WHEN THE LEVEE BREAKS

WHEN THE LEVEE BREAKS

If you’ve been at the coast this summer and gazed absent-mindedly at a stagnant estuary, you’ve probably wondered if, how, and when estuaries should be breached open to the sea, and how that would impact your favourite angling species. In The Mission Issue 43, Matt Kennedy explores estuary breaches in South Africa with scientific experts and anglers.   

Estuaries and estuarine fish  

First we turned to Prof Alan Whitfield. He is a leading expert on estuaries and estuary breaches in South Africa. He broke down the protocol and logic behind estuary management and the impact that closed estuaries has on fish.  

South Africa has 258 functional estuaries, of which 182 are temporarily open/closed estuaries (TOCEs). This means that for part of the year, after the wet season, they are open to the sea. They are fully exchanging water and allowing the passage of nutrients and organisms. When the rainy season ends, the flow of inland freshwater is slowed. Eventually the estuary closes off, with the sea forming a sand berm at the mouth, thus preventing any further water exchange. As the rainy season starts again, rivers come down in flood. The estuary tops over the sand bank and creates an outflow channel, which then drains the system. “It’s like pulling a plug out of the estuary,” says Alan. 

When nature takes its course, breaching usually occurs during the wet season. For most of South Africa, this is during the spring and summer, from September to March. The Eastern Cape experiences winter and summer rainfall, and in the Western Cape it rains in winter, May to August.

“The timing of a breach and marine spawning is ideally congruent.”

Uilendskraal estuary breached to the sea. Photo: Jean Tresfon

Most of the fish in these estuaries are marine species that are spawned at sea. They then recruit into estuaries during post-larval and early juvenile stages. Adult breeding of many popular angling species normally takes place in nearshore marine waters. Alan says, “The larval offspring – fish in the first phase of life – are no more than 10mm long. They can’t swim but rather drift in the ocean, spending about a month at sea. They then move either directly into an open estuary, or a TOCE that is open. Or they migrate into the surf zone and move laterally along the coast until they hit an open estuary.”  

Where an estuary is closed, sea-side larvae can sense freshwater seeping out through the beach from inside a lagoon. These little fish will hang around the surf zone in high concentrations, waiting for the system to breach. In some cases, larvae will be swept into a closed estuary by riding a wave big enough to wash over the sand berm into the estuary, without breaching the system.  

“The timing of a breach and marine spawning is ideally congruent.” says Alan. “If an estuary was artificially breached and drained of its water at the wrong time of year, then there may not be enough water for a proper spring breach, which needs to coincide with post-spawning juvenile fish recruitment, and ideally is strong enough to scour out accumulated sediments and organic matter from the stagnant system.” 

On the subject of spawning, there is a general misconception among many anglers that marine fish breed in estuaries. Rather than delivery rooms, estuaries function as nurseries for developing marine fish. That prize leervis of yours was likely a sub-adult, as most adults occur at sea. Soz. 

“Maturing fish locked in an estuary cannot spawn, so they start reabsorbing their gonads.”

Alan says, “The larvae and early juveniles come into estuaries. They spend two, three or even four years there before returning to the sea where they spend most of the rest of their lives. There are exceptions, like adult grunter, which often return to estuaries after spawning.

Duiwenhoks estuary open to the sea. Photo: Jean Tresfon

“When you have a flood, many adult fish, and particularly the ones that have reached sexual maturity, return to the sea because that is where spawning takes place. They don’t want to be trapped in an estuary. The worst thing that can happen is for a growing population of fish to be locked into an estuary for more than five years without any marine connectivity. Maturing fish locked in an estuary cannot spawn, so they start reabsorbing their gonads.”

REABSORBING THEIR… WHAT? No, it’s not just because the water is cold. Gonad reabsorption is one version of how fish perform “skipped spawning”, often due to unfavourable environmental conditions like temperature, poor nutrition, or closed access to the sea. Alan says, “The energy saved during gonad reabsorption likely improves their chances of survival until a future spawning event. The downside is that a successful spawning event is forgone for that individual.” Think of it as an involuntary vow of celibacy that ends with a big flood. Like most of your sex life.

In South Africa’s Western Cape province, winter is wet. In larger systems such as the Breede River Valley, regardless of rainfall, a big enough tidal prism will keep the estuary open. Smaller systems tend to be closed in the dry summers and will only open towards the third quarter of the year, when they’ve been filled up from winter rains.  

