So why does the Zambesi River 'Fall' into such a deep, narrow crack in the Earth?
Well it literally IS falling into a crack in the 180 million year old basalts that cover the area. Part of the East Africa Rift System. As these basalts cooled they formed cracks or fissures. These weak zones are preferentially eroded to form the Falls. This is the most recent of perhaps 5 earlier water falls that have formed in the area, having an estimated age of about 100,000 years.