Yes, this is a case of 'Africa's all the same place, right?' - it's a generic African exhibit so the oryx are next to a waterfall.
Of course, there's no reason why an oryx exhibit shouldn't be next to a waterfall - the area isn't a desert-themed exhibit, just African and happens to have oryx.
Actually, underneath the falls are the stables of the oryxes. As from the two reall exhibits (a grassy exhibit and a sandy one) you can't see the falls. I really like this exhibit, but indeed: it's a bit unusual that the oryx exhibit is next to a waterfall.
HAHA that's hilarious and yet strangely correct, but historical context they are more desert dwelling.
now for the comment i was going to type before reading all the previously posted:
THIS IS AN AMAZING PHOTO. I was thinking of something similar to this i had drawn up several months ago for a exhibit featuring mule deer, Roosevelt elk, moose, mountain goats from Great Bear Rainforest in British Columbia Canada. Just a hobby right now designing these for a fictional zoo of mine.