137 Cs and 210 Pb derived sediment accumulation rates and their role in the long-term development of the Mkuze River floodplain, South Africa Marc S. Humphries a, ⁎, Andrew Kindness a , William N. Ellery b,c , Jeffrey C. Hughes b , Claudia R. Benitez-Nelson d a School of Chemistry, University of KwaZulu-Natal, Private Bag X54001, Durban, 4000, South Africa b School of Environmental Sciences, University of KwaZulu-Natal, South Africa c Department of Environmental Science, Rhodes University, Grahamstown, 6140, South Africa d Department of Earth and Ocean Sciences, University of South Carolina, Columbia. SC 29208, USA abstract article info Article history: Received 27 September 2009 Received in revised form 1 March 2010 Accepted 2 March 2010 Available online 7 March 2010 Keywords: Floodplain wetland Pb-210 Cs-137 Sedimentation rates Wetlands are well known to act as sinks for sediment and chemicals in the landscape. Within the Mkuze River floodplain in northern KwaZulu-Natal, South Africa, deposition of clastic sediment occurs predominantly in close proximity to the river channel, resulting in the formation of levees. Over time, this leads to channel avulsion, a process that may be accelerated by the activities of hippopotami whose trails create hydraulically favourable pathways. Sedimentation rates, determined using the isotopes 210 Pb and 137 Cs, indicate that the Mkuze River floodplain is a relatively rapidly aggrading system that should experience frequent avulsion, with average short-term rates in the order of 0.25 to 0.50 cm/y. Sediments on the floodplain are also an important sink for solutes, which concentrate in the groundwater and precipitate out because of evapotranspiration. Over long timescales, chemical sedimentation affects the landscape by influencing salinity, vegetation distribution, hydrological flows, and local topography. The Mkuze River floodplain is an actively evolving system, which continues to aggrade as a result of the combination of clastic and chemical sedimentation. In a region characterised by a strong annual water deficit, temporal patterns of clastic and chemical sedimentation are likely to exert influence on the long-term development of wetland systems elsewhere in southern Africa. This study, the first on sediment accretion rates using 210 Pb and 137 Cs dating for a southern African wetland, demonstrates that radioisotopic methods are an important tool that can be applied toward fully understanding wetland formation, evolution, and functioning in the region. © 2010 Elsevier B.V. All rights reserved. 1. Introduction One of the benefits of wetlands is their ability to act as sinks for sediment and chemicals in the landscape. Sedimentation within wetland systems has traditionally been investigated by considering only clastic deposition (Sanchez-Carrillo et al., 2001; Harter and Mitsch, 2003). However, studies carried out in the Okavango Delta have shown that ∼450,000 tonnes of chemical sediments accumulate annually in the wetlands of this system (McCarthy and Ellery, 1998), amounting to over twice the clastic load and, thus, the dominant form of aggradation. Within the Okavango Delta, chemical sedimentation occurs predominately as a result of evapotranspiration. With ∼ 96% of the water entering the Okavango Delta being lost to the atmosphere, solutes in the groundwater concentrate to levels that eventually result in the precipitation of minerals, such as CaCO 3 and SiO 2 . Mineral precipitation causes volume increase and expansion in the soil that plays a role in modifying the topography of the landscape (McCarthy and Metcalfe, 1990), while the development of highly saline ground- water results in marked vegetation zonation (McCarthy and Metcalfe, 1990; McCarthy et al., 1993). Similar chemical accumulation processes, although on a more limited scale, have also been documented on the Nyl River floodplain, South Africa (Tooth et al., 2002). Floodplain wetlands are common features of rivers in southern Africa, but have been insufficiently investigated from a geomorpho- logical perspective. Despite the fact that these wetlands fulfil important hydrological and geochemical functions in a region of strong seasonal water deficit, knowledge on such systems is limited to a few case studies (e.g. Tooth et al., 2002; Tooth et al., 2007; Grenfell et al., 2009). Recent studies of sediment, groundwater and porewater geochemistry have identified the Mkuze Wetland System as an important sink for solutes (Humphries et al., 2010; Barnes et al., 2002). This system, representing South Africa's largest freshwater wetland area, forms part of the iSimangaliso World Heritage Site and plays an ecologically important role in filtering sediment from water entering Lake St. Lucia. Sedimentation processes on the Mkuze River floodplain are, hence, of vital importance in understanding the role that the system plays in trapping material within the landscape. Geomorphology 119 (2010) 88–96 ⁎ Corresponding author. Tel.: + 27 312603090; fax: + 27 312603091. E-mail address: marchump@gmail.com (M.S. Humphries). 0169-555X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2010.03.003 Contents lists available at ScienceDirect Geomorphology journal homepage: www.elsevier.com/locate/geomorph