The origin and development of the Nyl River floodplain wetland, Limpopo Province, South Africa: trunk–tributary river interactions in a dryland setting

Abstract The Nyl River floodplain wetland, one of South Africa's largest floodplain wetlands and a Ramsar site of international conservation importance, is located in an area of long-term and still active valley sediment accumulation. Creation of accommodation space for sedimentation has previously been attributed to tectonic controls, but new investigations reveal that a more likely cause is progradation of coarse-grained tributary fans across the narrow river valley downstream of the main area of floodplain wetland. Obstruction of trunk river flow and sediment transfer by these tributary fans has led to backponding and upstream gradient reduction and to accumulation of valley fills up to ∼35 m thick. Chronological control for the timing and rate of sediment accumulation is limited, but we hypothesise that a semi-arid to arid climate, characterised by asynchronous trunk–tributary activity that results in marked discontinuities in downstream water and sediment transfer, is likely to have been a key control. These interpretations are supported by other studies of dryland rivers globally and the findings add to our growing understanding of the controls on the origin and development of southern African wetlands, particularly by demonstrating how the combination of a particular physiography and a dryland climate can impart some distinctive geomorphological characteristics. Keywords: alluvial sedimentationdrylandfloodplainriver channeltributarywetland Acknowledgements Financial support for this study was generously provided by the Skye Foundation, the University of the Witwatersrand, the National Research Foundation, the British Geomorphological Research Group and Aberystwyth University. We gratefully acknowledge fruitful discussions with Dr Warwick Tarborton concerning the dynamics of the Nyl River floodplain wetland. We thank Anglo Platinum for making borehole logs available for this study, Keith Scott of MSA and Dr Walter Ward for providing logistical support, Diane du Toit and Ian Gulley for preparing diagrams and two anonymous reviewers and Paul Sumner for comments that helped to improve the manuscript.