Interactions between stream channel incision, soil ...

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Interactions between stream channel incision, soil water levels and soil morphology in a wetland in the Hogsback area, South Africa a

b

a

Mohammed Y Omar , Pieter AL Le Roux & Johan J van Tol a

Department of Agronomy, University of Fort Hare, Alice, South Africa

b

Department of Soil, Crop and Climate Sciences, University of the Free State, Bloemfontein, South Africa Published online: 07 Nov 2014.

To cite this article: Mohammed Y Omar, Pieter AL Le Roux & Johan J van Tol (2014): Interactions between stream channel incision, soil water levels and soil morphology in a wetland in the Hogsback area, South Africa, South African Journal of Plant and Soil, DOI: 10.1080/02571862.2014.944593 To link to this article: http://dx.doi.org/10.1080/02571862.2014.944593

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SOUTH AFRICAN JOURNAL OF PLANT AND SOIL

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ISSN 0257-1862 EISSN 2167-034X http://dx.doi.org/10.1080/02571862.2014.944593

Interactions between stream channel incision, soil water levels and soil morphology in a wetland in the Hogsback area, South Africa Mohammed Y Omar1, Pieter AL Le Roux2 and Johan J van Tol1* Department of Agronomy, University of Fort Hare, Alice, South Africa Department of Soil, Crop and Climate Sciences, University of the Free State, Bloemfontein, South Africa * Corresponding author, e-mail: [email protected] 1

Downloaded by [Johan van Tol] at 23:26 09 November 2014

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Wetland degradation in the form of channel incisioning can significantly alter the hydrological functioning of a wetland. In this study in a small headwater wetland in the Hogsback area, Eastern Cape province, the impact of channel incisioning on soil water levels and soil morphology was examined. A good correlation (R 2  0.89) existed between the depth of channel incisioning and average water-table depths in most of the 21 installed piezometers. In localised cases the upslope supply of water was in equilibrium with drainage from the piezometers. Although all the studied soils showed hydromorphic characteristics, those continuously saturated close to the surface exhibited redox accumulations in oxygen-supplying macropores, whereas gleyic colour patterns occur deeper in soils where the water table has been lowered by channel incision. The nature and occurrence of different hydromorphic soil indicators observed confirmed the contribution of soil morphology as a valuable indicator of long-term averaged soil water conditions. Keywords: hydromorphic properties, water regime, wetland hydrology

Introduction Wetlands are defined as ‘…land which is transitional between terrestrial and aquatic systems where the water table is usually at or near the surface, or the land is periodically covered with shallow water…under normal circumstances’, where the ‘normal circumstances’ refer to environmental conditions without anthropogenic interference (Republic of South Africa 1998). Although hydrology is normally seen as the force behind the creation and maintenance of wetlands, it is considered the least useful delineator in wetland characterisation due to its dynamic nature that varies daily, seasonally and yearly (Ingram 1983). Long-term hydrometrical measurements of water levels are absent for most wetlands, and indirect indicators, such as soil morphology and vegetation, are normally used to identify wetlands (Grundling 1999). The process of soil formation is relatively long (102 to 104 years) and properties related to soil morphology are unlikely to change over decades, even with anthropogenic modifications such as artificial drainage (MacEwan 1997). As opposed to vegetation, soil morphology can therefore be used to delineate and identify wetlands whose hydrology has been altered by anthropogenic impacts (Mausbach and Richardson 1994). Hydromorphic soil properties relevant to wetland classification include the following: organic carbon (OC) accumulation (10% OC); peat layers; grey matrix colours (chroma  2); high chroma mottles in grey matrix; low chroma mottles (10%) in high chroma matrix; and oxidised root channels (Mitsch and Gosselink 1993; Vepraskas et al. 1994; Vepraskas and Faulkner 2001;

