rived from cane fields. Fine-grained, loamy sediments (e.g.,found in sampling intervals 1, 6, 7, 18, and 19) appear tohave been derived primarily from vegetable plots. Figure 8also shows that there is a notable increase in the contributionof sediment from wattle groves within and above sample 11,as well as vegetables and roads, at and above sample 8 fol-lowing a period of input primarily from cane fields betweensamples 12 and 16.

4.2.3 Sedimentation rates

Samples from two cores (WT-C1 and R1-C1) were analyzedfor 210Pb to (1) determine the age of the deposits as a func-tion of depth, and (2) estimate sedimentation rates for spe-cific time intervals. These two cores were selected on the ba-sis of their location (one from the upstream-most wetland andone from the reservoir) and their relatively complete strati-graphic nature (in which the core extended through historicto pre-historic sediments).210Pb in the relatively fine-grainedsediment of Core WT-C1 was measurable, but low (Fig. 9).A 226Ra measurement of 0.82 DPM g−1 in the deepest sec-tion (at 112–122 cm depth), which based on stratigraphicdata represent pre-historic sediment, is similar to the210Pbmeasurement of 0.60 DPM g−1 in the same section. This210Pb measurement suggests that background levels of210Pb

Fig. 9. Estimated age of the sediments in Core WT-C1 as deter-mined by210Pb analysis. The slope of the line in age-depth plot rep-resents the sedimentation rate. Sedimentation rates increase above41.2 cm, or after about 1992.

have been attained at 105 cm. Although210Pb values divergeslightly from the measured226Ra value, background mayhave been attained at the shallower depth of 70 cm depth(R. Flett, personal communication, 2008).

Age-depth relations were modeled using the constant rateof supply (CRS) method for background values at both105 and 70 cm, generating two age-depth curves (Fig. 9).Both curves are presented to provide the range of age-depthrelations that may exist, depending on the background depththat is utilized. The two analyses yield similar results for theupper 6 samples (last 20 yr), but progressively diverge afterthat (Fig. 9). Both analyses indicate that sedimentation rateswithin the wetland are relatively uniform until the end of the1980s, at which point sedimentation rates begin to increasesignificantly to the present.

The 210Pb content of the sediment from R1-C1 was verylow and irregular, showing no consistent pattern. This pat-tern is presumably related to the coarse-grained nature of thereservoir deposits as measurable concentrations were foundin the fine-grained units. In any case, it was not possible todetermine with confidence the date of deposition of any ofthe sediments within the core from the reservoir (R1-C1).

4.2.4 Controls on sediment source

An examination of the core data collected from the wet-land and reservoir shows that the deposits exhibit significantspatial (depth, areal) variations in grain size. These varia-tions between subcatchment areas are dramatic. The most up-stream reservoir within the catchment (located in the uppermiddle subcatchment) is dominated by sand, as was men-tioned earlier (Figs. 7 and 8). The wetland, however (lo-cated in the upper subcatchment, upstream of the reservoirs)consists primarily of loamy or sandy loam deposits. Theobserved variations in deposit grain size presumably reflectchanges in sediment source and the nature of the source ma-terials, hydraulic sorting of the sediment during transport and



Table 6.Brief description of the primarily soil types found in the study area (after Le Roux et al., 2006).

Soil type General characteristics

Avalon (Av) The Avalon soil type surveyed to 120 cm depth and consistsprimarily of soft plinthic B horizons which is a sandy yellow-brownB horizon underlain by hard plinthic horizons.

Cartref (Cf) Shallow, sandy soils with very little water holding capacity found onsteep, short, convex hillslopes.

Clovelly (Cv) Associated with, and similar to, Longlands soil type.

Glencoe (Gc) Similar to Avalon soil type, but dominated by hard plinthicsubhorizon; found on steeper slopes of higher relief. Parent materialis thought to be the Natal Group Sandstone (NGS).

Hutton (Hu) Found near crest and midslopes of high relief, steep hillslopes.Moderately drained, underlain by NGS.

Katspruit (Ka) Clayey, strongly gleyed soils found on low-relief (10–15 m) terrain,particular valley bottoms.

Longlands (Lo) The Longlands soil type was surveyed up to 120 cm depth andconsists of soils that are sandier than the Avalon soils with similarprofile of soft plinthic B horizons well developed underlain by hardplinthic horizons.

Westleigh (We) The Westleigh soil type was surveyed up to 110 cm depth andconsist a poorly drained hydrosequence dominated by clayey soilswith prominent mottling and deep, clayed subsoils.

deposition, differences in flow magnitudes (owning to dif-ferences in catchment size), or a combination of the threefactors.