“The late winter rain is really the straw that breaks the camel’s back, forcing many of the estuaries to breach in early spring,” says Alan. “Although the rainfall regime in the Western Cape is different to KwaZulu-Natal’s, the opening periods are similar. This is ideal and timely for fish recruitment, which is mostly a spring and early-summer phenomenon.”  

To the delight of anglers and anyone who cares about healthy ecosystems, Alan says, “The late-winter storms that hit the coast in 2023 from Cape Town to Port Elizabeth were extremely timely. That flood event would have forced many estuaries open, scoured them out, and will result in excellent recruitment for this coming summer.” 

Estuary breaches in South Africa:

To breach or not to breach… Is that the question? 

An artificial breach involves sending large yellow machines onto the beach to dig open an estuary mouth. This has a variety of consequences – good, bad, predictable and not. “When the natural functioning of the freshwater system has been interfered with to such an extent that an estuary cannot function under a naturally regulated cycle, that could warrant a breach,” says Alan. Interference includes damming of freshwater systems, draining rivers for agriculture, unchecked development, or the overall cascading effects of climate change.  

While there are a few schools of thought, the main opposing opinions around forced breaching are either we need to fix what we’ve broken, or we need to leave well enough alone.  

“Even with the right motivation and reasoning, artificial breaching can go wrong.”

As Alan explained, based purely on rainfall regime, sometimes an estuary is artificially breached at the wrong time of year. “Water needs to accumulate during the closed phase so that the flood event and breach can scour sediments and accumulated organic matter out of the systems, as well as combat potentially eutrophic scenarios where excessive growth of filamentous algae can proliferate, smothering submerged plant beds such as eelgrass.”  

Even with the right motivation and reasoning, artificial breaching can go wrong. For example, there are pollution issues from low-lying developments in many TOCEs. Alan says, “E. coli levels rise in these estuaries because of leaking septic tanks and French drains that don’t work when the water table rises. The local municipality will put pressure on the respective authority to breach the estuary and supposedly get rid of the pollution. However, if the berm is breached when the water is too low, you do not get proper scouring of sediments out of the estuary. And what’s worse is that the system will not reset in the same way as it would reset if a proper flood came through the system, hindering the natural cycle for seasons to come.” 

Estuary breaches in South Africa:

A list of well-known systems and their breaching histories 

Bot River 

Alan says, “Under certain circumstances, systems can become hyposaline, where the salinity is too low for marine fish to survive and would probably not have occurred in a natural situation. A good example of this is the Bot River, whose flow is compromised by the many dams and farms that are extracting freshwater from the system. Were these obstructions not present, the Bot’s estuary berm would be breached on a more regular basis. The question is, in what is a totally artificial situation, do you just leave it, allowing all the marine fish to die from low salinity and lack of marine connectivity? Or, do you implement an assisted breach, which is also artificial, but it’s trying to keep the system in balance between connectivity and the open/closed phase?” 

estuary breaches in south africa
Bot River closed off to the sea. Photo: Jean Tresfon

East and West Kleinemonde  

These two neighbouring estuaries in the Eastern Cape are good examples of where forced breaching could have done a lot to help. Alan says, “Recently,  the East estuary was open to the sea while the West estuary remained closed, and is still closed. Both estuaries are home to growing populations of dusky kob. Dusky kob reach maturity when they’re 1m long, and need to spawn at sea at this point. When the West Kleinemonde estuary didn’t breach, news travelled fast, and soon that population of mature kob was wiped out by anglers. We know from historical records that South Africa’s dusky kob stocks are 95% depleted – we’ve only got 5% left. This could have been a good reason to breach that system artificially.”  

Alan says that a situation like this would still require a scientific justification, and that water levels would need to be sufficiently high to facilitate a realistic breach. “I don’t think the height of the lagoon behind the berm was sufficient even for an artificial breach. So, it might not have been feasible. However, one could justify an artificial breaching. The West Kleinemonde had not breached in five years and all the marine fish sitting in the system need to get to sea to breed.”  

estuary breaches in south africa
Klein River breached and overflowing with freshwater. Photo: Jean Tresfon

uMhlanga River

During the KZN riots of July 2021, fire fighters combatted arson at an agro-chemical warehouse, which resulted in toxic runoff into the uMhlanga River system via stormwater drainage. Alan says, “Everything downstream in the estuary was killed and, to help get rid of the pollution, the estuary was breached artificially. This slowed down the accumulation of those toxic chemicals into the sediments of the estuary. A breach was justified, regardless of the time of year, to prevent further fish kills in the future.” 