Vepraskas and Lindbo 2012). These morphological properties should occur within 500 mm from the surface in order for the soils to classify as wetland soils. Hydromorphic soil properties related to wetlands are formed under anaerobic conditions. Anaerobicity occurs when (1) there is organic matter present, (2) microorganisms are actively oxidising organic material, (3) the soil is saturated and (4) dissolved oxygen is removed from the pores (Vepraskas et al. 2012). When one of these four conditions is not present, hydromorphic properties in soils will not form. Although hydromorphic properties indicate that anaerobic conditions existed, these do not indicate the duration of saturation and reduction (Vepraskas and Lindbo 2012). Comparision in terms of the relative duration of saturation can, however, be conducted in terms of the prominence of various hydromorphic properties in a given area (Lindbo et al. 2010). The distribution of hydromorphic properties depends on the distribution of oxygen in the soil and it is controlled by the depth and duration of the water table and aeration of the soil through interpedal and biopores (Veneman et al. 1976; Vepraskas et al. 1976). The rapid and continuous loss and degradation of wetlands threaten human well-being through the loss of goods and services provided by these ecosystems (Millennium Ecosystem Assessment 2005). These services include lodging of a large and diverse number of animal and plant species (Gislason and Russell 1997), water purification by trapping sediments and excessive nutrients and heavy metals (Mitsch and Gosselink 1993) and water

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Omar, Le Roux and van Tol

storage, thereby enhancing groundwater recharge and prolonging baseflow as well as reducing peak flows (Kotze et al. 2005) and carbon sequestration (Badiou et al. 2011). Channel incision is one of the major causes of wetland and river ecosystem degradation (Naiman et al. 2005; Steiger et al. 2005; Loheide and Booth 2011). With the lowering and widening of the streambed, wetlands are often dewatered as groundwater will flow towards incised streams as opposed to parallel to unincised streams (Shields et al. 2009, 2010). Water flow through incised wetlands is concentrated to the stream channel and high turbidity flow with high erosive energy are often recorded (Shields et al. 2009). Increased sediment loss and reduced base flows results in increase in pollution levels, degradation of the physical habitat and consequently a reduction in biodiversity (Shields et al. 2010). Channel incision can be caused by direct channelisation and straightening of stream networks or by land-use change that result in an enhanced peak discharge (overland flow) or reduced sediment in discharging the water (Shields et al. 2010). In this study the influence of channel incision on water levels and soil morphology were studied on a stream with varying degrees of channel incision. This research addressed two important questions: (1) how did the degree of channel incision influence water levels? (2) Were changes in the water regime evident in the soil morphology? Study area and methodology The study area is a wetland of approximately 5 ha at the foot of Gaika’s head (1 963 m above sea level) in the Hogsback region (Figure 1). The site is the property of Amathole Forest Company. Hogsback has a cool climate with mean annual temperatures of approximately 14 °C. Cold winters with mean minimum temperatures of 1 °C and frequent snowfall are characteristic of this region. The mean annual rainfall is approximately 1 200 mm, the bulk of which falls during summer. The geology of the area is sedimentary

rocks of the Balfour formation, part of the Beaufort Group (Coleman 1999). The wetland is densely vegetated with grass and some shrubs with some of the dominant species being Restio spp. and sedges, Carex and Pycreus, with ground orchids commonly occurring (Coleman 1999). A first-order tributary to the Klipplaats River (which is a tributary to the Swart Kei River), with various degrees of incision, drains the wetland in a south-eastern direction over a rehabiltation weir where streamflow was recorded (Figures 1c and 2). Rainfall was recorded with a David rain gauge (0.2 mm tipping bucket) and temperature with a Barologger Edge (Solinst), both installed next to a rehabilitation weir in the south-eastern corner of the study site (Figure 2). Rainfall for the study period is presented as daily totals (mm) and temperatures as daily minimum and maximum (Figure 3). The study period stretched over approximately five months, commencing at the end of the wet rainy season (20 March 2013) throughout the relatively dry winter, and extended and ending on 16 August 2013). During the study period a total of 344 mm rain was recorded, the average maximum temperatures was 17.5 °C and the average minimum temperature was 0 °C (Figure 3). The depth of channel incision was directly measured with a tape measure for the main stream channel. The channel was then grouped into three classes based on the degree of channel incision (Figure 2): Severe, where the channel was incised deeper than 60 cm; Moderate, when the channel was incised between 10 and 60 cm; and None, where no significant incision was measured (10 cm). The impact of expansion of the road network, to facilitate afforestation, on surface hydrology is considered to be the major cause of channel incision in the Hogsback area. Afforestation around the site commenced around 1984 and roads concentrating overland flow might have altered the natural streamflow regime resulting in channel incision (SANBI 2012).

