The artificial breaching of the mouth of Lake St Lucia in early January has elicited a surprising volume of negative coverage. But the previous (2016) attempt to repair damage done by human intervention had been badly botched. It had left a situation where there was no estuary at all and no chance of one being re-established by natural processes. All the latest intervention has done is restore the status quo ante and put the question of how to manage it in future squarely back on the table.
The estuary is a critical component of the 70km-long St. Lucia lakes system. When the area was proclaimed a World Heritage Site by the United Nations Educational Scientific and Cultural Organisation (Unesco) in 1999, its estuarine nature (‘shifting salinity states’) was singled out as one of three outstanding natural characteristics of the system. But by 2016 there was no estuary, only a beautiful but much-less unique fresh-water lake system separated from the sea by a sand dune. The lake had been deliberately sealed from the ocean since 2002 when a marine accident threatened an influx of pollution.
But estuaries are delicate ecosystems. Since 2002, estuarine species such as mangroves and euryhaline fish species (which move between fresh and saline water) had either become endangered or gone extinct. The 35 species of fish which depended on the estuary for breeding (it is a nursery for grunter and mullet) had disappeared. The prawn beds of the estuary were no more and recent reports suggest that an invasive alien gastropod (snail), Tarebia granifera, has overwhelmed and replaced local species.
The problem has been long in the making. Drought conditions after 2002 saw the volume of the lake drop below critical levels, which led to a 2012 decision by the iSimangaliso Wetland Authority to reroute the Mfolozi River in order to bring in much needed fresh water. The mouth of the Mfolozi had previously been canalised directly into the ocean. The fresh water was successfully re-routed and the decision made to allow the mouth to find its course naturally. This was backed by a plethora of scientific research, funded mostly by the World Bank, between 2009 and 2017. The research generated considerable insight and will be an invaluable resource in managing the estuary going forward.
Cut off from the ocean
Because the estuary was cut off from the ocean, there was a marked accumulation of sediments behind the dune separating the lake from the sea. Scouring the sediments required a breach but the ‘natural process’ philosophy insisted that this could not be artificially induced. The previous process of using a dredger to keep the mouth open was never considered although the assumed shortcomings of this mechanism were never clarified by the authorities beyond a 2011 statement which referred to ‘excessive dredging in the narrows’.
But, in 2016, the actions of the authorities rendered ‘natural processes’ redundant and appeared to permanently lock in the sedimentation problem.
An enormous pile of dredger spoil had accumulated over 50 years in the form of the levee which had rerouted the canalised Mfolozi mouth southwards. A decision was taken to mechanically pump this obstacle into the ocean where it would be dispersed by currents. But the pumping system failed and mechanical excavators (‘yellow machines’) were deployed. Instead of taking the spoil to the sea, however, these machines dumped it on the existing dune and, in so doing, made a ‘natural breach’ impossible.
It is all too easy to underestimate the scale of this blockage. According to the World Bank’s 2017 project closure report, the volume of dredge spoil dumped on the beach totalled 140 million cubic metres. If this volume of mud and sand were piled into a cube, each side would be more than twice as long as an Olympic swimming pool. It has recently become apparent that transitional vegetation like casuarina trees have taken root and thus stabilised the obstacle even more.
The impact of the problem on human activities – commercial and subsistence farming and the local leisure and hospitality industry – was immense. But while there was pressure from these quarters to artificially breach the dune between St Lucia and the sea, the overwhelming case was environmental. The estuary was effectively dead and if the ocean was not allowed to scour river sediment deposits, the lake system would all too soon follow.
Ongoing quest
The breach is far from the total solution to the threat. It is only a single moment in the ongoing quest to manage the estuary in ways which restore its unique character. What it does signal is the end of a fairly brief experiment which consisted of an intense intervention followed by hands-off management intended to ‘allow natural processes’ to maintain the system. Among the problems with this approach was that it did not adequately account for 70 years of the impact of human intervention in the system.
The entire catchment area, running as far inland as Vryheid is intensely utilised and much degraded. There is simply no comparison between the volume of sediment generated from run-off today with the situation 70 years ago. More intense human activity has also seen more water abstracted from the Mfolozi, which has reduced the volume reaching St Lucia. In winter the river actually dries up. This reduced flow is another factor which makes a natural breach unlikely. What the St Lucia system requires is a way of living with this new reality, not a sentimental harking back to the days when nature ‘managed’ the system without human intervention.
The question now is how to manage the system? What would seem clear is that the ‘hands off’ approach has failed and that active management is needed. Far from this reflecting some sort of ignorance of science, it re-opens the possibility of redeploying science in the design of a functioning management system. The most recent experiment has not worked and a fundamental reappraisal is needed.
[Picture: Darren Glanville, https://commons.wikimedia.org/w/index.php?curid=38739264]
The views of the writer are not necessarily the views of the Daily Friend or the IRR
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