The utilized sediment mixing model suggests that nearlyall of the sand-sized sediment within the reservoir is derivedfrom Longlands and, to a much lesser degree, Clovelly soils.Both of these soil types exhibit sandy textures within thecatchment (Table 6; Le Roux et al., 2006). They tend to besandier than the clay-rich Westleigh or Avalon hillslope soils,and much sandier than valley bottom soils such as the clayeyKatspruit soil (Le Roux et al., 2006). The geographic dis-tribution of Longlands and Clovelly soils has been mappedin detail for only the headwaters of the Mkabela Catchment(Fig. 3). Here Longlands and Clovelly soils are located alongthe eastern corner of the catchment, and are shown to abutCartref soils on the 1 : 100 000 soils map (Fig. 3). The ar-eas shown as Cartref soils on the 1 : 100 000 scale map isdominated by Cartref types soils, but also include localizedareas of other soil types including Longlands and Clovellysoils that could not be shown on a map of this scale. Thus,the Longlands and Clovelly soils shown on the detailed soilsmap extend further south and underlay a portion of the east-ern hillslopes which drain into the wetland and the reservoir.It appears reasonable, then, that Longlands and Clovelly soilsserve as the primary source of sand-sized sediment withinthe reservoir, particularly given the relatively steep slopes (4–7 %) upon which they occur.

The hillslopes underlain by Longlands and Clovelly soilsare primarily covered by sugar cane. This is also consistentwith the land-use based mixing model results which indicatethat the majority of the sand-sized sediment was derived fromcane fields.

Several loam-textured layers occur within the reservoircore (R1-C1) (Figs. 7 and 8). These finer-grained units weremodeled to consist of sediment primarily derived from Kat-spruit, and to a much lesser degree, Cartref, Glencoe, andAvalon soils. As would be expected, Katspruit soils are richin clay as are Avalon and Glencoe soils (although not to thedegree of Katspruit soils) (Le Roux et al., 2006).

The Katspruit soils are primarily covered by vegetablefields on relatively flat sections of the valley bottom, andthe land-use based modeling suggests that the loamy de-posits within the reservoir are primarily derived from veg-etable fields, with minor contributions from roads (with aclay-plinthic base) and cane fields (presumably underlain byAvalon or other clayey hillslope soil).

Similar texture, soil type, land-use associations occurwithin all three of the cores obtained from the wetland.These associations are particularly apparent for the lowerportions of the deposits. For example, sediments withinCore WT-C1 below approximately sample 10 exhibit a finesandy-loam texture. Modeling suggests the sediments werederived from Katspruit soil (fine component) and Clovellysoils (sand component), covered primarily by vegetables and



cane, respectively (Figs. 7 and 8). Finer grained deposits(loam textured) between samples 7–9 were derived, accord-ing to the model, from Katspruit and Westleigh soils (bothfine grained) overlain by vegetables (including Maize, whichcan be found on Westleigh hillslopes).

A detailed examination of the modeling results indicatesthat a significant change in sediment source near the mid-dle to top of the wetland and reservoir cores is superimposedon the texture, soil type, land-use association. In Core WT-C1, the change in source begins at a depth of approximately55 cm (sample 8) with a progressive increase in the contri-bution of sediment from pastures, wattles, and, to a muchlesser degree, roads. Avalon and Longlands soil contributionsalso become more prevalent. The top of Core WT-C3 (abovesample 6) also exhibits an increase in the contribution ofsediment from wattle groves, and an increase in Longlandsand Cartref soil types (the latter of which underlies wattlegroves). In Core WT-C2, sediment above sample 10 (62 cm)is derived almost exclusively from areas composed of Kat-spruit soils and vegetable fields, with a rather abrupt inputof material from roads. Further downstream in the reservoir,the change in source is characterized by an increase in sed-iment from wattle groves, and an increase in sediment fromGlencoe soils.

Interestingly, changes in sediment source coincide with anotable increase in sedimentation rates from approximately0.67 to 2.21 cm yr−1 as determined using210Pb data in CoreWT-C1. The210Pb data suggest that the change occurred be-tween approximately 1988 and 1992.

