Refereed publications
Tooth, S. (2007), Arid geomorphology: investigating past, present and future changes, Progress in Physical Geography, 31 (3), pp.319-335 (DOI: 10.1177/0309133307079057).
Tooth, S., Rodnight, H., Duller, G.A.T., McCarthy, T.S., Marren, P.M. and Brandt, D. (2007), Chronology and controls of avulsion along a mixed bedrock-alluvial river, Geological Society of America Bulletin, 119 (3), pp.452-461 (DOI: 10.1130/B26032.1).
Tooth, S. (2006), Virtual globes: a catalyst for the re-enchantment of geomorphology?, Earth Surface Processes and Landforms, 31, pp.1192-1194 (DOI: 10.1002/esp.1383).
Tooth, S. (2005), Splay formation along the lower reaches of ephemeral rivers on the Northern Plains of arid central Australia, Journal of Sedimentary Research, 75, pp.634-647 (DOI: 10.2110/jsr.2005.052).
Tooth, S. (2004), ‘Floodout’, Invited Contribution to Goudie, A.S. (Ed.), Encyclopedia of Geomorphology, London: Routledge, pp.380-381 (Vol. 1).
Tooth, S. (2000), Process, form and change in dryland rivers: a review of recent research, Earth-Science Reviews, 51, pp.67-107.
Tooth, S., (2000), Downstream changes in dryland river channels: the Northern Plains of arid central Australia, Geomorphology, 34, pp.33-54.
Tooth, S. (1999), Floodouts in central Australia, In Miller, A.J. and Gupta, A. (Eds), Varieties of Fluvial Form, Chichester: John Wiley and Sons, pp.219-47.
Tooth, S. (1999), Downstream changes in floodplain character on the Northern Plains of arid central Australia, In: Smith, N.D. and Rogers, J. (Eds), Fluvial Sedimentology VI, International Association of Sedimentologists, Special Publication No. 28. Oxford: Blackwell Scientific Publications, pp.93-112.
Tooth, S. and McCarthy, T.S. (2006), Wetlands in drylands: key geomorphological and sedimentological characteristics, with emphasis on examples from southern Africa, Progress in Physical Geography, 31 (1), pp.3-41 (DOI: 10.1177/0309133307073879)..
Tooth, S. and McCarthy, T.S. (2004), Anabranching in mixed alluvial-bedrock rivers: the example of the Orange River above Augrabies Falls, Northern Cape Province, South Africa, Geomorphology, 57 (3-4), pp.235-262.
Tooth, S. and McCarthy, T.S. (2004), Controls on the transition from meandering to straight channels in the wetlands of the Okavango Delta, Botswana, Earth Surface Processes and Landforms, 29 (13), pp.1627-1649 (DOI: 10.1002/esp.1117).
Tooth, S., Brandt, D., Hancox, P.J. and McCarthy, T.S. (2004), Geological controls on alluvial river behaviour: a comparative study of three rivers on the South African Highveld, Journal of African Earth Sciences, 38 (1), pp.79-97.
Tooth, S., McCarthy, T.S., Brandt, D., Hancox, P.J. and Morris, R. (2002), Geological controls on the formation of alluvial meanders and floodplain wetlands: the example of the Klip River, eastern Free State, South Africa, Earth Surface Processes and Landforms, 27, pp.797-815.
Tooth, S., McCarthy, T.S., Hancox, P.J., Brandt, D., Buckley, K., Nortje, E. and McQuade, S. (2002), The geomorphology of the Nyl River and floodplain in the semi-arid Northern Province, South Africa, South African Geographical Journal, 84, pp.226-237.
Tooth, S. and Nanson, G.C. (2004), Forms and processes of two highly contrasting rivers in arid central Australia, and the implications for channel-pattern discrimination and prediction, Geological Society of America Bulletin, 116 (7-8), pp.802-816.
Tooth, S. and Nanson, G.C. (2000), Equilibrium and nonequilibrium conditions in dryland rivers, Physical Geography, 21 (3), pp.183-211.
Tooth, S. and Nanson, G.C. (2000), The role of vegetation in the formation of anabranching channels in an ephemeral river, Northern Plains, arid central Australia, Hydrological Processes, 14, pp.3009-3117.
Tooth, S. and Nanson, G.C. (1999), Anabranching rivers on the Northern Plains of arid central Australia, Geomorphology, 29 (3-4), pp.211-33.
Tooth, S. and Nanson, G.C. (1995), The geomorphology of Australia’s fluvial systems: retrospect, perspect and prospect, Progress in Physical Geography, 19 (1), pp.35-60.
McCarthy, T.S. and Tooth, S. (2004), Incised meanders along the mixed bedrock-alluvial Orange River, Northern Cape Province, South Africa, Zeitschrift für Geomorphologie, 48 (3), pp.273-292.
Nanson, G.C. and Tooth, S. (1999), Arid-zone rivers as indicators of climate change, In Singhvi, A.K. and Derbyshire, E. (Eds), Palaeoenvironmental Reconstruction in Arid Lands, Rotterdam: A.A. Balkema, pp.175-216.
Nanson, G.C., Tooth, S. and Knighton, A.D. (2002), A global perspective on dryland rivers: perceptions, misconceptions and distinctions. In Bull, L.J. and Kirkby, M.J. (Eds), Dryland Rivers: Hydrology and Geomorphology of Semi-Arid Channels, Chichester: John Wiley and Sons, pp.17-54.
Springer, G.S, Tooth, S. and Wohl, E.E. (2006), Theoretical modeling of stream potholes based upon empirical observations from the Orange River, Republic of South Africa, Geomorphology, 82 (1-2), pp.160-176 (DOI:10.1016/j.geomorph.2005.09.023).
Springer, G.S, Tooth, S. and Wohl, E.E. (2005), Dynamics of pothole growth as defined by field data and geometrical description, Journal of Geophysical Research, 110, F04010 (DOI:10.1029/2005JF000321).
