Blinded by Nature
The state of the Swartvlei estuary mouth is a topic that polarizes the community of Segdefield. Located in the Garden Route between Wilderness and Knysna, the coastal village of Sedgefield flanks the northern shore of the estuary south of the N2 motorway.
Like most estuaries along the Southern Cape and Eastern Cape coastline, most estuaries are classified as blind estuaries referring to the regular process where a sand bar develops across the mouth and blocks the direct connection of the estuary with the ocean.
Estuaries in this region are vital components of marine life, providing nurseries for some fish species. Adults will enter an estuary to spawn. The fingerlings in blind estuaries will grow, with the altered salinity stimulating more somatic growth and temporally suppressing gonadal growth.
Then when the sand bar washes away, either after heavy rain or stormy sea conditions, schools of fish return to the ocean and populate the inshore regions and reefs.
Most of the blind estuaries are left to the natural cycle to open to the ocean, while a few are managed either by municipalities or conservation authorities when human development is threatened by high levels of a closed off estuary.
Over the past three decades controversy over the level that the water in the Swartvlei Estuary must reach before it is artificially opened has become an annual topic of debate in the community of Sedgefield. I recall when I first moved to the Garden Route in 1994 that the then manager of the Goukamma Nature Reserve, Rhett Heisman, joked how the topic of the mouth opening polarized the village of Segdefield with the view of letting nature run it’s course strongly challenged by the view that human interference must be carried out. Amazingly, it still does.
Cycles of opening.
In the mid 1990’s the cycle of the Swartvlei Estuary being open and closed ranged from an average open phase of 18 to 22 months and sometimes as much as 24 months, while only being closed for 3 to 5 weeks and at a push for 6 weeks.
In the past decade the cycle has changed with the mouth being closed for 12 months and open for 3 to 5 weeks.
In considering what course of action needs to be followed with regards to the state of the Swartvlei Estuary mouth, it is pertinent not to focus on the managerial team responsible for the process of monitoring the water level and the physical process of opening the mouth.
Instead, it is vital to consider the processes that have driven the change in the duration of the mouth being open. It is well established internationally that to manage any portion of a river, especially the lower reaches flowing into the ocean, efforts have to be made up stream, not in the lower reaches, and now more than ever, the principle needs to be considered for the Swartvlei Estuary.
So what are the contributing factors that contribute to the reversal of the period of the mouth being open and closed?
Nature in action.
When considering the natural processes that can contribute to the reversal of the period of the Swartvlei Estuary mouth being open and closed climate change is an obvious candidate. There are two aspects to this.
Firstly the shift in rainfall patterns. In the 1990’s Sedgefield has an average annual rainfall in the region of 480mm to 540mm with a cycle of floods where 100mm of rain would fall in a 72 hour period. This cycle continues as late as 2007. For the past decade our rainfall has been slightly lower with fewer occurrences of 100mm in a few days. This lower rainfall translates into reduced flow of water into Swartvlei and a slower rate of the water level rising. This has the effect of extending the period when the mouth reaches the trigger height of being opened as well as a prolonged period of high water levels.
Globally, climate change is driving a rise in sea levels. In South Africa, the mean rate of sea level increase was in the region of 4cm per decade. Globally the rate has accelerated in the past decade and the South African sea level rising now exceeds the 4cm per decade. If we consider the impact of this using the previous levels, since the mid 1990’s the sea level has risen over 12cm, probably close to 15cm. While that doesn’t sound like much, the impact on the height of the sand bar across the estuary mouth is significant as well as the point of reference in the management of the height to manage the opening of the mouth.
While these two aspects play a vital role in the condition of the estuary mouth, even more significant are the human influences.
Water extraction and habitat transformation play a major role in the rate of water level rising in Swartvlei. In the past three decades South Africa has seen almost a doubling in the national population while due to semigration driven by economic and lifestyle pressures, the Garden Route has seen more than a threefold increase in population. This alone has resulted in a dramatic increase in the extraction of water for domestic consumption from the river systems that feed into Swartvlei, further reducing the rate of flow into the estuary.
Agriculture transformation is another major contributor to reducing the amount of water entering Swartvlei. The mass installments of center pivots for irrigation across the heights, the change from pastures to Macadamias that require large volumes of water and the increase in surface area under irrigation all further reduce water reaching Swartvlei.
The Garden Route was devastated by both the 2017 and 2018 wildfires. Analysis of the fires pointed to the rampant ingress of alien tree species which had changed the vegetation type. South Africans are well aware of high rate of evapotranspiration that Black Wattle, Blackwood and Eucalypts have and that they consume far greater volumes of ground water than indigenous trees.
Anyone taking a drive along the Seven Passes road or kloofing down the rivers in the Garden Route will be only too aware at the high levels of alien tree infestation in the area. Our indigenous Fynbos and Afro-montane forest are well documented as being a reservoir of water, the root mass and the humus layer acting like a sponge which stores water and releases it slowly so that it can feed into the subterrain water table and streams, traditionally providing a perineal supply of water.
The combination of over extraction (for agriculture and domestic use), lower rainfall figures and reduced water storage in the natural reservoir systems of Fynbos and forest humus all contribute to the reduced flow into Swartvlei and ultimately the slower rate of water level rising.
Look Upstream for the Cause
The management of opening the Swartvlei Estuary mouth is not the cause of the mouth not being opened as regularly as it used to be. Instead the impact of habitat change, population increase, climate change and over extraction is.
Beyond the Village Boundry
This is not a local issue, but rather a regional issue that extends from False Bay to the Great Fish River. There are many blind estuaries, some which haven’t opened for more than 5 years, and looking forward, it will become more of and issue as populations increase.
Does the mouth being open prevent flooding? Not necessarily. In 2006, the August flood was a good example of how the mouth being open doesn’t prevent flooding. The Touw River Mouth was open while the Swartvlei Estuary mouth was closed. Both villages were severely flooded up over the 70 year flood line. Of course nature doesn’t read municipal planning documents and the flood of 2007 again exceeded the 70 year flood line.
Considering that rate od sea level rising, it is not hard to forecast that future floods will cause damage to properties around the 70 year flood line.
Food for thought – if you had visited the Myoli dune 20,000 years ago, the beach would have been over 60km to the South. Even 2000 years ago, you would have had to travel approximately 2km south to get to the beach. With climate change, it is going to continue rising.