26°57ƍ30Ǝ E

AFRICA

EASTERN CAPE

SOUTH AFRICA

Hogsback

South Africa 32°33ƍ30Ǝ S

32°33ƍ30Ǝ S

26°57ƍ30Ǝ E

Figure 1: Location of the study area and instrumental layout in the study area

EASTERN CAPE

INDIAN OCEAN

South African Journal of Plant and Soil 2014: 1–8

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system for South Africa (Soil Classification Working Group 1991). Of special interest were micromorphological properties such as iron (Fe) accumulations and depletions and where these occur. Samples were taken at 10-centimetredepth intervals and analysed for OC following the Walkley– Black method (Walkley and Black 1934). All statistical analyses were conducted using XLSTAT version 2013.6.01 (Addinsoft, Paris, France, 1995–2013). To simplify representation and interpretation of the data the 21 piezometer sites were divided into nine groups of similar taxonomic class, channel incision, water levels and hydromorphic properties. These groups are presented in Table 1.

Figure 2: Experimental layout and degree of channel incision in the study area (none = incision < 10 cm; moderate = incision 10 – 60 cm; severe = incision > 60 cm)

Rainfall (mm)

Measurements

Maximum temperature

Minimum temperature

60 50 RAINFALL


A total of 29 piezometers were installed on the study site, marked HP1 to HP29 in Figure 2. The piezometers are 55-millimetre-diameter PVC pipes partially slotted at the bottom, following the methodology of Sprecher (2000). The piezometers were installed 1 m from the channel to the depth of refusal (bedrock) at different localities to capture the variation in of the influence of the degree of channel incision on water levels (Figure 2). Water levels in all the piezometers were manually recorded during field visits (Figure 3). Only data from the piezometers directly next to the stream channel (21 piezometers) were reported in this study. The soils at the 21 piezometer sites were described and classified in accordance with soil clasification, a taxonomic

40 30 20 10 20 25 30 04 09 14 19 24 29 04 09 14 19 24 29 03 08 13 18 23 28 03 08 13 18 23 28 02 07 12

Mar

Apr

May

Jun DATE 2013

Jul

Aug

Figure 3: Dates of field visits, daily rainfall and minimum and maximum temperatures recorded during the study period

HP 6

HP16/HP17

HP28/HP29

HP1/HP4/ 70/70/55/100 54/66/57/62 HP18/HP22

HP25/HP26

HP 20

HP21/HP23/ HP27

3

4

5

6

7

8

9

12/9/7

81

69/68

41/23

15/31

15

1/3/1

56

49/53

34/36/42/44

27/21

10/24

4

1/7

8/6/3

70

58/60

47/57/50/55

35/22

13/27

11

4/9

2

1

OC  Organic carbon ot, Orthic A horizon; gs, E horizon; gc, G horizon; sp, Soft plinthic B horizon 3 Kd, Kroonstad; We, Westleigh; Cf, Cartref

120/95/90

95

80/88!

53/53

40/40

40

7/9

HP19/HP24

2

20/29

HP7/HP10/ HP11/HP12

1

Water-table depth (cm) Maximum Minimum Average 1/1/1/6 1/0/1/2 1/1/1/3

Piezometer

Group

Channel incision (cm) 5/2/2/6

0–30 30–50 50

0–30 30–60 60–100

0–30 30–70 70–100

0–40 40–90 90–110

0–30 30–60

0–20 20–40

0–40 0–60 60–160

0–20 20–40

Depth (cm) 0–40 40–80 80–140

3.06 2.95 –

2.55 2.20 2.20

2.71 2.58 2.43

2.48 2.38 2.28

2.71 2.61

2.33 2.15

3.62 3.52 3.43

3.11 2.97

OC1 (%) 3.97 3.60 2.69

10YR 3/2 10YR 4/1 –

10YR 3/1 10YR 4/3 10YR 3/2

10YR 3/1 10YR 3/1 10YR 2/2

10YR 3/1 10YR 3/1 10YR 3/1

10YR 4/1 10YR 4/2

10YR 3/1 10YR 2/2

10YR 3/2 10YR 3/1 10YR 3/2

10YR 3/4 10YR 2/1

10YR 5/2 10YR 6/1 –

10YR 5/2 10YR 6/2 10YR 6/3

10YR 5/1 10YR 7/1 10YR 5/2

10YR 5/1 10YR 6/1 10YR 4/1

10YR 6/2 10YR 7/2

10YR 5/1 10YR 6/2

10YR 5/2 10YR 6/1 10YR 5/2

10YR 6/4 10YR 5/1

Munsell colour Moist Dry 10YR 3/2 10YR 6/2 10YR 3/1 10YR 7/2 10YR 3/1 10YR 6/2

Table 1: Vertical channel incision, water table characteristics and soil morphological properties for the different piezometers

