Morphodynamics of a gully and floodout system in the Sneeuberg Mountains of the semi-arid Karoo, South Africa: Implications for local landscape connectivity

The aim of this study was to investigate sedimentary processes in a discontinuous gully system, and to establish how they impacted upon overland flow and erosion in the short-term, while also considering their impact on long-term landscape connectivity. This manuscript documents the effect that a series of discontinuous gullies, located in the semi-arid Karoo (South Africa), had on local valley morphology and landscape evolution. The longitudinal and cross-sectional morphology of two discontinuous gully systems were accurately surveyed. In addition, the sedimentology of eight cores was described and sediment particle size and percentage organics determined, while the activity of the radioactive element 210Pb was measured in three of the cores. In contrast to previous studies, floodouts on the Karoo gullies formed progressively downstream, as avulsion occurred on the floodout more quickly than upstream backfilling. 210Pb geochronology suggests that the most recent avulsion occurred following one of the wettest seasons on record, where double the mean annual rainfall fell in just a few months. A series of palaeo-floodouts along the length of the gullies suggests that the process of gullying and depositional floodout formation is not restricted to the recent past, and is a long-term landscape process. Thus, despite the lack of characteristic cut-and-fill features, such as terraces, the long-term evolution of the Karoo landscape may be related to episodic gullying. In the short term, palaeo- and current floodouts alter the overland flow hydrology of the landscape, and prevent the local formation of badlands by diverting overland flow away from gully thalwegs. This results in the formation of amphitheatre-type gullies, with laterally eroded areas draining into narrow outlet channels incised into palaeo-floodouts. ► Sedimentology and processes of discontinuous gully network investigated. ► Floodouts comprise debris-flow and sieve deposits. ► Floodouts formed progressively downstream, as avulsion favoured over upstream backfilling. ► Palaeo-floodouts diverted overland flow from gullies, preventing lateral expansion. ► Palaeo-floodouts result in formation of amphitheatre-type gullies.