From Chirundu to the Mozambique border, the Zambezi descends 42m, from 371m to 329m above sea level, over a distance of over 150km. That very gentle gradient (about 1:3,500) explains why the Zambezi flows so slowly and spreads out across the wide valley, making such a gentle course for canoeing.
From the river, look either side of you into Zambia and Zimbabwe. In the distance you will spot the escarpment, if the heat haze doesn't obscure it. At around 1,200m high, it marks the confines of the Lower Zambezi Valley and the start of the higher, cooler territory beyond which is known as the 'highveld' in Zimbabwe.
The valley is a rift valley, similar to the Great Rift Valley of East Africa (though probably older), and it shares its genesis with the adjoining Luangwa valley. The original sedimentary strata covering the whole area are part of the karoo system, sedimentary rocks laid down from about 300 to 175 million years ago. During this time, faulting occurred and volcanic material was injected into rifts in the existing sediments.
One of these faults, the wide Zambezi valley, can still be seen. In geologically recent times, the Zambezi has meandered across the wide valley floor, eroding the mineral-rich rocks into volcanic soils and depositing silts which have helped to make the valley so rich in vegetation and hence wildlife. These meanders have also left old watercourses and ox-bow pools, which add to the area's attraction for game.
So look again from one side of the valley to the other. What you see is not a huge river valley: it is a rift in the earth's crust through which a huge river happens to be flowing.