The noted changes in sediment source and sedimentationmay be related to (1) changes in land use and crop typethrough time, both in terms of the absolute area covered andtheir position on the landscape, and (2) changes in manage-ment strategies. It is more likely, however, that the alterationsare associated with a major alteration in the geomorphic con-nectivity of headwater drainages that increased sediment de-livery to the wetland and reservoir. Discussions with a localsugar cane farmer revealed that in the early 1990s, small ar-eas of maize were changed to sugar cane. This would haveinvolved contouring and water way development associatedwith cane, in order to limit off-site sediment yields.

At the time of the conversion of these maize fields tosugar cane, the valley bottom upstream of the cored wetlandalso consisted of a wetland that was consistently flooded, re-sulting in the deposition of sediment in an area which thefarmer was attempting to pasture. Thus, a ditch was exca-vated through this wetland immediately upstream of the largetributary entering from the west (and which drains the wattlegrove) (Fig. 5). The net result was an increase in geomor-phic and hydraulic connectivity that allowed drainage fromthe fields within the headwater areas of the catchment and thewestern tributary to be transported further downstream. Thechange appears as (1) an increase in sedimentation rates inCore WT-C1, and (2) an increase in sediment from the wat-tles and Cartref soils from the tributary, as well as a road that

previously limited downstream drainage. The increased con-tribution of sediment from pastures, present in Core WT-C1,is probably due to bank erosion along the excavated ditch.

Core WT-C2 exhibits a significant increase in sedimentfrom vegetable fields underlain by Katspruit soils, at the ex-pense of sediment from cane fields. Given the conversion ofsome maize fields to sugar cane upstream of the cored wet-land around 1990, the change in sediment provenance is sur-prising. However, it may be related to better sediment controlpractices on the cane fields which allowed a larger proportionof the sediment to be derived from the vegetable plots (pri-marily cabbage). It is also important to remember that sedi-ment source is texture dependent, so that the contribution ofsand-sized sediment from the cane fields was shown to in-crease within Cores WT-C3 and WT-C1 as a result of thedrainage alteration.

It is significant to note that the relative contributions tothe cored wetland and reservoir as determined from the mix-ing model cannot be quantitatively extrapolated to lower-subcatchment areas. Ideally, we would have performed theanalysis on other downstream depositional environments, butfinancial constraints, particularly the costs of analyzing alarger number of upland samples, inhibited our ability to doso. We suspect that the relative contributions of sedimentfrom the defined sediment sources will change along the val-ley to a limited degree as a result of (1) minor changes inthe spatial distribution of primary soil and land-use types be-tween the upper, middle, and lower subcatchments, (2) sed-iment storage within downstream reservoirs, and (3) differ-ences in the relative percentage of runoff contributed to thechannel from the upper, middle, and lower subcatchment ar-eas. With regards to the latter, stable isotopic data (H, O)show that between 22 and 75 % of the discharge in the chan-nel near the coring site within the lower subcatchment is de-rived from basin areas located downstream of the reservoirs.These isotopic data suggest that a non-negligible portion ofthe sediment is likely to be derived from downstream areasduring at least some flows.

While the relative percentages of sediment derived fromeach source may change along the channel, the primarilyconclusions put forth earlier are likely to apply to the en-tire catchment that was studied. These conclusions include(1) that fine-grained sediment within the wetlands and reser-voirs are primarily derived from fine-grained lowland soilsfrequently used for vegetable plots and/or, in downstream ar-eas, pastures, (2) that sand-sized sediment is geochemicallydistinct from sampled fine-grained sediment, and is predom-inantly derived from coarse-textured soils found on steepslopes, (3) that the coarser sand-sized particles are trans-ported to and through the drainage system during relativelymoderate to high magnitude runoff events, and (4) that theconstruction of a ditch through the upstream most wetlandled to a significant and abrupt change in the source of sedi-ment toall downstream areas of the drainage system.



4.3 Insights into geomorphic connectivity from othergeochemical tracers

Several trace elements provide useful information regardingthe downvalley transfer of sediment within the entire sectionof the studied catchment. The two of most importance areCu and Zn. Both elements are contained in fertilizer knownto have been used on vegetable fields (cabbage, maize) pre-dominantly located within the upper subcatchment (Fig. 2).In fact, the utilized fertilizer is reported (on its bag) to contain2.5 % Zn. The potential impact of the fertilizer on Zn concen-trations in the soil is illustrated by comparing the amount ofZn within pasture and vegetable fields underlain by the samesoil type (Katspruit). The pasture samples exhibit a meanZn concentration of 3.58 µg g−1, compared to a concentra-tion of 139 µg g−1 for the vegetable plots, the latter higher bytwo orders of magnitude (Fig. 10). In addition, Zn concentra-tions within soils of the vegetable plots are much higher thanis generally observed for uncontaminated bedrock (16–105µg g−1) or soils (60 µg g−1), (Turekian, 1971; Buonicore,1996; Miller and Orbock Miller, 2007). Similar trends arefound for Cu, although differences between background ma-terials and the vegetable plots are not as significant (Fig. 10).

