African Wildlife & Environment Issue 81

CONSERVATION

is part of a large offshore marine protected area of enormous biodiversity.The upper limit of the estuary is St Helens Rock, which is the location of a large pump station for Ugu District Municipality. It is at St Helens Rock that the natural flood pulse of the river meets the natural tidal pulse from the ocean. The tidal range at the mouth is just under two meters, and this daily pulse pushes upstream, where sediment prevents it from reaching the St Helens Rock Pump Station. Sand mining operations, on an industrial scale, have fundamentally altered the bed, bank and flow of the river, to a point upstream of the pump station. The altered channel has created a larger volume for the estuary, and this means that each tidal pulse does not drain the entire estuary.The altered river channel in the immediate vicinity of the pump station, has enabled saline intrusion to occur, for short periods of time. Those short periods coincide with the attenuation of streamflow arising from abstraction upstream in excess of the Existing Lawful Use (ELU) defined by formalWater Use Licenses (WUL). Sand mining on an industrial scale has fundamentally altered the bed, bank and flow of the river The interaction of the flood pulse with the tidal pulse is complex. In essence, salt water is denser than

In general, all rivers with a functional pulse tend to be healthy. In the WCD context, it was the attenuation of the flood pulse in rivers that became the key driver of ecological integrity. The flood pulse defines the aquatic ecosystem, by sending signals to the various organisms that collectively comprise a functional environment. In this regard, most rivers globally have lost their flood pulse, primarily because of dams.Therefore, theWCD highlighted the intrinsic value of free-flowing rivers, with intact flood pulses, as being a national asset worthy of protection. In an African context, one of the last free-flowing rivers is the Okavango, which is shared between Angola, Namibia and Botswana. But in a South African context, the last free-flowing river of any significance is the uMzimkhulu. The Okavango and the uMzimkhulu are similar for specific reasons. Both still have a flood pulse that is largely intact. Both sustain an enormous biodiversity. Both present huge potential for future generations, as livelihood creators in areas of otherwise limited economic activity. Both represent inter-generational equity at its very best, and both are treasures worthy of protection. The Helens Rock Pump Station at Port Shepstone The uMzimkhulu river sustains the ecological integrity of an estuary, some ten km in length. This

freshwater, so it sinks to the bottom. The freshwater, being less dense, flows over the salt water. This results in a saline wedge, well described in coastal aquifers as the Ghyben-Herzberg Principle.This wedge pulses back and forth along the estuary. The location of the actual halocline – the plane separating salt from freshwater – is dependent on several factors. The simplest explanation is to think of the position of the saline wedge as being the balance of the tidal pulse pushing upstream, and the flood pulse pushing downstream. This is where fresh and salt water collide. When the saline wedge is unable to go out to sea, there is a volume of saltwater that is retained in the estuary. This becomes more saline over time,

The alteration of the river profile by industrial scale sand mining in the immediate vicinity of the St Helens Pump Station has enabled saline intrusion to put the drinking water supply of the Ugu District at risk.

12 | African Wildlife & Environment | Issue 81 (2022)