Siltation threatens national dams

Seithati Motšoeneng

SILT accumulation is emerging as a major threat to Lesotho’s dams, with authorities warning that reservoirs across the country’s lowlands, foothills and highlands are increasingly at risk, potentially compromising water supply and critical infrastructure.

The warning follows years of severe silt build-up at Rasebale Dam in Mafeteng, where sediment clogged intake pipes and significantly reduced water supply, particularly during the 2024 drought when communities experienced prolonged water shortages.

The Water and Sewage Company (WASCO) has estimated that about 900,000 cubic metres of sediment had accumulated in the dam — an amount described as being equivalent to five times the height of Qiloane Mountain.

The worsening situation prompted the arrival of a new dredging machine this week, which will be used to rehabilitate Rasebale Dam by removing the accumulated sediment and restoring its storage capacity.

Speaking during the reception of the dredging machine in Maseru, WASCO Chief Executive Officer, Lebohang Mpeka Makara, said the acquisition marked an important milestone in the utility’s efforts to fulfil its mandate of ensuring reliable water supply for Basotho.

“This is the first step towards achieving our goal through the acquisition of this equipment, which is the first of its kind in the country,” Mr Makara said.

He said the rehabilitation programme was aimed at addressing the effects of siltation that had contributed to the drying up of Rasebale Dam and left many communities without adequate water supplies.

Minister of Natural Resources, Mohlomi Moleko, said the problem extends far beyond Mafeteng, warning that soil erosion from the lowlands, foothills and rangelands is washing excessive sediment into dams across the country.

“The problem of siltation is not confined to Rasebale Dam. It is affecting dams throughout Lesotho. Even our major dams are already feeling the pressure,” Mr Moleko said.

He cited Metolong Dam and Katse Dam as among the country’s strategic reservoirs already experiencing significant siltation. Both play a vital role in supplying domestic water, while Katse Dam is central to the Lesotho Highlands Water Project, which transfers water to South Africa.

Mr Moleko said the deployment of the dredging machine at Rasebale forms part of broader efforts to restore the dam’s storage capacity and extend the lifespan of reservoirs through regular maintenance.

WASCO Board Chairperson, Steven Monyamane, acknowledged that dam maintenance had previously been carried out without a structured plan.

“Previously there was no clear maintenance plan for servicing Lesotho’s dams. Going forward, we have resolved to implement a clear and coordinated maintenance programme for all our dams,” he said.

Mr Monyamane said the utility company had, for more than a year and a half, received numerous complaints from Mafeteng residents over persistent water shortages, highlighting the consequences of inadequate maintenance.

He explained that Rasebale Dam was constructed in 1983 with capacity to accommodate future population growth. However, by 2012 it had become apparent that the dam required redevelopment to meet increasing demand. Instead, two additional dams were constructed in 2015, while the siltation problem at Rasebale continued to worsen.

He said authorities had since resolved that dredging Rasebale Dam was the most effective long-term solution to restoring reliable water supply in Mafeteng.

“Water is not just a necessity; it is a basic human right for every Mosotho,” he said.

Mr Monyamane added that the experience at Rasebale had demonstrated how severe soil erosion in the lowlands, foothills and rangelands accelerates sediment accumulation in reservoirs.

He called for stronger collaboration among the government, communities and other stakeholders to protect catchment areas and safeguard water resources from their sources to the country’s dams.

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