Process, form and change in dryland rivers: a review of recent research
Introduction
‘Drylands’ is a collective term for the world's hyper-arid, arid, semi-arid and dry-subhumid regions, which cover almost 50% of the global land area and contain almost 20% of the global population UNEP, 1992, Middleton and Thomas, 1997. Drylands, which include many regions commonly referred to as deserts or semi-deserts, currently exist across a wide spectrum of settings from cold, high latitude, high altitude environments to warm, low latitude, low altitude environments. Warm dryland regions, which are mainly located in certain areas of the tropics and sub-tropics, form the largest expanse of drylands Glennie, 1987a, Goudie, 1987, Cooke et al., 1993, Thomas, 1997a and are the focus of interest in this paper (Fig. 1). Whilst by definition the term ‘dryland’ covers a wide range of environments, warm drylands are characterised by high (but variable) degrees of aridity, reflecting low ratios between precipitation and potential evapotranspiration, and by sparse, unevenly distributed, or temporally variable vegetation covers. Given their vast areal extent, an ever-growing human population, the significant resource potential of many areas, and issues such as the threat of anthropogenically induced climate change and land degradation, warm drylands have increasingly become a focus of attention Thomas and Middleton, 1994, Millington and Pye, 1994, Williams and Balling, 1996.
There are a number of scientific and practical imperatives for the study of dryland fluvial environments. First, despite their general aridity, many drylands support numerous, albeit often infrequently flowing, rivers. General observations and a small number of monitored flood events in drylands indicate that fluvial processes are significant agents of erosion and deposition and thus, over time, dryland rivers can be active land-forming agents Frostick and Reid, 1987, Reid and Frostick, 1997. Second, by virtue of their aridity and general distance from the direct effects of the Quaternary glaciations, many dryland fluvial deposits have survived subsequent reworking and are preserved as sedimentary successions Merifield, 1987, Reid and Frostick, 1997. In combination with associated aeolian, lacustrine and pedogenic deposits, these fluvial deposits provide great potential for palaeoenvironmental reconstruction Reid, 1994, Nanson and Tooth, 1999. Third, water resource issues such as river channel and catchment management are rapidly assuming major environmental, economic and political importance in the drylands of both the developed and developing world, as evidenced by symposia and conferences devoted to dryland hydrology (e.g. IAHS-AISH, 1979, French, 1990).
Clearly, if these scientific and practical imperatives are to be met, then empirical research and fluvial theory for drylands need to be strengthened. In particular, recent research in central Australia has indicated greater diversity in dryland river process, form and change than has hitherto been appreciated (Tooth, 1997), and has highlighted the need for an up-to-date assessment of the present state of knowledge. Recent summaries and reviews of various aspects of dryland hydrology and fluvial geomorphology have been provided by Graf (1988a), Cooke et al. (1993), Thornes, 1994a, Thornes, 1994b, Reid and Frostick (1997), Knighton and Nanson (1997) and Nanson and Tooth (1999); these provide perspectives and information additional to that presented here. The purpose of this review is to provide a general overview of fluvial research in drylands, to highlight the distinctive yet diverse characteristics of dryland fluvial environments and establish the context for findings emerging from recent research in the Australian drylands, and to identify gaps in understanding that may help to prioritise future research efforts.
Section snippets
Location of dryland fluvial research
Dryland rivers featured strongly in the early development of the science of fluvial geomorphology, largely as a result of early work in the American southwest Graf, 1988a, Reid and Frostick, 1997. For example, semi-arid field sites provided the setting for much of the early research on the processes of erosion and deposition Gilbert, 1876, Gilbert, 1877, Schumm, 1960a, Schumm, 1960b, Schumm, 1961a, Schumm, 1961b, hydraulic geometry (Leopold and Miller, 1956), and the behaviour of arroyos and
Distinctiveness and diversity of dryland fluvial environments
Despite the importance of the early studies in the American southwest, and the results of later research both in this and other drylands, the development of theory for dryland rivers to some extent has relied on ideas imported from more humid regions. This approach can bring problems because in drylands, the relative importance of many fluvial processes, and the magnitude and frequency of their operation, often differ considerably from more humid regions Graf, 1988a, Thornes, 1994a.
Whilst there
Process–form relationships in dryland rivers
From this review of dryland fluvial research, a number of general principles emerge regarding the nature of dryland river process, form and change. Understanding the interaction between process and form over various timescales remains basic to fluvial geomorphology but in drylands a range of complicating factors impinge upon attempts to achieve this objective.
In particular, given the practical problems of directly observing and instrumenting flow and sediment transport processes during floods
Deficiencies in dryland fluvial research
In addition to the general principles emerging from consideration of the nature of dryland river process, form and change, a number of deficiencies in dryland fluvial research can also be identified. These relate to three main areas: first, to short-term, process–form studies; second, to studies of longer term river history; and third, to links between these two areas of research and concepts and theories regarding dryland river behaviour.
