CSIR warns of destructive impact of invasive plants on dam catchment inflows

1st April 2019 By: Marleny Arnoldi - Deputy Editor Online

The Council for Scientific and Industrial Research (CSIR) and the Water Research Commission have determined that up to 50% of the yearly inflows into the Western Cape’s Berg river dam catchment and Limpopo’s De Hoop dam catchment could be used up by alien plants, if uncleared, over the next 45 years – the average lifespan of dams in South Africa.

The study was published in the Water South Africa journal, titled “Impact of invasive alien plants on water provision in selected catchments” and authored by water experts and academics Dr David Le Maitre, Dr James Blignaut, Professor Lynette Louw, Professor Tally Palmer and Ian Preston.

The authors analysed the impact of failing to control invasive alien plants on the future water supply in the catchments of these important dams.

“The dynamics of alien invasive plants are such that repeated follow-up clearing is required to counter the regeneration of the plants,” explains Le Maitre, adding that a follow-up fire within two years of the initial clearing can kill all the seedlings and limit the need for further treatments.

“It is important to note that the results show the estimated effects of the uncontrolled growth of invasive alien plants on river flows into the dam, rather than on yield.”


The Berg River catchment, in Franschhoek, provides water to Cape Town. The government’s Working for Water programme has been tasked with clearing invasive alien plants and, at the end of 2013, had spent more than R90-million clearing the equivalent of 3 600 ha of dense stands of pines and acacias in the catchment area.

The study assumed the worst case scenario, where the average tree is 20 years, and the invasion would increase from 3% to between 49% and 99% of the invadable area in the catchment area.

The result would be that river flows into the dam would be reduced by up to 50%, with significant implications for Cape Town and irrigation schemes that depend on water from the dam. To clear the invasion at this point, would cost around 3 000% more.

Meanwhile, the De Hoop dam is located on the Steelpoort river, and is predominantly invaded by eucalyptus and wattle species. At the end of 2013, the Working for Water programme had spent close to R3-million clearing the equivalent of 180 ha of dense alien invasive plants in the catchment area.

In the worst case scenario, the study found that, the invasion in the De Hoop catchment would increase from 7% to between 53% and 55% of the invadable area of the catchment area.

River inflows into the dam would be reduced by between 42% and 44% and the cost to clear the plants at this point would be around 700% more.

These results, intended to be conservative and illustrative, rather than precise, demonstrated the inevitable outcome of the rapid spread and densification of invasions in areas of natural and semi-natural land cover, when control is not effective or not done at all.

These are only two dams whose inflows are at risk from unmanaged alien plant invasions.

“If these problems are not tackled, we will see a major water security problem develop over time across the country,” said Le Maitre, adding that building a dam without a plan to clear its catchments of invasive alien plants was tantamount to fruitless expenditure.