Most rivers in greater Cape Town are badly degraded from over-extraction, pollution and sewage overflows and are in need of rehabilitation, a new study has found.
The State of Rivers Report for the city, compiled by CapeNature, is part of the Western Cape River Health Programme funded by the department of water affairs and forestry.
The report found that generally only a few of the upper reaches of greater Cape Town's rivers are still in a good state.
The report also found that current water demand already exceeds the water yield available for greater Cape Town, which has been exacerbated by recent dry winters.
Climatologists predict that the recent droughts in the Western Cape are likely to become recurring events as global climate change intensifies.
The Western Cape is set to become hotter and drier.
Sewage effluent from blocked sewerage works had resulted in spills into the Kuils, Black and Vygekraal rivers, creating serious environmental and health problems.Invertebrates and indigenous fish had been lost as a result, and the pollution posed a risk to human health.
Nationally, about 15 people out of every 1 000 died of diarrhoea-related causes in areas where water supply and sanitation were below standard, the report said. Most were children under five years old.
In 2000, the cost of poor water quality was R2,2-billion in direct health costs and R64-million in water treatment costs. Environmental costs were excluded.
Litter and other pollution from urban areas washed into rivers via stormwater drains.
Many of the city's river habitats had been destroyed by canalisation or the building of weirs, gabions and levees. This was most evident in the Black, Elsieskraal and Keysers, the Big Lotus and the Little Lotus rivers.
Canalisation reduced a river's ability to attenuate floods and decompose pollutants, while levees intensified flood flows, disrupted the natural ability of the floodplain to absorb flood water and increased siltation.
Because of this, the report said any further canalisation of Cape Town's rivers should be discouraged, as well as the building of levees, weirs and gabions.
Alien invasive plants had seriously reduced the summer flows of the Sand, Silvermine and Sir Lowry's Pass rivers.
Alien weeds had invaded many of Cape Town's rivers, particularly the Black, Lotus, Sand, Eerste and Kuils rivers. This not only destroyed the integrity of the habitat, but also reduced the flow of rivers.
Rivers provided goods and services for society, which were reduced by pollution, canalisation and over-extraction.