Gregory, K.J., Davis, R.J. and Tooth, S. (1993), Spatial distribution of coarse woody debris dams in the Lymington basin, Hampshire, UK, Geomorphology, 6, pp.207-24.
Marren, P.M., McCarthy, T.S., Tooth, S., Brandt, D., Stacey, G.C., Leong, A. and Spottiswoode, B. (2006), A comparison of mud- and sand-dominated meanders in a downstream coarsening reach of the mixed bedrock-alluvial Klip River, eastern Free State, South Africa, Sedimentary Geology, 190, pp.213-226 (DOI: 10.1016/jsedgeo.2006.05.014).
Rodnight, H., Duller, G.A.T., Tooth, S. and Wintle, A.G. (2005), Optical dating of a scroll bar sequence on the Klip River, South Africa, to derive the lateral migration rate of a meander bend, The Holocene, 15 (6), pp.802-811.
Rodnight, H., Duller, G.A.T., Wintle, A.G. and Tooth, S. (2006), Assessing the reproducibility and accuracy of optical dating of fluvial deposits. Quaternary Geochronology, 1, pp.109-120 (DOI: 10.1016/j.quageo.2006.05.017).
Gregory, K.J., Gurnell, A.M., Hill, C.T. and Tooth, S. (1994), Stability of the pool-riffle sequence in changing river channels, Regulated Rivers: Research and Management, 9, pp.35-43.
Other documents
Tooth, S. (Ed.) (2007), Flood Processes and Products in Southern Crete: Guide to the Field Excursions, 4th International Palaeoflood Workshop, Crete, Greece, 24th-30th June 2007, 68 pp.
Tooth, S. (2004), Influence of bedrock properties on the forms and processes of the middle Orange River. Report for Project no. 2003-06-18STOO, South African National Parks, 9 pp.
Tooth, S. (2003), Aspects of the geomorphology of the middle Orange River, Northern Cape, South Africa: a report on fieldwork undertaken during July-August 2003. Report prepared for De Beers Africa Exploration, RESCOM project 31R+D66176, 54 pp.
Tooth, S. (2003), Review of Dyer, F.J., Thoms, M.C. and Olley, J.M., (Eds) (2002), ‘The Structure, Function and Management Implications of Fluvial Sedimentary Systems’, International Association of Hydrological Sciences (IAHS) publication no. 276, IAHS Press. Progress in Physical Geography, 27 (2), pp.306-308.
Tooth, S. (2002), Dryland feature of interest – December 2002. Waterholes: wetlands in Drylands?. Contribution to the Dryland Rivers Research website. Available at: http://www.abdn.ac.uk/~gmi196/DrylandRivers/feature/FoM_waterholes/FOM_waterholes.htm [Last access date: 26th July 2004]
Tooth, S. (2001), Dryland river floodplains: issues of diversity and distinctiveness. In: Mason, J.A., Diffendal, R.F., Jr. and Joeckel, R.M. (Eds), Program with Abstracts, 7th International Conference on Fluvial Sedimentology, Open-File Report 60, Conservation and Survey Division, University of Nebraska-Lincoln, p.270.
Tooth, S. (1997), The Morphology, Dynamics and Late Quaternary Sedimentary History of Ephemeral Drainage Systems on the Northern Plains of Central Australia, PhD. thesis, University of Wollongong, 433pp.
Tooth, S. and Coulthard, T.J. (2004), Channel-vegetation interactions along Magela Creek, Northern Territory. Report for Kakadu Research Permit RK 603, Kakadu National Park, Parks Australia, 11 pp.
Tooth, S. and Nanson, G.C. (Eds) (1997), A Field Guide for Physical Environments in Eastern Central Australia, Quaternary Deserts and Climatic Change Conference (IGCP-349), Post-Conference Field Excursion Guide, School of Geosciences, University of Wollongong, 151 pp.
Tooth, S., McCarthy, T.S., Brandt, D. and Hancox, P.J. (2002). A guide to the geology and geomorphology of the Klip River valley, Contribution to ‘Bird and Nature Guide to the Memel District’, Birdlife South Africa, Johannesburg. Also available via: http://users.aber.ac.uk/set/ [Last access date 26th July 2004].
Recent (post-2000) conference abstracts (full abstracts for some provided below)
Tooth, S., Brewer, P.A., Macklin, M.G. and Noble, P. (2007), Flood processes and products in gorge-bound rivers of southern Crete: a pre-excursion primer, Programme and Abstracts, 4th International Palaeoflood Workshop, Crete, Greece, 24-30 June 2007, p.58.
Tooth, S., McCarthy, T.S. and Ellery, W.N. (2006), Quantifying environmental change in southern African wetlands: principles and priorities, Workshop on Spatial, Temporal and Spatio-Temporal Analyses: Quantitative Solutions for Environmental Change Problems, University of the Witwatersrand Institute for the Study of the Environment (WiSe), 14 July 2006.
Tooth, S., McCarthy, T.S. and Ellery, W.N. (2006), Wetland geomorphology: a neglected area of earth system science, Abstracts, Geomorphology and Earth System Science, British Geomorphological Research Group Annual Meeting, 28-30 June 2006, Loughborough University, p.131.
Tooth, S., Coulthard, T.J., Jansen, J.D. and Nanson, G.C. (2005), Channel-vegetation interactions in the development of anabranching along Magela Creek, northern Australia, Abstracts, British Geomorphological Research Group Annual Meeting, 19-21 September 2005, University of Southampton, p.42.
Tooth, S., Rodnight, H., Duller, G.A.T., Brandt, D., Marren, P.M., McCarthy, T.S. (2005), Mechanisms and history of avulsion in a South African floodplain wetland, Abstracts, 8th International Conference on Fluvial Sedimentology, Delft University of Technology, The Netherlands, 7-12 August 2005, p.283-284.
