Contact the authors:
markr@gecko.biol.wits.ac.za

Five main channel patterns (viz. alluvial single thread, mixed (bedrock and alluvium) pool-rapid, mixed braided, mixed anastomosing and bedrock anastomosing) were identified in a study of channel pattern changes in a mixed bedrock/alluvial, semi-arid river system in the Kruger National Park, South Africa. The study made use of the 50 year aerial photographic record. A channel type classification system developed for morphologically similar rivers (van Niekerk et al., 1995) was used to classify the changing channel patterns between large flooding events. The results showed that there was frequent alternation between the mixed braided and pool-rapid channel type states; that the bedrock anastomosing channel type gradually became more alluviated and that the mixed anastomosing channel type had a highly stable planform (Rountree et al., 2001). From these data, a conceptual model (see figure) was developed which explains the differing rates and pathways of the channel pattern changes during the recovery period between large stripping events.

The magnitude of a flood required to strip (or 'reset') the different channel types is the main driver of this model. Very large floods would be required to exceed the thresholds of change in the wide anastomosing channel types and they are therefore the most stable channel patterns. Relatively small events can scour sediment from the braided and pool-rapid channel types, hence the comparative instability of these channel patterns. During the "recovery phase" between large flooding events, sediment deposition and vegetation establishment in and adjacent to the active channels increases the stability of the channel pattern. As the recovery phase progresses, the magnitude of flood required to strip the channel segment increases with increasing sediment deposition and vegetation development. A project is currently underway to validate this model.