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"The Vaal Dam doesn't 'overflow' in the way some other dams, like Gariep will do."
Vaal Dam. Picture: The reservoir
The Vaal Dam’s water levels have risen dramatically over the past week, climbing to 87.93% as of Wednesday, according to the latest data.
This marks a significant increase from the previous Tuesday, when levels stood at 63.34%.
According to the Department of Water and Sanitation’s weekly water reports, the dam was at 68.3% last year around the same period.
Vaal Dam levels steadily increase
Recent heavy rainfall across the country has significantly filled the dam, with water levels rising more than three percent in just 24 hours earlier this week.
The dam stood at 80.22% on Sunday, increased to 83.45% on Monday, and reached 85.73% on Tuesday.
“On 26 February 2025, the Vaal Dam’s water level reached 87.93%, reflecting a steady increase from earlier in the week, as shown in the latest flow data,” stated Midvaal councillor Pieter Swart.
“This rise is supported by consistent inflows, though the barrage discharge has been reduced, a decrease attributed to lower rainfall and reduced inflows received this week, helping to conserve water in the dam.”
The persistent rainfall has dramatically improved the dam’s capacity, which stood below 30% early in the year.
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Vaal Dam flow data
The Water Resource Information Centre for the Catchment Management Forums of the Upper Vaal Water Management Area has been tracking the dam’s daily increases of approximately 2% to 3% since Monday.
Their data shows that on Monday, the Vaal Dam was at 83.45% with an inflow of 720.7m³/s and outflow of 16.8m³/s.
The Vaal Barrage level was at 7.4m with an outflow of 88.8m³/s and water temperature at 21.7°C.
By Tuesday, the dam level had risen to 85.73% with a slightly reduced inflow of 658.5m³/s and unchanged outflow of 16.8m³/s.
The Vaal Barrage level increased to 7.6m with a higher outflow of 134.3m³/s and water temperature of 23.1°C.
The latest figures from Wednesday show the dam at 87.93% with inflow further reduced to 559.3m³/s and constant outflow of 16.8m³/s.
The Vaal Barrage level dropped slightly to 7.5m with significantly reduced outflow of 30.0m³/s and water temperature at 22.2°C.
Swart also noted that “the Grootdraai Dam continues to operate above 100% capacity, significantly contributing to the Vaal Dam’s supply, while the Sterkfontein Dam remains close to 100%, ensuring robust regional water reserves.
“However, the Katse Dam pipeline is still undergoing maintenance, which may affect water transfer in the system.”
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Controlled release procedures
Addressing concerns about potential overflow, Swart explained that the Vaal Dam doesn’t overflow in the conventional sense.
“The Vaal Dam doesn’t ‘overflow’ in the way some other dams, like Gariep dam will do. It makes use of sluice gates to release water in a controlled manner, once the dam reaches 100%.
“If the Vaal Dam is at around a 100%, and we are expecting big rains, the sluices will strategically be opened,” Swart told The Citizen.
He further explained that water releases help prevent flooding.
“The strategic release of water, in advance of the Vaal Dam becoming over full and the expectation of coming rain and inflows, allows Rand Water to open sluice gates to control the amount of water released downstream.
“In South Africa, building near rivers like the Vaal is governed by national and local laws that aim to mitigate flood risks.
“The key consideration is the 1-in-100-year flood line, which marks the boundary of an area expected to be inundated by a flood with a 1% chance of occurring in any given year.”
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Potential impact on surrounding areas
Swart noted that various properties along the banks of the river downstream of the dam wall could be affected by controlled releases.
“The closest major suburb is Three Rivers, and on occasion, with a lot of rain and a few sluice gates open, these lower laying areas does flood,” he said.
He emphasised the dam’s importance beyond water storage considering that the dam also feeds the river downstream, where plenty of farms are situated.
He said it was essential to have the dam at a high but regulated level.
“It ensures a stable supply to Gauteng’s taps, sufficient supply to the agricultural sector and also provides tourism opportunities, like for example water sports activities like in Parys.”
For residents concerned about potential flooding, Swart added: “I place daily updates on my various socials, especially when the level changes daily. When it gets close to capacity, the various authorities, including Rand Water, will send out notifications and updates on the possibility of flooding downstream.”
Vaal dam expert assessment
Water expert Carin Bosman said it was difficult to predict whether the Vaal Dam will overflow in the next week or months.
“Normally we see our dams overflow towards the middle end of March, beginning of April, and that’s the dams designed to handle overflow,” Bosman said.
She emphasised that the concept of “overflow risk” is technically incorrect.
“The dam is designed to overflow so there is no risk associated with the overflow. The risk of overflow is not a risk. The dam is designed to overflow.”
Regarding flood management strategies, she explained: “The measures that are floodline determinations downstream and people staying outside of the one in 100 year floodline.
“If people have built construction of infrastructure, houses, etc within the one in 100 year floodline, that is the risk that those people took to construct inside a floodline and it’s not a risk of overflow.”
The Citizen reached out to the Department of Water and Sanitation for comment, any response received will be included in updates to this article.
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