Land degradation and sediment dynamics in the South African Karoo

Land degradation has led to sedimentation in farm dams in the headwaters of the Seekoei River. Two small catchments (<10 km2) with farm dams were selected for investigation, but detailed results are only presented for one of them. Caesium-137 and unsupported 210Pb are present in the dam sediments; event-driven layers are apparent from fine gravels contained within layers deposited over the last -50 years. The 137Cs reference inventories are low by northern hemispheric standards (-53 mBq cm-2). Activities in dam sediments frequently exceed 4 mBq g-1, and the total inventory for the dam master core exceeds 470 mBg cm-2. While extensive gully systems exist in both catchments, the high 137Cs activities in the sediments suggest that sediment delivered to the dam over the last five decades is dominated by topsoil erosion. This conclusion is generally supported by the mineral magnetic data.