Cu and Zn concentrations within the wetland cores varysystematically with depth, but the trends are distinctly differ-ent (Fig. 11). Variations in observed trends can be explained,however, by differences in sediment provenance. In CoreWT-C1, for example, both Cu and Zn concentrations increasefrom the bottom of the core toward the surface (from sam-ples 16 to 7). The concentrations then abruptly decrease by afactor of 5 before remaining relatively constant until reach-ing the ground surface. The change in concentration is coin-cident with the observed increase in sedimentation rates (dis-cussed early), and a change in sediment sources documentedby the mixing model. More specifically, Zn and Cu con-centrations tend to increase as contributions from vegetablefields increase, and decrease as contributions from pasturesand cane fields increase (compare Figs. 8 and 11). The influ-ence of sediment provenance on Cu and Zn concentrationswithin the cores is illustrated more directly in Fig. 12. Withthe exception of three outliers (discussed below), there is aweak tendency for Zn and Cu concentrations to increase asthe modeled contribution of sediment from vegetable fieldsincreases. In contrast, inverse relations exist for cane and pas-ture. The dramatic decrease in concentration above sample 7in Core WT-C1 can therefore be explained by (1) increasingcontributions of sediment from pasture and cane fields, and(2) higher rates of sand-sized particle sedimentation whichpresumable exacerbated the effects of dilution on Cu and Znconcentrations.

In contrast to Core WT-C1, contributions of sediment fromvegetable fields in Core WT-C2 increase toward the surface(decreasing age) above sample 12 (Fig. 11). As expectedfrom the paragraph above, concentrations of Cu and Zn in-crease as the contributions of sediment from vegetable fields

Fig. 10. Mean concentrations of Cu and Zn calculated for(a) up-land soil and(b) land use. Dashed lines represent global averageCu (blue) and Zn (orange) concentrations within soil reported byBuonicore (1996).

increase. It is also notable that the lowest Cu and Zn con-centrations are associated with sample 12 which the sourcemodel suggests contains the most sediment from the canefields.

The indirect relationship between Cu and Zn concentra-tions and the relative contribution of sediment from pas-tures is understandable given the limited use of fertilizer onpastures. However, the indirect relationship between Cu–Znconcentrations and sediment from cane fields is surprisinggiven the relatively high mean concentrations of the two el-ements in cane field samples (Fig. 10b). This inverse rela-tionship may be related the use of fertilizers on maize fieldswhich were later converted into cane. This hypothesis issupported by (1) highly variable Cu and Zn concentrationswithin samples collected from the cane fields, and (2) Cu andZn concentrations in a few samples that exceed those typi-cally found in soil and bedrock (Fig. 10). The hypothesizedinflux of sediment to the wetland from previously fertilizedcane fields with high Cu and Zn concentrations would alsoexplain the outliers on Fig. 12 (high Cu and Zn with no sig-nificant input of sediment from vegetable fields, and high Cuand Zn with high input from cane fields and pastures).

The difference in Cu and Zn concentrations in Longlandssoils is interesting as the parent material for it is thoughtto be the same as that for the other soil types in the catch-ment (Natal Group Sandstones). Moreover, if the high Cuconcentrations consistently observed for Longlands soil sam-ples were related to fertilizer, high Zn concentration wouldalso be expected as shown on Fig. 11. It is unclear at this



Fig. 11.Variations in Cu and Zn concentrations with depth in wetland Cores WT-C1(a), WT-C2 (b), R1C1(c), and B2WTC1(d).

time why such large differences exist, when they do not forthe other soil types. It is possible, however, that the sandynature of Longlands soils, combined with their occurrenceon relatively steep slopes, allowed the more mobile Zn to beleached from the sampled surface sediments.

Figure 11 shows that Cu and Zn concentrations are rel-atively low from the bottom of Core R1-C1 (sample 21 tosample 9). Concentrations of both elements above sample 9are generally 3 to 5 fold higher. The change in concentrationsis roughly coincident with the modeled change in sedimentprovenance that was attributed earlier to the construction of adrainage ditch through an upstream wetland. In other words,higher Cu and Zn concentrations appear to result from an in-crease in system connectivity and the capacity for sedimentderived from headwater vegetable fields and other sedimentsources to be transported downstream through the wetlandsand to the reservoir.

