For a long time this storage dam ranked as the biggest in South Africa, but now is only the fourth-largest, after the two big dams on the Orange River and the JG Strijdom Dam on the Pongola River. The Vaal Dam was constructed in the Vaal River 32 km south-east of Vereeniging and 77 km south-east of Johannesburg. The concrete wall is only 10 km (as the crow flies) north-west of the infall to the Wilge River into the Vaal, so that the storage basin extends up both rivers for long distances. The selection of this site on the farm Vaalbank followed extensive surveys of the Vaal down to Christiana. The final choice rested on two important considerations, namely the suitability of the basin and the foundations, and the fact that the dam would be so placed as to enable the Rand Water Board to draw upon a large water-supply.
The catchment area of the Vaal dam extends over 40 000 m2. Owing to tremendous post-War development on the Witwatersrand and in Pretoria and Vereeniging, it soon became apparent that the dam would have to be enlarged in order to store bigger proportion of the run-off of close on 2,6-million cubic metres. In the original design this had been provided for, and in 1952 a start was made with raising the concrete wall by 6 m, an additional 4 m being provided by gates on the crest. This has brought the capacity up to 2,4-million cubic metres, the surface area being 30 000 ha and the perimeter 700 km. The depth of the water when full is 47 m above riverbed.
The solid crest of the concrete wall is 1 480 m above sea level. Spanning the wall, with a length of 680 m, is the operating platform. In addition to the main concrete wall, there are two subsidiary earth embankments, closing gaps on the right flank.
The drainage area above the dam is nearly 40 000 m².
