Drought conditions and sediment transport in the Sabie River

Abstract

Drought conditions in the Sabie catchment in the eastern Transvaal (now called Mpumalanga), South Africa, has had an observable effect on the sediment dynamics of the river. Sediment production within the catchment is largely unaffected by a reduction in the frequency and magnitude of rainfall events, although the rate of translocation of the weathered material from the catchment into the river channel is noticeably altered. The infrequent storm events during drought conditions generate a greater sed- iment input to the river from the catchment than a similar-magnitude event under average conditions. This sediment is also less likely to be transported through the system due to the reduced frequency of intermediate flows which act to rework in-channel sed- iment accumulations. Thus, significant accumulations of alluvial material are likely to form at specific locations, particularly where the local sediment transport capacity of the channel is low. Studies of the transport dynamics of the Sabie River, under both nor- mal and drought conditions, reveal that there are major depositional zones between Kruger Weir and Skukuza, and in the area around Lower Sabie. The 1992 drought resulted in a significant build-up of sediment in these areas, with a consequent reduc- tion in geomorphic diversity. This sediment is becoming stabilised due to the lower and less variable flows of the recent drought and associated vegetative colonisation. An increase in the magnitude and frequency of high and intermediate flows is needed to mobilise this accumulated sediment and to prevent its stabilisation by riparian vegetation.