Conclusion
Given the vast geographic extent of drylands, which includes a wide range of climatic, tectonic, structural, lithological and phytological settings, variability in dryland river process, form and change is only to be expected Knighton and Nanson, 1997, Nanson and Tooth, 1999. Nevertheless, some common aspects of dryland fluvial environments can be readily identified. For instance, although variability in time and space is a feature of fluvial processes in all environmental settings, it appears
Acknowledgements
An earlier version of this paper formed part of a PhD thesis supervised by G.C. Nanson, School of Geosciences, University of Wollongong. Fieldwork in central Australia was supported by a postgraduate scholarship provided by the University, by the University's Quaternary Environments Research Centre and by the Australian Research Council via grants to G.C. Nanson and D.M. Price. The paper was prepared while the author was based in the Department of Geology, University of the Witwatersrand, with
Dr. Stephen Tooth graduated with a BSc Honours degree in Geography from the University of Southampton, England, and obtained his PhD from the University of Wollongong, Australia. His doctoral research focused on the morphology, sedimentology and late Quarternary history of ephemeral rivers in arid central Australia. In 1998 he joined the Department of Geology, University of the Witwatersrand, South Africa, as a NRF Research Fellow, and in 2000 will start a lectureship in the Institute of
References (382)
Channel-fill and sheet-flood facies sequences in the ephemeral River Gash, Kassala, Sudan
Sedimentary Geology
(1989)- et al.
Relation between infiltration and stone cover on a semiarid hillslope, southern Arizona
Journal of Hydrology
(1991) - et al.
Resistance to overland flow on desert slopes
Journal of Hydrology
(1986) - et al.
Effects of vegetation change on interrill runoff and erosion, Walnut Gulch, southern Arizona
Geomorphology
(1995) A hierarchical view of the interactions of runoff and infiltration with vegetation and microtopography in semiarid shrublands
Catena
(1998)Discontinuous ephemeral streams
Geomorphology
(1997)- et al.
Fire and aspect induced differences on the erodibility and hydrology of soils at La Costeram, Valencia, southeast Spain
Catena
(1995) Hydraulics and basin morphometry of the largest flash floods in the conterminous United States
Journal of Hydrology
(1987)- et al.
Stratigraphy and sedimentology of the lower Neales River, west Lake Eyre, central Australia: from Palaeocene to Holocene
- et al.
Mesozoic and Tertiary sediment supply to the western Cape Basin and palaeodrainage systems in south-western Africa
Marine Geology
(1984)
Principles for the sustainable utilization of the Okavango Delta ecosystem, Botswana
Biological Conservation
Channel responses to declining flow on the Rio Grande between Ft. Quitman and Presidio, Texas
Geomorphology
Regional flood frequency analysis in arid and semi-arid areas
Journal of Hydrology
Anastomosing channels and arroyo development on the Nogoa River, Central Queensland, Australia
Chemical characteristics of a desert stream in flash flood
Journal of Arid Environments
The integrated Nile
Late Quaternary history of the Nile
Nature
Palaeogeography of the Gezira and of the lower Blue and White Nile valleys
Climatic, eustatic, and tectonic controls on Quaternary deposits and landforms, Red Sea coast, Egypt
Journal of Geophysical Research
Recharge to groundwater from floods in a typical desert wash, Pinal County, Arizona
Transactions of the American Geophysical Union
The nature of saltation and of ‘bedload’ transport in water
Proceedings of the Royal Society of London A
Extending overland flow models to problems of slope evolution and the representation of complex slope surface topographies
Stream-channel response to floods, with examples from central Texas
Geological Society of America Bulletin
Questions raised by the Tucson flood of 1983
Hydrology and Water Resources in Arizona and the Southwest
Flood sedimentation in bedrock fluvial systems
Palaeohydrologic analysis of Holocene flood slack-water sediments
Desert palaeofloods in central Australia
Nature
A model for the development of Quaternary terraces and piedmont-terraces in the southwestern United States
Zeitscrift für Geomorphologie
Precipitation
Hyperconcentrations of suspended sediment
Journal of the Hydraulics Division, Proceedings of the American Society of Civil Engineers
Saltcedar influence on sedimentation in the Brazos River
Journal of Soil and Water Conservation
Mediterranean alluviation: new evidence from archaeology
Proceedings of the Prehistoric Society
Quaternary wadi, lacustrine, aeolian depositional cycles and sequences, Chott Rharsa basin, southern Tunisia
Fluvial response to late Quaternary climatic and environmental change, Edwards Plateau, Texas
Palaeogeography, Palaeoclimatology, Palaeoecology
On the evolution of the upper-Molopo drainage
South African Geographical Journal
Cyclical construction and destruction of flood dominated flood plains in semiarid Australia
Fluvial form variability in arid central Australia
Stepped-bed morphology in arid gravelly channels
Geological Society of America Bulletin
Introduction
Sand sheets of the eastern Sahara and ripple banks on Mars
Runoff and sediment discharge in a semi-arid drainage basin
Zeitschrift für Geomorphologie, Supplementband
The alluvial-fan environment
Progress in Physical Geography
Threshold of critical power in streams
Geological Society of America Bulletin
Geomorphic thresholds as defined by ratios
Geomorphic Responses to Climatic Change
Spatial variability in runoff processes in an ephemeral channel
Zeitschrift für Geomorphologie, Supplementband
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Dr. Stephen Tooth graduated with a BSc Honours degree in Geography from the University of Southampton, England, and obtained his PhD from the University of Wollongong, Australia. His doctoral research focused on the morphology, sedimentology and late Quarternary history of ephemeral rivers in arid central Australia. In 1998 he joined the Department of Geology, University of the Witwatersrand, South Africa, as a NRF Research Fellow, and in 2000 will start a lectureship in the Institute of Geography and Earth Sciences, University of Wales, Aberystwyth. Research interests include fluvial geomorphology and the geomorphology and sedimentology of drylands, with particular emphasis on Australia and southern Africa.