November 25, 1937. Vaal-Hartz Irrigation Scheme: The Vaal-Hartz Weir. This dam is one of a pair that is being constructed to irrigate the Hartz River valley The Hartz River empties into the Vaal some miles below this dam and the water is diverted here into the canal seen on the right bank and to an area about 40 miles northward, in the Hartz basin. The other dam, which is used only as a storage basin, is just above Vereeniging; it regulates the flow of water down the Vaal River. 6300 feet. 05:45.