Reconstructing historical sediment yields from the infilling of farm reservoirs, Eastern Cape, South Africa

In order to calculate sediment yields, a chronology was established for sediments accumulating in two farm reservoirs at single coring locations in each reservoir. The chronology has been transferred to six adjacent cores in each reservoir using magnetic core correlation, particle size and visual stratigraphy. Reservoir sediment volumes corresponding to variable surface levels behind the two dams were estimated using GPS-based measurement of the contemporary reservoir in-fill, combined with coarse assumptions about underlying reservoir geometry derived from aerial photographs and topographic maps. Upper limits on volume were calculated for the hypothetical case of a reservoir with near-vertical sides, and lower limits for the case of linearly-sloping sides (i.e. a v-shaped cross-section). The arithmetic mean of these two boundary cases was taken as the best estimate. Reservoir sediment volumes, and volumes of sediment accumulating between dated horizons in the sediment cores, were obtained from the best estimate of changing sediment volumes derived from these assumptions. The volume of sediment accumulating between each time zone was adjusted by combining sedimentation rates with trap efficiency estimates. These data were subsequently converted to specific sediment yields using the average sediment density. Results from the two reservoirs to which this methodology has been applied show that the historical timing of increased sediment yields is different and that yields have not declined significantly over the last 50 years, despite reductions in stocking density and the abandonment of rain-fed agriculture