The Orange - Senqu River originates in the highlands of Lesotho some 3 300m above mean sea level, and it runs for over 2300km to its mouth on the Atlantic Ocean. The river system isrnone of the largest river basins in Southern Africa with a total catchment area of more than 850,000km2 and includes the whole of Lesotho as well as portions of Botswana, Namibia and South Africa. The natural mean annual runoff at the mouth is estimated to be in the order of 11 500 million m3, but this has been significantly reduced by extensive water utilization for domestic, industrial and agricultural purposes to such an extentrnthat the current flow reaching the river mouth is now in the order of half the natural flow. The basin is shown in Figure 1. The Orange-Senqu system is regulated by more than thirty-one major dams and is a highly complex and integrated water resourcernsystems with numerous large inter and intra basin transfers.