When the dam wall was complete, it rose 171 feet (52 metres) above the river bed and impounded a lake of water with a shore line of 435 miles (700 kilometres). The present Vaal Dam wall has been raised twice. In 1955, the wall was raised ten feet (3.05 m) and the dam’s capacity increased to 344 007 million gallons (1 563 million litres or 1 563 Ml). The installation of flood gates in 1956 and the raising of the water level by another 3.05 m saw the capacity increase to 2 188 Ml. The ever increasing demand for water made it necessary for a further raising in 1985 and the wall was raised by a further 3,5 metres which increased the volume of the dam to 2 536 Ml with provision for a further 663 Ml which can be stored temporarily for flood attenuation.
The Vaal Dam has a catchment area covering 38 500 square kilometres, most of which is located in the Free State Province with the remainder in the North West, Gauteng and Mpumalanga provinces. The catchment area has an average annual precipitation of approximately 700mm with a corresponding annual potential evaporation in the order of 1 500mm. The dam has the capacity to hold 2575 million cubic metres of water which, under normal circumstances and without a substantial inflow, will last for 2 years. This dam has a surface area of 320 square kilometres and an average depth of 22,5m.