The Chobe-Zambezi Channel-Floodplain System:Anatomy of a Wetland in a Dryland

In this chapter, the Chobe-Zambezi channel-floodplain system is defined as the fluvially influenced area that is located around and between the Chobe and Zambezi rivers approaching their confluence. This area is located in the ‘Four Corners’ region, the informal term given to the region where the Botswana, Namibia, Zambia and Zimbabwe borders meet. The large-scale structure and medium-term (102–105 years) development of the channel-floodplain system is related to a combination of tectonic activity and climatically-driven changes to flow and sediment supply. The system has developed in a region of subsidence that is related to the East African Rift System. Upstream of the Mambova Rapids, the modern sinuous, alluvial channels are flanked by extensive floodplain wetlands, with crevasse splays, gullies, oxbows, scroll plains, abandoned channels and backwaters (stagnant or slow-flowing, channel-like depressions) all widespread. Collectively, these fluvial landforms create the physical template for shorter-term water, sediment and ecosystem dynamics. A strong flood and drying season dynamic is evident; river stages typically rise from January and peak around April, before subsequently falling again. The Zambezi provides the largest flow volumes, with flow spreading gradually from north to south through a complex system of active and partially active channels and floodplain wetlands towards the Chobe. Along the two rivers, lateral channel migration and extension of splays, gullies and backwaters has been negligible over at least the last 40–50 years, with few new oxbows forming. To the east, both rivers cross the uplifting Chobe fault, with each river forming complexes of steeper, bedrock anabranching channels in the region of the Mambova Rapids. The two rivers ultimately coalesce ~10 km farther downvalley, and continue as the Zambezi River. A longer term (>106 years) developmental model is outlined, which posits that headward retreat of the Victoria Falls, at present located ~80 km downstream of the Chobe fault, will initiate a phase of erosion that will cross the fault in ~1–2 million years’ time. This phase of erosion will initiate deep channel incision, river network reorganisation and wider landscape denudation.