Addressing this issue of siltation of the nation’s water reservoirs – which is made worse by unregulated mining for sand in rivers all over the country – is no longer optional but imperative for Botswana’s long-term water sustainability. Special Correspondent DOUGLAS RASBASH warns of parched repercussions in the non-option of ignoring the issue
Botswana’s water supply is in the spotlight following the unprecedented rainfall experienced recently.
It is not because the dams are full, which of course is wonderful, but because the water quality is affected by fine particles of silt suspended in the water.
The water supply has been cut off in many areas (as this is being written) until the problem is solved. This unfortunate situation has prompted a rethink about dam capacity and the issue of siltation.
While official reports cite dam capacities, the reality is that these figures are overestimated. The figures being cited are based on the original design capacities and not the current operating capacities.
Over decades, vast amounts of silt accumulate in our reservoirs, gradually reducing their effective water storage. This crisis is exacerbated by widespread, unregulated river sand mining, which accelerates soil erosion and sediment transport.
Addressing this issue is no longer optional but imperative for Botswana’s long-term water sustainability.
The science behind siltation
Siltation occurs when soil, sand, and organic matter are transported by fast-moving rivers and settle in slower-moving or still water, such as a dam or reservoir.
The sediment carrying capacity of a river is proportional to the third power of its flow. Therefore, fast-flowing turbid waters of our rivers carry a lot of suspended sediment.
Naturally, all rivers carry sediment, but human activities, especially deforestation and mining, significantly increase the sediment load. In Botswana, where rivers often experience high seasonal flows, the problem is compounded by extreme weather events that accelerate erosion.
Illegal sand mining along riverbanks worsens the situation. Sand acts as a natural buffer, stabilising riverbeds and slowing water velocity. When removed recklessly, the river’s energy increases, scouring the bed and carrying more sediment downstream into our dams.
Over time, this process results in a steady loss of storage capacity, reducing the volume of water available for agriculture, industry and domestic consumption.
Current status of Botswana’s dams
A significant step in understanding the extent of siltation is evaluating the actual storage capacities of Botswana’s major dams. Below is an estimated assessment based on assumed siltation rates:
| Dam Name | Design Capacity (Million m³) | Year of Construction | Assumed Siltation Volume (Million m³) | Estimated Current Capacity (Million m³) |
| Gaborone Dam | 141 | 1971 | 20 | 121 |
| Shashe Dam | 85 | 1973 | 15 | 70 |
| Letsibogo Dam | 100 | 2000 | 10 | 90 |
| Dikgatlhong Dam | 400 | 2012 | 20 | 380 |
| Thune Dam | 90 | 2013 | 8 | 82 |
These figures, though estimates, highlight the critical need for precise measurement and action to restore lost capacity.
Proposed Modalities for Improvement
To combat siltation and restore the efficiency of our dams, Botswana must adopt a multi-pronged approach:
- Nationwide Siltation Assessment
The government should commission an independent hydrogeological study to determine the true storage capacities of major reservoirs. This should include bathymetric surveys (underwater topography mapping) to assess sediment deposition over time. - Regulated Dredging for Commercial Use
Instead of allowing illegal sand mining to destroy river ecosystems, Botswana should formalise and regulate the industry by issuing controlled dredging licences. These companies would extract silt from reservoirs and riverbeds in a sustainable manner, restoring storage capacity while supplying the construction industry with much-needed sand. - Strict River Sand Mining Controls
Unregulated sand extraction must be curtailed through stricter enforcement of environmental laws. Designated mining zones should be identified, ensuring that riverbanks remain intact in key ecological areas. Hefty penalties should be imposed on illegal operators who degrade watercourses. - Catchment Management Strategies
Siltation is best prevented at the source. Reforestation and soil conservation initiatives must be implemented in key river catchments. Vegetative cover slows runoff, stabilises soil and reduces erosion. - Public Awareness and Community Involvement
Local communities often engage in sand mining out of economic necessity. Alternative livelihood programmes should be developed to provide sustainable employment options while reducing environmental damage.
Conclusion
Botswana’s water security depends on the integrity of its reservoirs, yet their capacity is being silently eroded. Without intervention, the country risks water shortages in the coming decades. A proactive approach – combining scientific assessment, sustainable dredging, strict regulation, and catchment management – can ensure that our dams remain viable.