Rodnight, H., Duller, G.A.T., Wintle, A.G. and Tooth, S. (2005), Optical dating of palaeochannels to establish avulsion frequency, Abstracts, 8th International Conference on Fluvial Sedimentology, Delft University of Technology, The Netherlands, 7-12 August 2005, p.252.
Rodnight, H., Duller, G.A.T., Wintle, A.G. and Tooth, S. (2005), Investigating the reproducibility and accuracy of equivalent dose distributions from fluvial deposits, Book of Abstracts, 11th International Conference on Luminescence and Electron Spin Resonance Dating, 24-29 July 2005, University of Cologne, Germany, p.163.
Tooth, S., Hancox, P.J., Brandt, D., McCarthy and Woodborne, S. (2005), Controls on the genesis and sedimentary architecture of a dryland river floodplain: Modder River, Free State, South Africa, Final Programme & Abstract Book, Drylands: Linking Landscape Processes to Sedimentary Environments, Joint BGRG/BSRG International Conference, 2-4 February 2005, Geological Society, London, p.30.
Springer, G.S., Tooth, S. and Wohl, E.E. (2004), A geometry-based, theoretical estimation of the role of abrasion by suspended sediment in a pothole-dominated knickpoint: Orange River, Republic of South Africa, American Geophysical Union Fall Meeting, 13-17 December 2004, San Francisco, California, Eos Transactions of the American Geophysical Union, 85(47), Fall Meeting Supplement, Abstract H54B-01, p. F930.
Springer, G.S., Tooth, S. and Wohl, E.E. (2004), Inner channel development and knickpoint retreat in an anabranching river by potholing, Geological Society of America Annual Conference, 7-10 November 2004, Denver, Colorado, GSA Abstracts with Programs, 36(5), pp. 11-12.
Tooth, S. (2004), A geological perspective on the erosion ‘problem’ in the interior of South Africa (Invited plenary talk), Abstracts, Geosciences Africa 2004, University of the Witwatersrand, 12-16 July 2004, pp.639-640.
Tooth, S. and McCarthy, T.S. (2004), Wetlands in drylands: geomorphological and sedimentological characteristics, with examples from southern Africa, Abstracts, Geosciences Africa 2004, University of the Witwatersrand, 12-16 July 2004, pp.641-642.
Hancox, P.J., Brandt, D., Kendall, G., McCarthy, T.S., Pickering, R., Tooth, S. and Woodborne, S. (2004), The Maandagshoek alluvial fan system and evidence for widespread periods of alluviation and stasis across the interior of South Africa during the past 150 000 years, Abstracts, Geosciences Africa 2004, University of the Witwatersrand, 12-16 July 2004, pp.248-249.
Marren, P.M., McCarthy, T.S., Brandt, D. Stacey, G.C. and Tooth, S. (2004), Sedimentology and geomorphology of the Klip River, eastern Free State, South Africa: downstream coarsening, mud-dominated meanders and avulsion control, Abstracts, Geosciences Africa 2004, University of the Witwatersrand, 12-16 July 2004, pp.421-422.
McCarthy, T.S., Tooth, S., Brandt, D., Marren, P. and Hancox, P.J. (2004), On the origin of the Nylsvlei wetland, Abstracts, Geosciences Africa 2004, University of the Witwatersrand, 12-16 July 2004, pp.435-436.
Rodnight, H., Duller, G.A.T. and Tooth, S. (2004), Optical dating of a scroll-bar sequence from the Klip River, South Africa, to obtain rates of lateral migration, Abstracts, Geosciences Africa 2004, University of the Witwatersrand, 12-16 July 2004, p.557.
Springer, G.S., Tooth, S. and Wohl, E.E. (2004), Theoretical modelling of stream potholes based upon empirical observations from the Orange River, Republic of South Africa, Abstracts, Geosciences Africa 2004, University of the Witwatersrand, 12-16 July 2004, pp.610-611.
Tooth, S. (2002), Splays on the Northern Plains of arid central Australia, Programme and Abstracts, Dryland Rivers: Process and Product, University of Aberdeen, 8-9 August 2002, p.27.
Tooth, S. and Nanson, G.C. (2002), Forms, processes, and products of two highly contrasting rivers on the Northern Plains in arid central Australia, Programme and Abstracts, Dryland Rivers: Process and Product, University of Aberdeen, 8-9 August 2002, p.28.
Tooth, S. and McCarthy, T.S. (2002), Channel pattern transitions in the Okavango wetlands, Botswana, Abstract Volume, 16th International Sedimentological Congress, International Association of Sedimentologists, Rand Afrikaans University, Johannesburg, South Africa, 8-12 July 2002, p.369.
Tooth, S., Brandt, D., Hancox, P.J. and McCarthy, T.S. (2002), Natural cycles of floodplain wetland formation and destruction on the South African Highveld, Abstract Volume, 16th International Sedimentological Congress, International Association of Sedimentologists, Rand Afrikaans University, Johannesburg, South Africa, 8-12 July 2002, p.370.
Hancox, P.J., Brandt, D., McCarthy, T.S. and Tooth, S. (2002), The stratigraphy and sedimentology of Pleistocene fluvial fossil bearing strata in the Free State, South Africa: implications for dryland river evolution, Abstract Volume, 16th International Sedimentological Congress, International Association of Sedimentologists, Rand Afrikaans University, Johannesburg, South Africa, 8-12 July 2002, p.144.
Hancox, P.J., Brandt, D., Kendall, G., McCarthy, T.S. and Tooth, S. (2002), The stratigraphy and sedimentology of the Maandagshoek fan system: a cyclic history of erosion, sedimentation and pedogenic overprint, Abstract Volume, 16th International Sedimentological Congress, International Association of Sedimentologists, Rand Afrikaans University, Johannesburg, South Africa, 8-12 July 2002, p.145.
