Optical dating of a scroll-bar sequence on the Klip River, South Africa, to derive the lateral migration rate of a meander bend

Problems are often encountered in accurately establishing the rates of processes associated with river channel change. In this study, optical dating was used to obtain the lateral migration rate of an abandoned meander bend on the Klip River, eastern Free State, South Africa. Five samples were collected by augering along a transect across an approximately 200-m-wide scroll-bar sequence on the inner meander bend, and an additional sample was collected from the channel fill of the related oxbow lake. Optically stimulated luminescence (OSL) ages were derived for each sample using the single-aliquot regenerative-dose protocol on small aliquots of quartz. Incomplete bleaching was evident in some of the samples and thus the appropriate statistical analysis was performed using the 'finite mixture model' for the scroll-bar samples and the 'minimum age model' for the channel-fill sample. The ages calculated were in the correct chronological order for the depositional sequence, which was formed over the last ∼ 1000 years. The average rate of lateral migration across the scroll-bar sequence was found to be ∼ 0.16 m/yr but with an apparent increase in the rate occurring during the early sixteenth century. Where quartz-rich alluvium exists, the methods used in this study can potentially be applied to other river systems, enabling rates of channel change processes to be determined.