The water supply to Bloemfontein is, therefore, augmented from the adjacent Caledon River by means of the Caledon - Modder River Government Water Scheme (CMRGWS).
For various reasons, however, constructing a smaller scheme on the Caledon River was implemented before the completion of the Vanderkloof Dam.
The dam has a catchment area of some 15 245 km2 with a natural mean annual runoff (MAR) of around 1210 million m3/a (1920 to 1987) and was completed in 1973.
Its purpose was to supply water to the city of Bloemfontein via the 115-km-long Caledon-Bloemfontein pipeline, which has a capacity of around 1.157 m3/s.
This reduction in storage created problems in meeting the Bloemfontein demand at an acceptable level of reliability and as a result, the 50-m-high Knellpoort Dam was completed in 1988.