McCarthy, T.S., Brandt, D., Hancox, P.J. and Tooth, S. (2002), The geomorphology and sedimentology of the Nylsvlei wetland, Abstract Volume, 16th International Sedimentological Congress, International Association of Sedimentologists, Rand Afrikaans University, Johannesburg, South Africa, 8-12 July 2002, p.251.
Noble, P.L., Brewer, P.A., Macklin, M.G. and Tooth, S. (2002), River response to environmental change in the Anapodaris Gorge, southeast Crete. Palaeofloods, Historical Floods & Climatic Variability: Applications in Flood Risk Assessment, Barcelona, Spain. 16-19th October 2002.
Noble, P., Brewer, P.A., Macklin, M.G. and Tooth, S. (2002), Geomorphology and sedimentology of rivers in gorges: the example of the Anapodaris gorge, southern Crete, Abstract Volume, 16th International Sedimentological Congress, International Association of Sedimentologists, Rand Afrikaans University, Johannesburg, South Africa, 8-12 July 2002, pp.285-86.
Tooth, S. (2001), Dryland river floodplains: issues of diversity and distinctiveness, 7th International Conference on Fluvial Sedimentology, University of Nebraska, Lincoln, USA, 6-10 August 2001, p.270.
Tooth, S., McCarthy, T.S., Brandt, D., Hancox, P.J. and Kendall, G.J. (2001), Controls on late Quaternary river channel and alluvial fan activity in the semi-arid Steelpoort region, Mpumalanga/Northern Province, South Africa, Dryland Change 2001 International Conference, Upington, Northern Cape, South Africa, 30 August - 2 September 2001, p.52.
Abstract for the BGRG Annual Meeting, 'Geomorphology and Earth System Science', Loughborough University, 28th-30th June 2006
Wetland geomorphology: a neglected area of earth system science
Tooth, S.1, McCarthy, T.S.2 and Ellery, W.N.3
1 Institute of Geography and Earth Sciences, University of Wales, Aberystwyth, SY23 3DB
2 School of Geosciences, University of the Witwatersrand, Wits 2050, South Africa
3 School of Environmental Sciences, University of KwaZulu-Natal, South Africa
Wetlands cover 4-6% of the globe and represent areas where biological activity is adapted to wet conditions resulting from shallow water or saturated soil. Located at the interface between terrestrial and fully aquatic environments, wetlands exemplify in microcosm the complex interactions between the atmosphere, hydrosphere, geosphere and biosphere implicit in the earth system science (ESS) concept, playing important roles in hydrological, sedimentary and biogeochemical cycles whilst supporting high biodiversity. Scientific and applied interest in wetlands is increasing, yet the geomorphological contribution to wetland science is limited. Wetland research is dominated by the perspectives of ecologists and hydrologists who tend to emphasise short-term (decadal or less) processes relevant to biota rather than longer term earth-surface processes that shape wetlands. Geomorphologists have tended to neglect wetlands, perhaps because they are difficult to characterise. Hence, wetlands remain poorly understood landscape features, despite growing pressures for data to inform decisions regarding wetland conservation, rehabilitation or artificial construction, especially in the context of global environmental change.
Examples from our interdisciplinary studies of wetlands in southern African drylands illustrate the diverse interactions between hydrology, sedimentology, hydrochemistry, biota and landforms. We demonstrate how geomorphological perspectives are central to improved understanding of wetland origin, structure, function and dynamics, but argue that these must be embedded within a broader ESS framework. Rapidly developing technology means the opportunities for wetland geomorphologists are considerable but many challenges remain, notably developing our ability to translate research findings into information that can be assimilated by other earth system scientists, policy makers and managers.
Channel-vegetation interactions in the development of anabranching
along Magela Creek, northern Australia
Tooth, S.1, Coulthard, T.J.1, Jansen, J.D.2 and Nanson, G.C.2
1 Institute of Geography and Earth Sciences, University of Wales, Aberystwyth, SY23 3DB, UK
2 School of Geosciences, University of Wollongong, NSW 2522, Australia
Australian anabranching rivers are characterised by strong interactions between fluvial processes and riparian vegetation dynamics but many uncertainties still surround the processes and timescales of anabranch development. This study investigated a 7-8 km reach of Magela Creek in the seasonal tropics of northern Australia to determine spatial and temporal trends in the development of an anabranching planform. Along the creek, many riparian trees and shrubs establish and survive on the sandy beds and lower banks, and anabranches divide and rejoin around numerous ridges and islands that form either by accretion in the lee of in-channel vegetation or by excision from formerly continuous island or floodplain surfaces. Over the Holocene, Magela Creek has vertically aggraded and extended in length by delta progradation into Madjinbardi Billabong, so that a broad age sequence of anabranches, ridges and islands exists along the creek from older (upstream) to younger (downstream). The older, upstream reaches are characterized by narrow, deep anabranches (w/d ~10-30) with few in-channel obstructions whereas the younger, downstream reaches tend to have wider, shallower anabranches (w/d ~20-70) with more obstructions, and splays and partial avulsions are common. A combination of geomorphological evidence, previous flow and sediment transport measurements, flume experiments, and theoretical analyses suggests that this indicates a decline in anabranch efficiency from an upstream equilibrium system in mass flux balance to a downstream disequilibrium system characterised by bed aggradation and island/floodplain erosion. In the downstream reaches, inefficient anabranches do not persist over time as they either aggrade or subdivide into narrower, deeper and less obstructed anabranches owing to interactions between in-channel vegetation growth and ridge/island accretion or excision. Consequently, a more efficient anabranching system gradually develops, evolving characteristics similar to those in the upstream reaches. This enhances downstream sediment transfer, thus enabling ongoing delta progradation and providing fresh sediment surfaces for vegetation to colonise and initiate new anabranches. By extrapolating recent rates of delta progradation back in time, we propose that a recognizable but relatively inefficient anabranching system can develop within a few decades to a century, whilst adjustment to a more efficient system probably occurs within a few centuries to a millennium. Flume experiments, cellular modelling and age control from luminescence dating are providing further insight into channel-vegetation interactions, changing channel efficiencies, and anabranch development.
