Development of badlands and gullies in the Sneeuberg, Great Karoo, South Africa

Abstract

The study aims to examine the origin and development of land degradation with particular emphasis on badland and gully systems in the Sneeuberg uplands of the Great Karoo. This is an area of semiarid extensive stock farming where land degradation in the form of rill and gully erosion has accompanied the replacement of grassland by shrub vegetation. Species diversity has declined and ground cover has been reduced, leading to a positive feedback loop which exacerbates the degradation. Many foot slopes developed in shales, clays and colluvium have extensive, incipient badland development with closely spaced gullying up to 1.5 m deep. In valley-bottom and valley-side depression locations gullies up to 8 m deep have developed, usually cut to bedrock through valley fills of mainly Holocene colluvium. Both badlands and gullies appear to have developed since European settlement and to be part of the same hydrological system with extensive areas of bare ground (badlands) feeding water to incising gullies. Experiments using simulated rainfall throw some light on current processes. Badland areas are active under high-frequency, low-magnitude rainfall events. Major gullies are likely to be the result of occasional, high-magnitude events, but these have not been observed. Overgrazing in the past is the most likely cause of the degradation.