The main purpose of this study is to determine the difference in water quality of the rivers between the Katse and Vaal Dams (Wilge River and Vaal Dam reservoir sub-catchments) after the construction of the Lesotho Highlands Water Project. These rivers include the Ash, Liebenbergsvlei and Wilge Rivers. The temporal changes in water constituents, namely: electrical conductivity, chemical oxygen demand, pH, turbidity, ammonia, calcium, manganese and chlorophyll a, at selected water sampling points were analysed to clarify if Katse Dam water has had any impact on the water quality of the Ash, Liebenbergsvlei and Wilge Rivers and the Vaal Dam. The water quality was studied over an eleven-year period from November 1994 until December 2005. This includes a five-year period prior to, and a six-year period following the completion of the Katse Dam. The Ash, Liebenbergsvlei and Wilge Rivers fall within the Wilge sub-catchment, and the Vaal Dam falls within the Vaal Dam reservoir sub-catchment. Both the aforementioned sub-catchments form part of the Vaal River catchment. Physical, chemical and microbiological sampling results were obtained from Rand Water. The results were compared with the in-stream water quality guidelines as set by the Vaal Barrage Catchment Executive Committee. The results of the selected constituents were depicted visually in the form of graphs. Trends in the constituents over the period were then determined. The graphs were divided into two sections namely, pre-Katse Dam (before 1999) and post-Katse Dam (1999 to 2005). Differences in water quality before and after the construction of the Katse Dam were determined from sampling and chemical analysis at six locations, and hence evaluations were made whether the release of Katse Dam water has had a significant effect on the water quality results in the Vaal River System. The water quality results with respect to the different water constituents illustrated a distinct change in water quality over the period. Northwards, towards the Vaal Dam, the difference in water quality became less apparent. Sampling points throughout the study area experienced decreases in: electrical conductivity, chemical oxygen demand, turbidity, ammonia, and manganese. Hence, the release of Katse Dam water into the Vaal River system has had a ii positive influence on the water quality and thus changed the riverine environments in the Vaal River system. The high quality water from the Katse Dam that enters the Vaal River system thus initially increases the quality of the water in the recipient system with a lesser effect downstream. The result is an improvement of water quality in the upper reaches of the Vaal River system and no significant influence on the Vaal Dam itself. However, the change in water quality may have a detrimental effect on the river environment as a result of the increased volume of water entering the system and the resultant soil erosion, which serves for further studies. Consequently, the advantageous high quality water from the Lesotho Highlands is not being optimally utilised, hence the proposed recommendation by Rand Water to alternatively transfer Katse Dam water via a gravity-fed pipeline to the Vaal Dam thereby receiving the full benefit of high quality water, leaving river environments unaltered and possibly lowering purification costs. / Prof. J. T. Harmse Prof. H. J. Annegarn