Abstract for the '8th International Conference on Fluvial Sedimentology', Delft University of Technology, The Netherlands, 7-12 August 2005
MECHANISMS & HISTORY OF AVULSION IN A SOUTH AFRICAN FLOODPLAIN WETLAND
Tooth, S.1, Rodnight, H.1, Duller, G.A.T.1, Brandt, D.2, Marren, P.M.2, McCarthy, T.S.2
1 Institute of Geography and Earth Sciences, University of Wales, Aberystwyth, Ceredigion, SY23 3DB, UK
2 School of Geosciences, University of the Witwatersrand, Johannesburg, Wits 2050, South Africa
Research interest in channel avulsions has increased rapidly in recent years, but there remains a need for further studies of avulsions in a range of different fluvial settings. In subhumid to semiarid eastern South Africa, avulsion is a common but poorly documented feature of channel dynamics in many floodplain wetlands. We present findings from an ~10 km reach of the upper Klip River, eastern Free State, South Africa, where geomorphological and sedimentological investigations and absolute dating have established the mechanisms and history of avulsion. The Klip transports a mixed load of mud, sand and pebble gravel, and meanders (P <1.75, w/d <30) within a floodplain wetland up to ~1500 m wide, but the channel bed is positioned just above sandstone/shale bedrock. Floodplain inundation occurs during summer floods (Qbf <60 m3/s, ω <15 W/m2) with gradual desiccation occurring during the drier winter months. Numerous abandoned meander belts up to 4 km long with weakly developed (<1.5 m high) alluvial ridges provide evidence of past avulsions. Crevasse splay complexes are not present in the reach, suggesting that avulsions occur primarily through the following stages: 1) accretion of muddy alluvium in an established meandering reach progressively reduces cross-sectional area and promotes increased overbank flooding; 2) during falling stage, floodwaters drain from the adjacent floodplains back to the channel through low points in channel levees, sometimes initiating small headcuts on the channel banks; 3) continued headward cutting results in erosion of a new, straighter channel through the floodplain that eventually reconnects with the original channel further upstream; 4) flow and bedload sediment is increasingly diverted along the topographically lower course of the new channel, which progressively enlarges and develops meanders while the original channel fails and is abandoned. Luminescence and radiocarbon ages from abandoned channel fills show that five major avulsions have occurred during the late Quaternary at ~23 ka, ~14 ka, ~7 ka, ~4.5 ka, and ~1 ka. The tectonically-stable setting, and the lack of close correspondence with southern African late Quaternary climatic changes, suggests that avulsions have not been allogenically forced but rather autogenically driven as a natural part of meander belt evolution.
In addition to past events, an avulsion is occurring at present. Failure of the modern Klip channel is being accelerated by exotic willow trees that have colonised the lower banks and form debris jams, thus reducing flow and sediment conveyance, and promoting overbank flooding. During the 1930s, excavation of an ~1200 m reach of this failing channel by local farmers appears to have initiated dendritic networks of headcutting channels in the adjacent floodplain wetlands. Some headcuts have evolved into a new channel that has rapidly extended (~40 m/yr) to drain an increasingly large part of the wetlands. In recent years, headward growth has continued along extensive trails that formed following re-introduction of hippopotami to the wetlands, and the new channel is now <1 km from reconnecting with the failing channel. Once complete, the avulsion will result in a radical redistribution of flow and sediment across the floodplain wetland.
Abstract for 'Drylands: Linking Landscape Processes to Sedimentary Environments', Joint BGRG/BSRG International Conference, 2-4 February 2005, Geological Society, London
Controls on the genesis and sedimentary architecture of a
dryland river floodplain: Modder River, Free State, South Africa
S. Tooth
Institute of Geography and Earth Sciences, University of Wales, Aberystwyth, SY23 3DB, UK. E-mail: set@aber.ac.uk
P.J. Hancox, D. Brandt, T.S. McCarthy
School of Geosciences, University of the Witwatersrand, Johannesburg, Wits 2050, South Africa
S. Woodborne
Environmentek, CSIR, Pretoria 0001, South Africa. E-mail: SWoodbor@csir.co.za
Alluvial floodplains, terraces and fans are prominent features in many drylands, harbouring records of palaeoenvironmental change, providing modern analogues for improved interpretation of the geologic record, and sometimes hosting economically-important minerals. Nevertheless, knowledge of the architecture of dryland alluvial successions is limited, in part owing to poor exposures typical of depositional settings. In many South Africa valleys, however, erosion by dongas (gullies) provides extensive exposure of alluvial successions, enabling an evaluation of the controls on their genesis and architecture. We focus here on the incised Modder River at Erfkroon, semi-arid Free State, where donga erosion has exposed a floodplain succession within a narrow (<500 m) valley carved predominantly in shale. Channel facies range from planar and trough cross-bedded, sandy gravel through to structureless sandy silt, and occur in association with overbank facies consisting largely of sandy mud with varying degrees of pedogenic overprinting. Younger facies are stacked upon, cross-cut, or onlap older facies, indicating a complex history of cut and fill. Preliminary luminescence ages, and changing assemblages of associated faunal fossils and archaeological artefacts, indicate sediment accumulation over the last ~180 ka under conditions both more arid and more humid than at present. In common with many rivers in interior South Africa, minor channel incision and donga erosion followed by extensive deposition has broadly coincided with relatively arid intervals identified from other environmental proxies. By contrast, the current phase of major channel incision and donga erosion is related to base level fall owing to breaching of downstream hard rock barriers (dolerite, andesite), resulting in a new phase of valley excavation. Although alluvial successions at sites such as Erfkroon have little chance of preservation in the stratigraphic record, further study nevertheless may yield important insights into the ‘temporary’ recording of floodplain processes in long-term degradational settings subject to fluctuating climate and base level fall.
