The Orange - Senqu River basin as shown in Figure 1 originates in the highlands of Lesotho some 3 300m above mean sea level, and it runs for over 2 300km to its mouth on the Atlantic Ocean. The river system is one 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 generated within the basin is estimated to be in the order of 11 900 million m3 based on the 1920 to 2004 period of record. The flow reaching the Orange river mouth has been significantly reduced by extensive water utilization for domestic, industrial and agricultural purposes as well as several major interbasin transfers , both into and out of the Orange/Senqu basin. Based on the most recent assessments, the average annual flow reaching the Orange River mouth is estimated to be approximately 3 700 million m3 which comprises the environmental requirements of the river mouth as well as the intermittent floods.