Interestingly, Cores WT-C2 and B2WTC1 exhibit similardepth trends in Cu and Zn concentrations to that observedfor approximately the top third of R1-C1 (Fig. 11b–d). Con-centrations are high at the surface and then systematicallydecrease with depth before increasing further down the core.The primary difference is that the abrupt decrease in Cu andZn concentrations (and their subsequent uniform distribu-tion) observed at depth within Core R1-C1 is not presentin the other two cores. The zone of relatively low Cu andZn concentrations corresponds sedimentologically to layerscontaining significant amounts of sand-sized sediment whichthe source modeling indicates was derived in part from canefields. The Cu- and Zn-enriched horizons are finer grainedand derived predominantly from vegetable plots in Cores

WT-C2 and R1-C1 (as noted earlier, source modeling wasnot performed on Core B2WTC1 because it was located welldownstream of the sampled upland sediment sources). Thesimilarities in depth trends in concentration suggest that allthree locations, spanning the entire study catchment, receivedsediment with similar Cu and Zn concentrations since about1990. Because these two elements are primarily associatedwith soil amendments used on vegetable fields concentratedin headwater areas, the observed similarities in elementalconcentrations as a function of depth within cores from theupper subcatchment wetland, the middle subcatchment reser-voir, and the lower subcatchment riparian wetland suggestthat sediment was transported through the drainage systemduring runoff events. Thus, some degree of geomorphic con-nectivity must have existed. It therefore appears that follow-ing the construction of the upstream drainage ditch throughthe upstream wetland, the axial drainage network was geo-morphically and hydrologically connected.

4.4 Sediment sources, runoff magnitudes, and basinconnectivity

Geochemical provenance studies show that the source of sed-iment deposited within wetlands of the upper subcatchment,and a reservoir from the middle subcatchment, varies as afunction of sediment size, stratigraphic layer, and time ofdeposition. Fine-grained sediment within both depositionalsettings was primarily derived from vegetable fields under-lain by fine-grained soils (e.g., Katspruit) that comprise thevalley floor. It seems reasonable to assume that these sedi-ments were delivered to the axial drainage network duringrelatively modest storm events when hillslope runoff over



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Fig. 12. Relationship between Cu and Zn concentrations and per-cent relative contribution from vegetable and pasture plus canefields.

sandier soils with higher infiltration capacities was limited(Fig. 13). In other words, the lack of sandier sediments fromcane fields on hillslopes within the wetland deposits suggeststhat the hillslopes were not significantly eroded during rel-atively low-magnitude events. Alternatively, what sedimentwas eroded was redeposited on the hillslopes as a result ofthe utilized management practices.

Very little of the fine-grained sediment from low-lyingvegetable fields was deposited and stored with the first down-stream reservoir of the middle subcatchment. The generallack of fine-sediment within the reservoir is presumablyrelated to (1) the hydrologically and geomorphologically dis-connected nature of the drainage network during low-flowconditions (prior to valley floor modification), and (2) the

minimal impact of the dams on the storage of silt- and clay-sized particles during larger events when the system is hy-drologically connected. The lack of influence of the dams onsediment storage is not surprising given that water overflowsthe dams during periods of high surface runoff, creating arapidly flowing system through the reservoir, and the increasein sediment transport capacity of the middle subcatchment(as described below).

Sandier sediments within both the wetland and the reser-voir were derived largely from hillslope cane fields. Presum-ably, these sediments were not only eroded from the val-ley bottom sediment sources, but from sandier hillslope soilscovered largely by sugar cane during larger storm events thatproduced significant runoff. What is important to recognizeis that the provenance of the sediment within the examineddepositional environments varied as a function of both sedi-ment size and runoff magnitude. Moreover, given the chem-ically reactive nature of fine-particles, and the association ofnutrients, particularly phosphate, with sediment, reductionsin sediment-associated nutrient loads may best be soughtthrough practices that address the erosion of sediment fromagricultural fields along the valley floor.

