Unravelling the recent history of sediment dynamics in the Karoo uplands of the Eastern Cape, South Africa based on reservoir sedimentation and sediment source tracing

We present results from an ongoing research programme in the Sneeuberg and Coetzeesberg ranges of the Eastern Cape, South Africa. The programme has focused on the econstruction of sediment yields from Cs-137 and Pb-210 dated deposits accumulating in small farm reservoirs. A number of tracers were used to determine whether changes in sediment yield were associated with changes in catchment connectivity and / or sediment sources in two of these small (<60 km-2) catchments. The analysis presented here compares the response of a steep upland catchment (Ganora; in the Sneeuberg) that has extensive badland development with that of a lower altitude catchment (Cranemere; located on a gently sloping plain south of the Coetzeesberg) that has limited badland development and significant internal sediment storage (alluvial fans and flood-outs). Results show that average sediment yields were higher by almost an order of magnitude in the Ganora catchment but increased significantly in the latter half of the 20th Century in both catchments. The exact timing of these increases was different in each of the two catchments as were the factors controlling sediment delivery to the reservoirs. At Ganora, a change in sediment source and small increases in sediment yield appear to have been initially driven by the development of badlands in the 1930s. More significant increases in sediment yield and a sustained change in sediment source was maintained by improved connectivity between the badlands and the main channel network in the 1960s. At Cranemere, increased connectivity between the two contributing sub-catchments and the reservoir occurred as a result of the construction of a causeway to carry a main road and by the construction of culverts beneath the road in the early 1950s, permanently improving the connectivity between the contributing catchments and the dam. Occasional changes in sediment source have also been identified in the sedimentary record at Cranemere but these are not linked directly to road construction but to sediment that is most likely derived from the much smaller eastern tributary 40 catchment. Our analysis has also explored the possible impacts of land use change (especially stocking density) and changes in the magnitude and frequency of daily rainfall in order to help explain some of the trends observed. Research to date shows the complexity of sediment delivery in these semi-arid catchment systems and emphasises the need to combine sediment yield reconstruction with sediment source tracing in order to better understand the dynamics of these systems.