Abstract for 'Dryland Rivers: Process and Product', University of Aberdeen, 8-9 August 2002
Splays on the Northern Plains of arid central Australia
Stephen Tooth
Institute of Geography and Earth Sciences, University of Wales, Aberystwyth, SY23 3DB, UK
Splays are common features of many river and delta systems but have rarely been described from modern dryland settings. On the Northern Plains of arid central Australia, numerous splays can be found in the lower reaches of ephemeral rivers that traverse extensive floodplains up to 6 km wide. While splay morphology varies widely, ranging from small, lobate or tongue-shaped features less than 1 km long, to larger elongate features up to 3 km long, in all cases splays emanate from channel bankline breaches and comprise well-defined channels that transport rare floodwater and gravelly sand. In their proximal and medial parts, splay channels are incised up to 1.5 m into the underlying finer-grained floodplain sediments but remain elevated (>1 m) above the level of the parent channel bed. Towards their distal margins, splay channels are less deeply incised and commonly terminate in sediment lobes up to 0.3 m thick that prograde over the floodplain. Aerial photographs show that most splays developed rapidly in one or several large floods in the mid 1970s and early 1980s, and field observations between 1993 and 2001 provide evidence for some splays having undergone minor modification during subsequent floods. Together, these findings suggest a general model for splay development on the Northern Plains: 1) a splay is initiated by overbank flow, with bankline breaching and channel incision being facilitated by slaking of the floodplain sediment; 2) the initial splay channel deepens further at the bankline breach until it starts to draw off gravelly sand from the parent channel. This sediment supply prevents further incision at the bankline breach but in the relatively sediment-starved medial reaches of the splay, incision may continue; 3) gravelly sand is transported through the incised medial reaches and progrades distally as a lobe across the floodplain; 4) subsequent large floods divert around the topographic high formed by the distal sediment lobe, lengthening the splay and depositing another lobe.
In the lower reaches of these ephemeral rivers, splays are a contributory factor to the characteristic downstream decreases in the size of parent channels, and may also lead to channel avulsion and distributary/anabranch formation. In addition, active and abandoned splay deposits form locally important accumulations of gravelly sand in the predominantly fine-grained floodplain sediments. While the developmental stages and certain features of splays (e.g. erosive bases) on the Northern Plains differ from those typical of many splays in more humid environments, they display few morphological or sedimentological features that are necessarily diagnostic of formation in a dryland setting.
Abstract for 'Dryland Rivers: Process and Product', University of Aberdeen, 8-9 August 2002
Forms, processes, and products of two highly contrasting rivers
on the Northern Plains in arid central Australia
Stephen Tooth1 and Gerald C. Nanson2
1 Institute of Geography and Earth Sciences, University of Wales, Aberystwyth, SY23 3DB, UK
2 School of Geosciences, University of Wollongong, N.S.W. 2522, Australia
On the Northern Plains in arid central Australia, ephemeral rivers are commonly of low sinuosity (P ~1.05-1.12) but vary from single-thread to anabranching. Comparison of the closely adjacent middle reaches of the Plenty and Marshall Rivers shows that while channel-bed gradient, discharge, and bank strength are essentially similar, bed material calibre and the patterns of tributary drainage are markedly different. This results in strong contrasts in channel cross-sectional geometry, planform and sedimentology. The Plenty River, which transports medium to coarse sand and is joined by few tributaries, remains predominantly single-thread but is variably wide (<350 to >800 m) and in places appears transitional to braiding. By contrast, the Marshall River, which transports coarse sand to granules and is joined by several minor tributaries, has numerous narrow (usually <60 m wide), anabranching channels which divide around vegetated ridges and broader islands.
Importantly, while river pattern discrimination diagrams predict that hydraulic, sedimentary and energy conditions are conducive to the development of meandering or braiding on these rivers, both the Plenty and the individual Marshall anabranches remain essentially single-thread, planar-bed, low-sinuosity (‘straight’) channels. This appears to be due primarily to the high degree of bank strength provided by riparian vegetation, and secondarily to certain flow and sediment transport conditions characteristic of these ephemeral rivers. In particular, field observations and theoretical model results suggest that while a wide, single-thread channel relatively free of in-channel trees is sufficient to convey a sandy bed material load (the Plenty), where tributaries provide small supplies of water that encourage the growth of obstructing in-channel trees or introduce some additional coarse sediment, then narrow anabranches form or increase in number in an attempt to minimise channel flow roughness and maintain sediment throughput (the Marshall). While preliminary observations show that the contrasting forms and processes of these two rivers are also reflected in different sedimentary bedforms and architectures, it is unclear whether, or to what extent, these sedimentological attributes of either river are truly diagnostic of dryland conditions or shared with many sandy/gravelly rivers in other (more humid) settings. Nonetheless, the findings from the Plenty and Marshall demonstrate how the forms, processes and products of dryland alluvial rivers can adjust dramatically in response to subtle downstream variations in flow hydrology, sediment supply or boundary roughness.
