When you go to Orpen again, have a look on top of the door at reception there is a whole explanation of why waterholes in the KNP have been closed.
And it was posted on the site by KNPSM, but I cannot find it!
But, google is your friend, here are a few snippets I did find:
In the 1960s, in an effort to boost game numbers, the Water for Wildlife project was started and erected about 300 windmills in the park. The waterholes attracted game into the area. At first this seemed a good thing; only decades later did the results show that with the impalas and zebras the waterholes attracted also brought more predators into the area. Before the waterholes, these dryer areas supported roan antelope, which are much easier for lions to catch - the roans weren't able to compete. The park has started to close the waterholes, and let nature take its course.
Angela Gaylard is doing her doctorate on the impact of elephants on Kruger's riverine vegetation through the University of the Witwatersrand. In an ideal world, she says, park conservationists should close the existing artificial waterholes and force the entire ecosystem, including elephants, to depend on the seasonal fluctuations of the rivers. This would keep numbers in check through natural forces, and the elephants' impact on vegetation would no longer threaten regional biodiversity. In fact, she maintains, it may even enhance it.
Even so, elephant expert and former director of conservation development at SANParks, Dr Anthony Hall-Martin, considers megaparks and closing the artificial waterholes in Kruger the most realistic, natural and potentially effective of the options.
: The photos from our trip! Overhere! Feel free to use any of these additional letters to correct the spelling of words found in the above post: a-e-t-n-d-i-o-s-m-l-u-y-h-c