Luminescence dating, sediment analysis, and flood dynamics on the Sabie River, South Africa Jasper Knight ⁎, Mary Evans School of Geography, Archaeology and Environmental Studies, University of the Witwatersrand, Private Bag 3, Wits, 2050 Johannesburg, South Africa abstract article info Article history: Received 2 October 2017 Received in revised form 14 July 2018 Accepted 14 July 2018 Available online 17 July 2018 Floods on sediment-rich semiarid rivers often lead to significant geomorphic change, but luminescence dating of sediments deposited by past flood events has hitherto been unable to meaningfully inform on past flood dynam- ics. This is important because luminescence dating has commonly been applied to flood sediments, but the cal- culated sediment ages are in almost all cases older than the ages of known floods responsible for sediment deposition. Correctly interpreting these older ages in the context of flood dynamics is therefore a key problem. This study addresses this problem by discussing relationships between luminescence dating, sediments, and reach-scale geomorphology along the semiarid Sabie River, northeastern South Africa. Twelve luminescence ages were obtained from near-surface sediments (b10 cm depth) from five sites across a 60-km distance of the lowland part of the river. Luminescence ages of 26–163 years suggest that sediments are differentially mobilized by successive floods and that high suspended sediment concentrations in the water column were insufficient to effectively zero the luminescence signal of the quartz grains. Analysis of equivalent dose values and distributions from individual dated samples using abanico plots shows the relative degree of bleaching, thus informing of the mixed composition of the sample and its likely source. Different landforms (bars, fans, flood drapes) were devel- oped during maximal and waning flow stages, followed by fallout from suspension across the flooded valley that formed a drape that thins toward the upper flood limit. Different grain size properties and luminescence signals are found in these different fluvial settings. These landform and sediment properties can help explain why differ- ential bleaching took place during fluvial sediment transport, which gave rise to the scatter in equivalent doses found within the dated samples. A model is proposed that describes these corelationships and provides a hypoth- esis of these relationships to test in future studies. © 2018 Elsevier B.V. All rights reserved. Keywords: Grain size analysis Fluvial geomorphology Flood sediments Luminescence dating 1. Introduction Rivers in semiarid environments tend to be characterized by high seasonality of precipitation and thus river discharge, high event-scale precipitation rates giving rise to rapid overland flow and river sediment yield, and flashy discharge regimes giving rise to high flow turbulence and sediment carrying capacity during maximal flood stages (Pickup, 1991; Ashworth et al., 2000; Grenfell and Ellery, 2009; Tooth et al., 2013). As such, semiarid rivers are often associated with high flood haz- ard. In the semiarid areas of northeast South Africa, incoming cyclones from the Indian Ocean have led to some significant flood events over the last 20 years (e.g., in 2000, 2012) that have resulted in loss of life as well as major geomorphic change within the river systems (Rountree et al., 2000; Heritage et al., 2004). Many previous studies of semiarid rivers globally have focused on the geomorphic impacts of flood events, using a combination of aerial surveys, satellite imagery, and LiDAR in order to produce geomorphological maps and/or for change analysis (e.g., Ashworth et al., 2000; Rountree et al., 2000; Heritage et al., 2001, 2004; Croke et al., 2013; Buraas et al., 2014; Hooke, 2016). However, reach-scale patterns may vary significantly even along the same river, reflecting spatial changes in bedrock controls and sediment connectivity (Heritage et al., 2003, 2015; Croke et al., 2013; Entwistle et al., 2015). The landforms and sediments (sediment type, sorting, mineralogy) found along individual reaches can also in- form on flood dynamics during maximal and waning flow stages (e.g., Pettit et al., 2005; Grenfell and Ellery, 2009; Keen-Zebert et al., 2013; Knight and Evans, 2017). More recently, luminescence dating of flood-transported sediments (either as slackwater drapes or maximal flood stage bars or fans) has been commonly used to provide chronologies of palaeoflood events over the late Holocene, linked to climate (e.g., Sim et al., 2014; Roskosch et al., 2015; Larkin et al., 2017; Zhao et al., 2017). Issues con- cerned with this approach include the age-model chosen, which can sig- nificantly affect the calculated age and thus climatic interpretation of age results (Kunz et al., 2013; Muñoz-Salinas et al., 2016). Other studies have also examined the luminescence signatures of flood layers Geomorphology 319 (2018) 1–14 ⁎ Corresponding author. E-mail address: Jasper.knight@wits.ac.za (J. Knight). https://doi.org/10.1016/j.geomorph.2018.07.011 0169-555X/© 2018 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Geomorphology journal homepage: www.elsevier.com/locate/geomorph