Abstract for the 16th International Sedimentological Congress, International Association of Sedimentologists,
Rand Afrikaans University, Johannesburg, South Africa, 8-12 July 2002
Channel pattern transitions in the Okavango wetlands, Botswana
S. TOOTH 1 and T.S. McCARTHY 2
1. Institute of Geography and Earth Sciences, University of Wales, Aberystwyth, SY23 3DB, UK
2. Department of Geology, University of the Witwatersrand, Johannesburg, Wits 2050, South Africa
Previous studies of rivers with banks formed in alluvium have shown how channel patterns form a continuum controlled by interactions among factors such as gradient, discharge, sediment size, and bank strength. Morphological, hydraulic and sedimentary data from channels draining the extensive (>12 000 km2) permanent wetlands of the Okavango Delta, Botswana, add significantly to these findings by focusing on pattern transitions in channels where the banks are formed almost entirely by sedges and emergent grasses rooted in peat. Channels are very well defined, with tranquil flows (depths <4 m, mean velocities <0.7 m/s) transporting bedloads of fine-medium sand between the vertical, vegetation-lined banks. Water depths, velocities, grain size, and bankline vegetation do not vary systematically downstream, but the permeable, vegetation-lined banks allow water to leak from the channels which contributes to an overall downstream decrease in discharge and channel width. In addition, as the channels flow from the <12 km wide entry corridor known as the Panhandle into the broader upper Delta, the Gomare Fault causes valley gradient to steepen by ~60% and stream power increases downstream.
These downstream changes result in an adjustment of channel pattern. In the Panhandle, the Okavango River is a relatively wide (~50-100 m), actively meandering, sinuous (P >2.0) channel, but further downstream in the Delta, narrower (<50 m) channels follow stable stable, lower sinuosity (‘straight’) courses (P <1.75). This runs counter to the findings of many previous studies which argue that meandering occurs on higher slopes and at higher stream powers than straight channels, and that sinuosity tends to increase in response to valley gradient steepening. Most traditional channel pattern threshold diagrams based on simple indices of gradient, discharge, sediment size, or stream power are inadequate for analyzing and explaining the meandering-straight transition in the Okavango but an approach proposed by Parker [1], based on ratios of depth-width and slope-Froude number, accurately predicts the transition. In combination with field data, this criterion suggests that as channel width-depth ratio decreases downstream in line with decreasing discharge, increasing hydraulic drag exerted by the bankline vegetation suppresses thalweg meandering and bankline erosion, and channels follow stable, straighter courses. Eventually, hydraulic drag slows velocity to such an extent that bedload transport can no longer be maintained, and the channels aggrade and ultimately fail. In the relatively low energy channels of the Okavango wetlands, where grain size and bank strength remain essentially constant downstream, the success of Parker’s criterion in predicting channel pattern transitions may be due to its emphasis on geometric and hydraulic factors as controls of channel pattern.
REFERENCE:
[1.] Parker, G., (1976) On the cause and characteristic scales of meandering and braiding in rivers. Journal of Fluid Mechanics, 76, 457-480.
Abstract for the 16th International Sedimentological Congress, International Association of Sedimentologists,
Rand Afrikaans University, Johannesburg, South Africa, 8-12 July 2002
Natural cycles of floodplain wetland formation and destruction on the South African Highveld
S. TOOTH 1, D. BRANDT 2, P.J. HANCOX 2 and T.S. McCARTHY 2
1. Institute of Geography and Earth Sciences, University of Wales, Aberystwyth, SY23 3DB, UK
2. Department of Geology, University of the Witwatersrand, Johannesburg, Wits 2050, South Africa
Floodplain wetlands are common features of rivers on the South African Highveld, but there has been relatively little study of the geological and geomorphological factors governing their origin, development and characteristics. Comparison of the upper Klip River, the Schoonspruit and the Venterspruit, three adjacent tributaries of the Vaal River, shows how dolerite sills and dykes form local base levels in the river longitudinal profiles and exert strong, but variable, controls on river behaviour in the upstream, dominantly alluvial reaches.
The upper Klip River flows largely over weakly-cemented Karoo Supergroup sandstones but also crosses highly-resistant dolerite dykes and sills. On the sandstones, vertical erosion rates are retarded by the slow erosion of the downstream dolerites, and in the interim the river is laterally planing bedrock and following a sinuous (P ~1.75), actively meandering course within broad (up to 1.5 km wide) floodplains marked by oxbow lakes and abandoned channels. These floodplains host extensive wetlands that remain in a near-pristine condition. The Schoonspruit flows largely over easily-erodible Karoo Supergroup shales in its upper and middle reaches but also crosses a resistant dolerite sill in its lower reaches. On the shale, vertical erosion previously was retarded by the downstream dolerite, and the river laterally planed bedrock and carved a valley up to ~1 km wide. The river also formerly meandered (P ~1.99) within a broad (up to 1 km wide) floodplain wetland but partial breaching of the dolerite has resulted in headward knickpoint migration, which in turn has led to river incision into shale, abandonment and desiccation of the wetlands, and secondary erosion of the valley fills by dongas (gullies). The Venterspruit also flows largely over erodible shales, apparently having completely breached a dolerite sill in its lower reaches. Vertical erosion has not been retarded to the same degree as on the Klip and the Schoonspruit, and the river follows a much straighter course (P ~1.53), which is now deeply incised into bedrock, with little surviving evidence of floodplain wetlands, and with numerous dongas having formed in the valley fills.
These three river reaches represent a temporal sequence whereby river superimposition first exposes and then breaches dolerite sills and dykes to create a natural cycle of floodplain wetland formation and destruction. Work in progress is attempting to obtain luminescence ages from the floodplain sediments to establish chronological control for late Quaternary phases of wetland formation and destruction. Recognition of the natural dynamism of these environments has implications for the design of effective strategies for managing wetlands and controlling donga erosion.
Abstract for the 7th International Conference on Fluvial Sedimentology, University of Nebraska, Lincoln, USA, 6-10 August 2001.