The above provenance studies, combined with data fromthe geomorphic investigations, show that the MkabelaBasin, and presumably other similar catchments within theKwaZula–Natal Midlands, can be subdivided into three ge-omorphologically distinct subcatchments. These subcatch-ments vary in relief, the nature of their drainage network(or process zones) and their ability to store and transportsediment. As a result, sediment transport and storage donot systematically vary along the axial drainage system, butare characterized by spatially abrupt changes in their natureand magnitudes. In headwater areas with intact valley floors,sediment eroded predominantly from low-lying areas duringlow-magnitude events are largely deposited within wetlandsthat comprise large segments of the valley floor. The gen-eral lack of fine sediment within the reservoir, prior to valleymodification, suggests that while the axial drainage networkmay be integrated during large floods, during low to moder-ate events the upper catchment areas were disconnected fromdownstream sections of the catchment (Fig. 14). Thus, thewetlands (in their natural state) serve as reservoirs of sedi-ment (and associated nutrients).

In contrast to the upper subcatchments, the mid-catchmentareas are dominated by relatively high gradient alluvial andbedrock channels, with fewer, natural depositional zones.This portion of the catchment, then, possesses a greater abil-ity to transport sediment downstream effectively (Fig. 14).The general lack of fine sediment within the reservoirs indi-cates that once silt- and clay-sized sediment is entrained, it istransported through this section of the catchment, althoughat least some of the transported material may be stored onor within floodplains that are more extensive than they areupstream.



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Fig. 13.Schematic diagram of the primary processes occurring in each of the three delineated subcatchments, and the variations in sedimentsize and source from varying runoff magnitudes.

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Fig. 14. Schematic diagram illustrating differences in geomorphic connectivity and sediment storage between subcatchment zones of theMkabela Basin.

The lower subcatchment is dominated by low gradient, al-luvial channels boarded by extensive riparian wetlands. Stor-age of sediment within this zone is extensive, as illustrated byCore B2WTC1, once again limiting the downstream transla-tion of sediment and nutrients that they may carry (Fig. 14).The natural division of the catchment into geomorphologi-cally distinct sections suggests that previously developed wa-tershed modeling routines that are used to predict sedimentexports from a basin may need to be modified before beingapplied to these catchments adequately to address the abruptchanges in sediment transport processes that occur.

5 Conclusions

The source, transport, and storage of sediment was evaluatedwithin the Mkabela Catchment using a combination of geo-morphic, hydrologic, and geochemical tracing analyses. Theintegrated approach resulted in the following conclusions.

1. The Mkabela Basin, and other similar catchmentswithin the KwaZulu–Natal Midlands, exhibit three dis-tinct morphological areas (referred to as upper, mid-dle, and lower subcatchments), each characterized bydifferences in its ability to produce, transfer, and store



sediment. Upper catchment areas are characterized byextensive wetlands along the valley floor which, intheir natural state, represent significant sediment sinks.Mid-catchment areas are characterized by less exten-sive riparian wetlands, and possess higher-gradient allu-vial and bedrock channels conducive to sediment trans-port. Lower catchment areas are characterized by lower-gradient alluvial valley floors, incised into the surround-ing terrain, that possess broad riparian wetlands thatstore considerable quantities of sediment and associatednutrients.

2. The complex interactions between runoff, soil type andcharacteristics, and land use (among other factors) cre-ate temporal and spatial variations in sediment prove-nance. Silt- and clay-rich layers found within the wet-land and reservoir deposits are derived from the ero-sion of fine-grained, valley bottom soils which are fre-quently utilized as vegetable fields. The deposits tendto exhibit elevated concentrations of Cu and Zn, pre-sumably from the use of fertilizers which contain bothelements. Coarser-grained sediments within the wetlandand reservoir environments are derived from the erosionof sandier hillslope soils extensively utilized for sugarcane. Erosion of these upland cane fields presumableoccurs during relatively high magnitude runoff eventsthat are capable of transporting sand-sized sediment offthe slopes. Therefore, sediment source varies as a func-tion of particle size and runoff magnitude.

3. Sediment source determination, carried out on multiplecores from the wetland, demonstrated that sediment par-titioning by particle size occurs during transport pro-ducing deposits of varying sedimentological and chem-ical characteristics. While general provenance charac-teristics (e.g., the fact that fine sediment was derivedfrom specific soils used primarily for vegetable fields)were similar between the cores, differences in sedi-ment provenance as a function of depth (time) exist.As a result, within highly variable depositional environ-ments, multiple cores should be collected and analyzedfully to determine sediment provenance from non-pointsources.

4. Comparison of sediment provenance between sites andthrough time revealed that the construction of a drainageditch through the upstream-most wetland significantlyaltered the geomorphic and hydrologic connectivity ofthe catchment. Prior to its construction, sediments (andthe nutrients that they carry) were largely depositedwithin wetlands which encompassed a majority of thevalley floors within the upper catchment. Thus, up-per catchment areas were disconnect

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