Chronology and controls of donga (gully) formation in the upper Blood River catchment,KwaZulu-Natal,South Africa: Evidence for a climatic driver of erosion

Abstract

Dongas (gullies and badland-type terrain) are found extensively across interior South Africa, but the controls on their formation remain contested. Establishing the timing of donga formation is essential for developing a clearer understanding of the relative importance of natural (e.g. climatic) and human (e.g. overgrazing) controls. Here, we present optically stimulated luminescence (OSL) ages for colluvial sediments exposed in the sidewalls of dongas and for tributary fan sediments deposited downslope of dongas in the upper Blood River catchment, eastern South Africa. The tributary fan ages enable the timing of donga formation to be established, since fan sedimentation is a direct result of upslope donga erosion. The OSL ages indicate that net colluviation and intervening palaeosol formation occurred from prior to 22 ka until ~1.6 ka. From ~0.89 ka to present, there has been a shift to net erosion characterised by widespread donga formation and tributary fan sedimentation. Contemporary donga formation in interior South Africa has most commonly been attributed to changing land-use practices associated with European settlement from the second half of the 18th century onwards, but the OSL chronology indicates that dongas in the study area began forming significantly prior to European settlement. Instead, the initiation and main phase of donga formation encompasses periods of abrupt climatic changes associated with the ‘Mediaeval Climate Anomaly’ (ad ~900–1300) and the ‘Little Ice Age’ (ad ~1300–1800). The magnitude and abruptness of climatic changes during these periods appear to have destabilised hillslopes and initiated the shift from long-term net colluviation to net erosion by donga formation. The OSL-based approach to constraining the timing of donga formation adopted in this study has broader implications for assessment of the drivers of historical and contemporary donga (gully) formation elsewhere in South Africa and other drylands, and may help to inform management strategies for degrading lands.