estuary breaches in south africa
uMglanga River. Photo: Alan Whitfield

Great Brak River

The Great Brak River estuary on the Garden Route is one of Jazz Kuschke’s home waters. Being a conservation-minded angler, and with a major in geography, he’s penned many pieces on estuaries over the years. Since his teenage years he has seen and understood the changes to his local waters better than most. Jazz says, “The mouth is mechanically opened, year after year, at the wrong location. The natural and more efficient mouth point would be via the path of least resistance, a flow that would leave the current river bend as an oxbow lake.” He’s resigned to the fact that it’s not a popular opinion. And that unfortunately, “It seems that making a pretty beach for holiday homes is given priority over proper estuary functioning.”

His personal leaning is towards a balanced management of these systems. “We’ve messed with these systems, their catchment areas, flows and drainages, and we’ve built on their banks. It’s a bit like the elephant populations in the Kruger National Park. We have to protect and nurture them and grow the population. But the other side of the coin is having to cull some animals to save the veld.” 

estuary breaches in south africa
Great Brak River. Photo: Jazz Kuschke

Zandvlei 

As highlighted in The Mission Issue 29 in Richard Wale’s “Open Secrets”, Cape Town’s Zandvlei, a highly modified estuary, has had its fair share of turmoil. Although the leervis fishery is making a turn for the better. As a local angler who is highly involved with the system’s conservation and management, Richard explains their approach.  

“The main purpose for the forced breaching here is to prevent a flooding of the houses in the marina. The mouth is kept open during winter so that the persistent rains do not cause a rise in water level. At the end of the rainy season, the mouth is closed, and managed throughout summer, where it is breached once a month during the second spring tide. This allows ‘fresh’ saltwater to push up into the system, maintaining salinity concentrations. Without this, the vlei would warm up too much. Combined with high nutrient levels, this can result in an algal bloom, which is bad news for anything with gills.”  

estuary breaches in south africa
Conrad Botes explores Zandvlei estuary. Photo: Matt Kennedy

St Lucia 

The St Lucia estuary and lake in KwaZulu-Natal accounts for 50% of South Africa’s estuarine surface area. It was (yes, past tense) a major nursery for many marine species, including dusky kob and spotted grunter. It was closed and isolated for almost two decades until a forced breaching in 2020. Marine scientist Dr Ryan Daly explains that many factors made this important estuary uninhabitable.  

“Previously,” says Ryan, “the Mfolozi River pumped straight into the St Lucia system. Then the river was channelised to combat flooding of sugarcane farms and a new mouth was created near Maphelane. During the extended closed phase of St Lucia, high temperatures and drought caused the lake to become hypersaline and it almost dried out. As a counter, St Lucia was reconnected to the Mfolozi River, where freshwater floods resulted in massive sediment deposition, accelerated reed growth and fragmentation between the lakes and narrows.”  

“In this case the two decade-long loss of estuarine-marine connectivity was the trigger and so that was the reason for supporting the artificial breach of St Lucia at that point. It doesn’t mean to say that we will automatically support another artificial breach at any time in the future.

Alan adds, “St Lucia was a very important marine fish nursery area and could no longer provide that function because the connectivity was broken. There were two schools of thought in the scientific community. The one said, ‘No, too bad, that’s a natural process.’ Which it wasn’t, because it had an artificial layer on top of it – agriculture, developments, dams, canals, etc. In the other camp you had a few others, including me, who said, ‘We’ve intervened with this system a lot. We agree that trying to restore natural functioning, including breaching regime, is the ideal. However, there comes a point where an assisted breach is justified.’

“In this case the two decade-long loss of estuarine-marine connectivity was the trigger and so that was the reason for supporting the artificial breach of St Lucia at that point. It doesn’t mean to say that we will automatically support another artificial breach at any time in the future.”   

Although the long-term consequences of the forced breaching at St Lucia will only be known through active monitoring and management, the short-term benefits (e.g. recruitment of bull sharks and mullet) have been encouraging.

estuary breaches in south africa
St Lucia breached. Photo: Ryan Daly

The solution to your local estuary may be different to what needs to happen several hundred kilometres up the coast. Alan’s take is that “estuaries should be treated like individuals, each with unique characteristics, problems, and solutions. We should not be dogmatic. We need to be flexible, but the flexibility mustn’t compromise the long-term health of the system.”  

This article was first published in The Mission Issue 43. Read the whole thing, mahala, below. All we ask for is your email address and the first chin hair of your second-born.

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