Few; medium; brown Few; medium; brown –

None Many; medium; red Few; medium; grey

None Few; small; brown Few; small; grey

None Few; small; brown Few; medium; grey

None Few; large; brown

Many; small; brown Few; small; brown

Few; small; red Many; small; brown Few; small; brown

Few; small; red Few; medium; grey

Mottles: frequency; size; colour None None Few; medium; brown


Rusty – –

Bleached BleachedRusty –

Bleached Rusty –

Bleached BleachedRusty –

Bleached Rusty

Rusty BleachedRusty

Rusty – –

Rusty –

– – –

Root channels

ot gs gc

ot gs gc

ot gs gc

ot gs gc

ot gs

ot gs

ot gs gc

ot sp

Kd

Kd

Kd

Kd

Cf

Cf

Kd

We

Diagnostic Soil horizon2 form3 ot Kd gs gc

4 Omar, Le Roux and van Tol


5

Results

Soil water levels Piezometer groups installed near severely incised channels (groups 1, 6 and 8) were associated with deeper maximum, minimum and average water-table depths when compared to those of moderately incised (groups 2, 3, 4 and 5) or group 1 with limited incision (Table 1). An exception to this was the piezometers of group 9. The relationship between average water-table depth and depth of channel incision is graphically illustrated in Figure 4. When piezometers of group 9 (Table 1) are excluded from the correlation, a good linear relationship (R 2  0.89), with a Pearson correlation coeficient of 0.94, between water-table depth and depth of channel incision existed. Piezometers of group 9 were installed next to some of the most severely incised parts of the stream. The fluctuation in the water-table depth measured in these piezometers were, however, minimal and water tables remained close to the surface throughout the study period. The period between field visits on 24 April and 2 May was marked by little rain (1 mm) following two relatively large rainfall events, i.e. 26 and 32.8 mm, on 20 and 21 April, respectively (Figure 3). This period was selected to illustrate differences in drainage rates from different piezometer groups.

AVERAGE WATER TABLE DEPTH (cm)


The groups of piezometers, the depth of channel incision, minimum, maximum and average water contents over the study period, OC contents, selected soil properties for different diagnostic horizons and soil forms for the piezometers are presented in Table 1.

Other groups

70 60 50

Group 9

y = 0.72x í 4.90 R 2 = 0.81 MSE = 121.58 RMSE = 11.03

40 30 20 10 20

40 60 80 100 CHANNEL INCISION (cm)

120

Figure 4: Relationship between average water-table depths (cm) and channel incision for 18 piezometers

The greatest change in the water-table depth was observed in groups 6 and 7 (Table 2). Both these groups were marked by severe channel incision (Table 1, Figure 2). The water table in group 9, with deeply incised channels, did not change during the 8 d period. The water table remained close to the surface (1 cm on average) in the piezometers of group 1, with virtually no channel incision, as well. The piezometer groups reflecting moderate incision showed slight decreases during the 8 d drying period (Table 2). Soil morphology The soils were classified as either Kroonstad (Kd) or Cartref (Cf) soil forms (Soil Classification Working Group 1991). In the Kd an orthic A horizon (ot), rich in OC, overlies an E horizon (gs), which overlies a G horizon (gc). This soil is similar to a Gleysol (IUSS Working Group WRB 2006). On shallower soils (piezometer groups 2, 4 and 5) a gc horizon did not form and the gs below the ot directly overlies impermeable bedrock. The moist colour of the soils was generally dark, with Munsel colour notation values of 3 or less dominating, only bleached in the dry state. The dry colours were without exception ‘grey’ when considering the definition of grey soil colours in the Soil Classification Working Group (1991). There was not a clear relationship between the moist or dry colour and the degree of channel incision, water-table characteristics or even the OC content (Table 1). There was, however, a decreasing relationship (Pearson correlation coefficient −0.71) between the average OC content of the topsoils of the different piezometer groups and the average water-table depths of the respective groups (Figure 5). The OC content for the topsoil of group 3, associated with limited incision, was the highest at approximately 4%, with groups 4, 6 and 8 being the lowest at around 2.5%. The OC contents of the soils were relatively high with a very small decrease with depth (Table 1). Brown, red and grey mottles occured in the soils of the study area (Table 1). Grey and red mottles occurred in a distribution pattern related to interpedal and biopores. Grey mottles occured in the reduction morphology of subsoil horizons (gs and gc), whereas red mottles occurred in the ot horizons of selected groups and gs horizons of group 6. Brown mottles occurred frequently, but their distribution appeared to be random. Bleached root channels were predominant in topsoils of piezometer groups with relatively deep water tables, i.e. group 5, 6, 7 and 8 (Figure 6a), as well as in gs horizons that were periodically saturated (groups 4, 6 and 8). In the latter these occurred together with rusty root channels (Figure 6b and c). Rusty root channels occurred in topsoils where the water table was relatively close to the surface

