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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 ~35m 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 …