Process, form and change in dryland rivers: a review of recent research

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Abstract

Many of the world's extensive warm dryland regions support numerous, albeit often infrequently flowing, rivers. Dryland rivers are increasingly a focus of scientific and applied interest but empirical research and fluvial theory for drylands need to be strengthened. Recent research in arid central Australia indicates greater diversity in dryland river process, form and change than has hitherto been appreciated, and highlights the need for a global review assessing the present state of knowledge. This review outlines the distinctive characteristics of dryland fluvial environments (hillslope and channel hydrological and sediment transport processes, river pattern and geometry, temporal and spatial aspects of channel change, sedimentary structures and bedforms), many of which contrast with more humid fluvial environments. Although features common to many dryland fluvial environments can be identified (extreme temporal and spatial variability of rainfall, runoff and sediment transport, poor integration between tributary and trunk channels, importance of large floods as a control on channel morphology, lack of equilibrium between process and form), the fluvial diversity that exists within drylands requires recognition of the limitations to these generalisations. In particular, research in central Australia illustrates the need to understand the rivers of this region using empirical relationships, terms, and concepts additional to those defined by earlier work in drylands. Key deficiencies in dryland fluvial research are identified, and relate to three main areas: limited study of some aspects of modern dryland rivers (floodplain characteristics, influence of vegetation, downstream changes, importance of scale); limited understanding of dryland river behaviour over longer (Cenozoic) timescales; and lack of integration between the results from short-term, process-form studies and studies of the longer term histories of river behaviour. Linking knowledge of past hydrological and channel changes to present-day changes in dryland rivers is suggested as a key research priority. This will help develop a sound theoretical basis for the assessment of future developments in dryland river systems which will contribute to their improved scientific understanding and environmentally sensitive management.

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

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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.

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