Table 2: Change in water-table depth (cm) of nine piezometer groups during eight dry days

Date 24 April 2013 2 May 2013  Depth (cm)

1 1 1 0

2 6 8 2

3 9 14 5

4 20 23 3

Water-table depth (cm) 5 6 31 50 32 58 1 8

7 54 63 9

8 79 81 2

9 6 6 0

6


TOPSOIL OC (%)

4.0

(a)

(b)

(c)

(d)

3.5 3.0 2.5 2.0 y = í0.017x + 3.431 R 2 = 0.504 MSE = 0.177 RMSE = 0.421

1.5 1.0 0.5

10 20 30 40 50 60 70 AVERAGE WATER TABLE DEPTH (cm)


Figure 5: Average water table depth (cm) vs topsoil organic content (%) for different groups

(groups 3, 4 and 9) as well as in gs horizons within the region of saturation. Reduction and depletion indicators on the root channels were absent from gc horizons as well as from gs horizons that were continuously saturated during the study period (groups 1 and 9) and from the ot of group 1. Oxidised Fe (orange water) was flowing on the surface at HP10 of group 1 (Figure 6d). Discussion Water levels Water levels are controlled by the balance between addition and removal of water. Both these factors are controlled by the permeability of the soil and difference in hydraulic head. The relationship between depth of water tables and depth of erosion gully is in line with the understanding (see Shields et al. 2009, 2010) of changing hydraulic head by lowering the outlet (Figure 4). However, the response of the observations in group 9 does not relate to the degree of drainage, implying localised hillslope aquifers supply water at a high rate, balancing the drainage. The constant water levels in the piezometers of group 9 after eight rain-free days (Table 2) suggest that the supply and release of water from these piezometers were in equilibrium. Localised variation in the supply of water by the hillslope can be explained by fast return flow by the fracture system. This impact of the fractured rock characteristics on the hydrologic response of the soil is expected to be more pronounced in shallow soils and buffered by deeper soils and larger wetlands. Soil morphology The formation of soil horizons under water-saturated conditions is expected to be limited. The standard concept of the formation of soil horizons requires water flowing from the surface downwards to the saprolite. In spite of stagnant water conditions, ot and gs horizons are visible in both soils and in the Kroonstad soil a gc horizon is also distinguishable. Under saturated conditions and more so under subaqueous conditions in the wet season, water cannot infiltrate surface horizons. It can only enter the profile from

Figure 6: Related distribution of grey and red mottles. Green arrows indicate grey zones of Fe depletion (a, b and c), yellow arrows indicate red zones of Fe accumulation (b and c) and iron oxide-rich water flowing over the surface at HP10 of group 1 (d)

the underlying saprolite profile by upward flow (capillary rise) to supply evapotranspiration losses and stream flow (van Tol et al. 2010). The diffuse transitions between horizons (see uniform colours of horizons in Table 1) of these soils are typical of wetland soils with long duration of saturation (van Huyssteen et al. 2005) and probably related to an upward water flux. The very homogeneous morphology is typical of wetland soils indicating long duration of conditions at or near saturation, implying a water table at or near the soil surface. Under stagnant saturated conditions, homogeneous reducing conditions are expected to stabilise in the soil including the matrix and pores. Typical of chemical homogeneous conditions is diffuse transitions between limited colour variations in the matrix. This explains the bleached and grey colours of the ot, gs and gc horizons of both soil forms. It must be taken into consideration that most processes of soil formation is active in most soils but that the dominant process is responsible for the dominant morphological feature. Although both gleyic and stagnic colour patterns (IUSS Working Group WRB 2006) occur in these soils they dominate in some horizons. A gleyic colour pattern, i.e. ferrans observed as iron accumulations in red pore linings (interpedal and biopores) in soils with shallow water tables represented by observation groups 4, 6 and 9, are indicative of conditions near saturation (Veneman et al. 1976). The distribution of iron from the matrix to the pore linings is driven by selective supply of oxygen to the soil through macropores. Above the water level near saturation of the soil matrix for significant periods results in reduction of iron in the soil matrix and precipitation of iron in the oxygen-supplying macropores. It also explains the presence of red pore linings as rusty root channels forming