Dryland river floodplains: issues of diversity and distinctiveness
TOOTH, Stephen, River Basin Dynamics and Hydrology Research Group, Institute of Geography and Earth Sciences, University of Wales, Aberystwyth, SY23 3DB, UK, set@aber.ac.uk
Floodplains are key features of many dryland fluvial environments which may contain economically-important minerals, harbour proxy records of Quaternary hydroclimatic change, and provide modern analogues for improved interpretation of the geologic record. Current knowledge of dryland floodplains, however, is based on a very limited range of case studies, resulting in a number of invalid generalisations and misconceptions regarding dryland floodplain landforms, processes and sediments. This paper presents the results of ongoing research in the Australian and southern African drylands which is contributing to a better understanding of the diversity and distinctiveness of dryland river floodplains.
Inaccurate statements regarding dryland floodplains in large part result from limited recognition of the global diversity of dryland rivers. Dryland climates vary from hyperarid to subhumid, and together with differences in catchment physiography and lithology, hydrology, sediment load and vegetation, result in river patterns and their derivative floodplains varying in character across a wide spectrum. Low gradient, anastomosing rivers traversing extensive muddy floodplains in central Australia contrast strongly with moderate-gradient, meandering rivers in sandy floodplains in South Africa, and with high-gradient, straight or braided rivers in gravelly floodplains in the Mediterranean, Middle East or American southwest. Some dryland floodplains may not even host river channels, as is characteristic of parts of central Australia and South Africa where owing to the downstream disappearance of channels, floodwaters spill across extensive, unchannelled alluvial surfaces termed ‘floodouts’.
Specific features distinctive to some dryland floodplains include intercalated alluvial and aeolian sediments, and subsurface accumulations of solutes which result from biological or evaporational concentration mechanisms. In southern Africa, solute accumulation sometimes results in swelling of floodplain soils to form ‘islands’ elevated above the level of flooding. Many other floodplain features, however, are commonly shared with rivers in more humid settings, thus casting doubts as to whether dryland floodplains can necessarily be regarded as truly distinctive. Such ‘shared’ features may result from the fact that floodplain landforms and sediments often do not carry a particular climatic signature, or because the floodplains themselves have been constructed under both relatively humid and arid climates. Detailed investigations of floodplains and terraces bordering moderate-gradient rivers in semi-arid South Africa, for instance, show that major changes in river hydrology and sediment load have occurred in response to late Quaternary climatic fluctuations involving conditions both more humid and more arid than at present. This has resulted in substantial changes in depositional style, and extensive reworking and climatic overprinting of previously accumulated sediments and soils. The resulting sedimentary facies and architectures are complex and encompass both dryland and humid region attributes.
Abstract for ‘Dryland Change 2001 International Conference’, Upington, Northern Cape, South Africa, August 30-September 2
Controls on late Quaternary river channel and alluvial fan activity in the semi-arid Steelpoort region, Mpumalanga/Northern Province, South Africa
S. Tooth1, T.S. McCarthy2, D. Brandt2, P.J. Hancox2 and G.J. Kendall2
1Institute of Geography and Earth Sciences, University of Wales, Aberystwyth, UK: set@aber.ac.uk
2Department of Geology, University of the Witwatersrand, Johannesburg, South Africa: 065mct@cosmos.wits.ac.za, 065dion@cosmos.wits.ac.za, 065pjh@cosmos.wits.ac.za
River channels and alluvial fans are common features in many mountainous dryland regions, and their landforms and sediments have been widely used for palaeoenvironmental reconstruction. In many cases, however, there is considerable debate as to the nature of the palaeoenvironmental signal recorded, such as whether phases of erosion and sedimentation are primarily controlled by tectonic, climatic or land-use change and, if climatically-controlled, whether these phases reflect conditions of relative aridity or relative humidity.
In the rugged, semi-arid Steelpoort region, Mpumalanga/Northern Province, South Africa, individual or coalesced fans cover many of the mountain footslopes, and grade downslope into the alluvial terraces of the major rivers draining the valleys. Many of these fans and terraces currently are being extensively dissected by ‘dongas’ (gullies), which is causing damage to many roads and settlements, and reducing the area available for cultivation. Investigations of the stratigraphy exposed in the dongas typically reveal successions of clay-rich soils intercalated with, and sometimes cross-cut by, gravelly sands. The absence of range-bounding faults or obvious dislocation of the sediments argues against a tectonic control on sedimentation/erosion, and indicates instead that the channels and fans have experienced complex, climatically-controlled histories of erosion, sedimentation, and pedogenesis during the late Quaternary.
The gravelly sands consist largely of unweathered plagioclase feldspar and pyroxene minerals derived from the local mafic source rocks, suggesting that fan and floodplain/terrace erosion followed by more extensive sedimentation occurs during relatively arid climatic intervals when a protective vegetation cover is reduced, and increased hillslope sediment supply occurs during occasional storms. In contrast, during subsequent relatively humid intervals when a protective vegetation cover is restored, the landsurface stabilises and the top of the gravelly sands weather to form clay-rich soils. Pedogenesis during these humid intervals is aided by rapid alteration of the unstable feldspar and pyroxene-rich minerals. A subsequent return to a relatively arid climate again results in erosion followed by extensive sedimentation, thus resulting in partial truncation and/or burial of the soils by gravelly sands. With a return to a relatively humid climate, the top of these gravelly sands then weather to form another clay-rich soil. Aerial photographs and field investigations show that the current phase of extensive donga erosion in the fans and terraces predates indigenous population expansion in the region, and demonstrates instead that it is a natural process closely controlled by base level fall along the major rivers.
The similar stratigraphy exposed in dongas throughout the Steelpoort region indicates that channel and fan erosion/sedimentation has responded in synchrony to late Quaternary climatic change, although it is presently unclear whether this activity is driven by submillenial climatic events such as the Little Ice Age (c.1300-1800 AD), or only to major climatic changes on the scale of stadial-interstadial cycles. Research in progress is attempting to obtain luminescence ages from the fan and terrace sediments to establish chronological control for phases of sedimentation, and to enhance our understanding of late Quaternary climatic change in a region where other sources of palaeoenvironmental data currently are largely lacking.
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