African Wildlife & Environment Issue 83

CONSERVATION

species that includes whales, sardines, billfish, turtles and gamefish that sustain a vibrant ecotourism industry of international significance if properly developed. The estuary provides an important sanctuary for marine turtles, who stop over during long distance migration to drop parasites that are intolerant of the fresh water. It is also a nursery for a Zambezi Shark population, as well as the Clawless Otter. The bird life is prolific and endemic species of fish include the Ox-eyed Tarpon. The distribution of the freshwater plume is dependent on the timing of the flood pulse and the prevailing wind. It is thus a highly dynamic process, with shorter time scales than those of the Okavango where tectonic activities are a key driver. In the Okavango Delta, change in the distribution of the flood pulse is measured in decades, or even centuries, while in the uMzimkhulu, change in the distribution of silt and nutrients can occur within hours. For example, observed changes to the Okavango Delta over the past century suggest that the preferential flow that used to exit on the western side, has shifted gradually towards the eastern side. This is ascribed to a combination of tectonic activities and the deposition of silt when flooding occurs. Both the silt and the water have mass, and it is thought that this is at least one of the forces driving the tectonic activities now being observed. The important aspect in the context of this

the Kwando River, and diverted the flow away from the Makgadikgadi depression, into the Zambezi River, has also created a hydraulic linkage between the two river basins in the form of the Selinda Spillway. The flood pulse through the Chobe-Linyanti is therefore highly complex, because it is driven by events in the upper Zambezi and Kavango systems.The biodiversity of the Delta and Chobe-Linyanti wetland systems are both extraordinarily rich because of this complex hydrology. In the case of the uMzimkhulu, the terrestrial portion of the basin is somewhat smaller than the Okavango, but is also fed by two different rivers. The uMzimkhulwana, or ‘Little uMzimkhulu’ is a distinctive sub-basin that feeds into the mainstem of the uMzimkhulu upstream of the Helens Rock Rapids, where a large weir is being planned. This will alter the hydrology and sediment transport of the estuary in a fundamental way. The natural flood pulse drives estuarine processes up to Helens Rock, which is the upper limit of tidal activity. Stated differently, a reach of river, almost ten kilometres long, is an ecologically functional estuary of a scale that is unrivalled in KZN. The flood pulse is characterised by the discharge of a silt laden plume, into the Indian Ocean, adjacent to several ecologically diverse Marine Protected Areas that includes the Protea Banks, Trafalgar and the more distant Aliwal Shoal. The biomass production in this area is massive, with a diversity of

article is that while the Okavango is a dynamic system, changes to the silt plume, and therefore nutrient loads, are generally slow and are driven by tectonic forces. This factor is exacerbated by the low nutrient status of the Okavango, which is described as ‘hyper oligotrophic’. This means that nutrient distribution by the flood pulse is derived more from the release of nutrients bound in the peat alongside the waterways during fires, rather than from the headwaters of the river basin. More importantly, it means that the water is crystal clear, enabling predators to hunt by visual

Umzimkhulu Estuary high flood with North Easterly wind

14 | African Wildlife & Environment | Issue 83 (2023)