in periodically saturated gs horizons of observation groups 3, 4 and 8, which can form when the water table is slightly lowered. Where recent erosion lowered the water table, gleyic colour patterns occur deeper in the soil related to the current water-table level as the zone of aeration moved lower down the profiles. More aerated topsoils of soils with lower water tables are more suitable for vegetation growth and simulates root development and development of reducing conditions in the biopore while oxygenated matrix conditions prevail. Albans of depleted iron and a stagnic colour pattern, i.e. grey pore linings as recorded in observation groups 5, 6, 7 and 8 where root residues enhance microbiological activity in root pores, reduce the environment, dissolve and deplete the iron to form bleached pore linings to precipitate in an oxygenated soil matrix. The duration of the process limits the pronounced presence of iron precipitation as a quasi-ferran recorded in mature soils (Le Roux and du Preez 2004). The high OC contents is well above the average OC contents of Kroonstad and Fernwood soils and indicative of a constantly waterlogged soil-water regime more like that of Champagne soils (Le Roux al et. 2013). The generally dark colours of all horizons support the statement. The small decline in OC content with depth is not normal of mineral soils. It can be due to oxidation caused by lowered water-table conditions enhancing oxidation of humus in the surface horizon. Alternatively, it may indicate the formation of organic soil characteristics. Conclusions The depth of soil water levels correlates well with the degree of channel incision for most piezometers. Constant supply of water to piezometers of group 9 was in equilibrium with drainage from these piezometers. Soil morphology is a useful indicator of ancient and recent soil water regimes of wetland soils. Although soil morphology is generally considered an ancient indicator of the soil water regime and may be out of phase with current terrain, climate and hydrology, the colour variation observed in the studied soils serves as a sensitive indicator of recent changes and by implication confirm the contribution of soil morphology as a valuable indicator of long-term averaged soil water conditions. Future studies should focus on quantification of the redox-potential of the water in relation to soil morphological properties. Acknowledgements — The authors acknowledge the Govan Mbeki Research and Development Centre (GMRDC) for funding this research as well as the National Research Foundation (NRF) for a bursary to the first author. Amathole Forest Company is also thanked for supplying the research site.

References Badiou P, McDougal R, Pennock D, Clark B. 2011. Greenhouse gas emissions and carbon sequestration potential in restored wetlands of the Canadian prairie pothole region. Wetlands Ecology and Management 19: 237–256. Coleman JA. 1999. Sustainable management in a disturbed environment: a case study of the Hogsback Working for Water project. Unpublished MSc thesis, University of KwaZulu-Natal, Pietermaritzburg, South Africa.

