Olifants River faces extreme warming by 2100, threatening fish and macroinvertebrates

The Olifants River, one of South Africa’s most troubled waterways, is facing an escalating threat from climate change. 

Already heavily polluted, overused and ecologically stressed, new research suggests that the river could experience dramatic warming over the coming decades, with potentially devastating consequences for freshwater ecosystems.

The study, published in the latest issue of the South African Journal of Science, models how rising air temperatures could affect the Olifants under a high-emissions scenario. 

The authors said the simulations presented in their study suggest that rising air temperature because of climate change will translate into rising water temperature in

the Olifants River, with both daily and monthly water temperatures rising by up to 5°C by 2100. 

The study projects that by the end of the century, daily water temperatures on some extreme days could reach 42°C to 44°C, while monthly averages could rise by about 3.6°C and daily averages by about 3.7°C. Summer temperatures could regularly reach 34°C to 35°C.

The researchers warn that these results do not just indicate a large rise in water temperature but allude to the Olifants River undergoing other changes, such as increased evaporative losses and possible changes in flow regime. 

“The river will experience changes in both thermal and physical habitats, which will have detrimental effects on the survival and breeding of freshwater species which are sensitive to disruptions in flow regime and temperature change.”

The scientists, from the University of Pretoria’s department of zoology and entomology, the Mammal Research Institute and the Association for Water and Rural Development, focused on a stretch of the river flowing through Kruger National Park – a section that remains ecologically important despite heavy pressures accumulating upstream.

Rather than directly measuring future water temperatures, the researchers used validated statistical water-temperature models that estimate river temperatures from air temperatures. 

They then applied future climate projections from 16 general circulation models under representative concentration pathway (RCP) 8.5, a high-emissions or “business as usual” scenario in which greenhouse gas emissions remain elevated and warming continues largely unchecked.

The researchers modelled both monthly and daily average water temperatures from 2025 to 2100 in the heavily polluted and over-abstracted Olifants River in the Kruger.

Freshwater systems at risk

According to the paper, this approach adds to the still limited body of research on freshwater climate change in Africa, where the impacts of warming rivers remain comparatively understudied.

The significance of their findings lies partly in the condition of the river itself. The Olifants is not an intact or lightly used river system facing climate change in isolation. It is already shaped by decades of human pressure, including pollution, altered flows, over-abstraction, mining, habitat degradation and river modification.

The study notes that freshwater systems are under pressure from “direct and indirect anthropogenic [human-caused] effects” and that these disturbances are often occurring at the same time.

As the researchers noted, “increasing air temperature due to anthropogenic climate change will cause water temperatures to rise”.

The study stresses that water temperature is one of the most important variables shaping life in rivers. It influences how much oxygen water can hold, how fish and other aquatic organisms grow and reproduce and whether some species can survive at all. When rivers warm too far or too fast, the consequences can ripple across the food web.

Among the most vulnerable are macroinvertebrates – the tiny spineless river creatures, such as insect larvae, that underpin much of the freshwater food web.

To illustrate the risks, the paper draws on previous work on the critical thermal maxima (CTmax) of certain aquatic species, the upper temperatures they can tolerate before suffering severe physiological stress.

For example, the critical thermal maximum for blackflies (Simuliidae) is 31.6°C, while that of mayflies (Baetidae) is 36.7°C. 

Based on the study’s temperature projections, the scientists suggest that by the end of the century blackflies could be exposed to prolonged periods above their thermal limit, placing them under chronic thermal stress and potentially pushing them towards local extinction. 

Mayflies, by contrast, may remain below their threshold for longer periods but could still experience acute thermal stress during exceptionally hot days, raising the risk of mass die-offs. 

“Although these are only two macroinvertebrate families that will be affected by the rising water temperatures, it is likely that there are many more species, including fishes,

other vertebrates and algae that will also be either acutely or chronically

affected by climate change,” the study said. 

Compounding pressures

This drastic rise in water temperatures will ultimately have negative effects on freshwater life. The researchers argue that the threat is not limited to temperature alone. Rising air temperatures are also likely to intensify evaporation, reduce surface-water flow and increase pressure on already scarce water resources.

Southern Africa is likely to warm faster than the global average, with air temperatures projected to rise sharply by the end of the century under low-mitigation scenarios.  

Evaporation rates in the region are already extremely high, reaching 65% in some areas, which reduces effective rainfall in a part of the world already prone to recurring drought.

Freshwater species often rely on specific flow conditions for breeding, migration and survival and the study warns that climate change could disrupt both the thermal and physical habitat of the river.

The Olifants also partly depends on groundwater, which helps supply water for mining, agriculture, drinking and rural livelihoods, while also providing a degree of thermal buffering. 

“However, the increase in demand for water due to a growing human population and consequences of climate change such as changes in precipitation, land use and flow rate have altered the rate of groundwater recharge,” the study said.

It has been predicted that, within the Olifants River Basin, groundwater recharge will decrease by 2100 under RCP 4.5 and RCP 8.5 scenarios “and this is likely to reduce thermal buffering in the region”.

The paper also highlights likely impacts beyond the main river channel. The Ga-Selati River, a lower Olifants tributary, is projected in other research cited by the authors to experience increased evaporation and reduced rainfall, compounding water stress across the wider basin.

The Olifants River Catchment spans roughly 75 000 km² across South Africa and Mozambique and forms an important tributary of the larger Limpopo Basin, which stretches across four countries.

Highly fragmented

The study notes that this is not only an ecologically important system but also a heavily used one. The river supports mining, agriculture and commercial forestry, while also flowing through the Kruger, South Africa’s largest protected area and a key tourism drawcard.

But it is also a highly fragmented system. According to the paper, the Olifants catchment contains 37 major dams, about 300 minor dams and roughly 4 000 small dams. 

These structures disrupt river connectivity, making it harder for fish and other aquatic species to move in response to changing environmental conditions, including rising temperatures.

The study places the Olifants within a much broader concern: that freshwater ecosystems are among the most vulnerable on earth, despite receiving far less public attention than terrestrial conservation crises.

Freshwater systems cover just 0.8% of the Earth’s surface but face intense pressure from pollution, overuse, invasive species, altered nutrient loads, salinisation, eutrophication and climate change.

The researchers said that freshwater ecosystems are already under severe pressure from multiple human-driven impacts, with climate change adding to those stresses.

The authors point to some existing resilience work, including the RESILIM-O project in the Olifants Basin, which has focused on reducing climate vulnerability, improving biodiversity management and strengthening local capacity for water governance.

They emphasise that more research is urgently needed, especially on what rising river temperatures could mean for biodiversity in the country’s freshwater systems.