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Gislason GM, Russell RC. 1997. Oviposition sites of the saltmarsh mosquito, Aedes vigilax (Skuse) (Diptera: Culicidae), at Homebush Bay, Sydney, NSW—a preliminary investigation. Australian Journal of Entomology 36: 97–100. Grundling PL. 1999. Challenges for the IMCD in the conservation of mires and peat lands in southern Africa (and other developing countries). Pretoria: Council for Geoscience Geological Survey. Ingram HAP. 1983. Hydrology. In: Gore AJP (ed.), Ecosystems of the world 4A. Mires: swamp, bog, fen and moor. General studies. New York: Elsevier Scientific Publishing Company. pp 67–118. IUSS Working Group WRB. 2006. World reference base for soil resources 2006. World Soil Resources Reports no. 103. Rome: Food and Agriculture Organization of the United Nations. Kotze DC, Marneweck GC, Batchelor AL, Lindley DS, Collins NB. 2009. WET-EcoServices: a technique for rapidly assessing ecosystem services supplied by wetlands. WRC Report no. TT 339/09. Pretoria: Water Research Commission. Le Roux PAL, du Plessis MJ, Turner DP, van der Waals J, Booyens HB. 2013. Field book for the classification of South African soils. Durban: Reach Publishers. Le Roux PAL, du Preez CC. 2004. Indications of ferrolysis and structure degradation in an Estcourt soil and possible relationships with plinthite formation. South African Journal of Plant and Soil 22: 199–206. Lindbo DL, Stolt MH, Vepraskas MJ. 2010. Redoximorphic features. In: Stoops G, Marcelino V, Mees F (eds), Interpretation of micromorphological features of soils and regoliths. Amsterdam: Elsevier. pp 129–147. Loheide SP, Booth EG. 2011. Effects of changing channel morphology on vegetation, groundwater and soil moisture regimes in groundwater-dependent ecosystems. Geomorphology 126: 364–376. MacEwan RJ. 1997. Soil quality indicators: pedological aspects. In: Gregorich EG, Carter MR (eds), Soil quality for crop production and ecosystem health. New York: Elsevier. pp 143–166. Mausbach MJ, Richardson JL. 1994. Biogeochemical processes in hydric soils. Current Topics in Wetland Biogeochemistry 1: 68–127. Millennium Ecosystem Assessment. 2005. Ecosystems and human well-being: wetlands and water synthesis. Washington, DC: World Resources Institute. Mitsch WJ, Gosselink JG. 1993. Wetlands (2nd edn). New York: John Wiley and Sons. Naiman RJ, Décamps H, McClain ME. 2005. Riparia: ecology, conservation, and management of streamside communities. Burlington: Elsevier Academic Press. Republic of South Africa. 1998. National Water Act, Act no. 36, 1998. Government Gazette 398, no. 19182, 26 August 1998. Cape Town: Office of the President. SANBI (South African National Biodiversity Institute). 2012. Draft Rehabilitation Plan. Prepared by Jenny Youthed, Aurecon South Africa (Pty) Ltd as part of the planning phase for the Working for Wetlands Rehabilitation Programme. SANBI Report no. 5686a/107406. Shields FD Jr, Lizotte RE, Knight SS, Cooper CM, Wilcox D. 2010. The stream channel incision syndrome and water quality, USA. Ecological Engineering 36: 78–90. Shields FD Jr, Simon A, Dabney SM. 2009. Streambank dewatering for increased stability. Hydrological Processes 23: 1537–1547. Soil Classification Working Group. 1991. Soil classification: a taxonomic system for South Africa. Memoirs on the Agricultural Natural Resources of South Africa no. 15. Pretoria: Department of Agricultural Development. Sprecher SW. 2000. Installing monitoring wells/piezometers in wetlands. WRAP Technical Notes Collection. ERDC TN-WRAP00-02. Vicksburg, Mississippi: US Army Engineer Research and Development Center.

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Vepraskas MJ, Wilding LP, Drees LR. 1994. Aquic conditions for soil taxonomy: concepts, soil morphology and micromorphology. In: Ringrose-Voase AJ, Humphreys GS (eds), Soil micromorphology: studies in management and genesis. Amsterdam: Elsevier. pp 117–131. Vepraskas MJ, Faulkner SP. 2001. Redox chemistry of hydric soils. In: Richardson JL, Vepraskas MJ (eds), Wetland soils: genesis, hydrology, landscapes and classification. Boca Raton: Lewis Publishers. pp 85–105. Vepraskas MJ, Lindbo DL. 2012. Redoximorphic features as related to soil hydrology and hydric soils. In: Lin H (ed.), Hydropedology: synergistic integration of soil science and hydrology. Oxford: Elsevier. pp 143–172. Walkley A, Black CA. 1934. Estimating organic carbon by chromic acid titration method. Soil Science 37: 29–38.


Steiger J, Tabacchi E, Dufour S, Corenblit D, Peiry JL. 2005. Hydrogeomorphic processes affecting riparian habitat within alluvial channel-floodplain river systems: a review for the temperate zone. River Research and Applications 21: 719–737. van Huyssteen CW, Hensley M, Le Roux PAL, Zere TB, du Preez CC. 2005. The relationship between soil water regime and soil profile morphology in the Weatherly catchment, an afforestation area in the north-eastern Eastern Cape. WRC Report no. K8/1317. Pretoria: Water Research Commission. van Tol JJ, Le Roux PAL, Hensley M, Lorentiz SA. 2010. Soil as indicator of hillslope hydrological behaviour in the Weatherley catchment, Eastern Cape, South Africa. Water SA 36: 513–520. Veneman PLM, Vepraskas MJ, Bouma J. 1976. The physical significance of soil mottling in a Wisconsin toposequence. Geoderma 15: 103–118.


Received 1 October 2013, revised 3 June 2014, accepted 22 June 2014 Associate Editor: Nebo Jovanovic