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Rivers & Wetlands

Author: Dr Heather Malan and Dr Jenny Day ~ Freshwater Research Unit, UCT

( Article Type: Overview )

Rivers and wetlands provide a range of goods and services for the benefit of people, but need a healthy ecosystem in order to provide these.

People need water in their homes for drinking and washing and for watering their gardens. We also rely on agricultural produce like meat, milk and vegetables, and on industrial products from motorcars to paper and computers. The production of all of these goods requires large amounts of water, which comes from our rivers, and aquifers (underground water sources) and sometimes even from our wetlands. We also use the water that runs in rivers to generate electric power, and for diluting and removing much of our waste.

But natural aquatic ecosystems like wetlands and rivers supply us with many more services other than just actual water. They support populations of fish and other resources that we eat; they provide grazing for our cattle and sheep; they provide timber and reeds for building and weaving; in many parts of the world they act as major transport systems. They also cleanse the water that flows in them. Living ‘decomposer’ organisms – microbes and invertebrates (insects, worms, snails and crustaceans) – break down and thus remove a lot of waste material, so that as water flows down a river it becomes cleansed. Thus water some distance downstream of a pollution source is a lot cleaner than it was where pollutants entered it. This is not merely the effect of dilution because rivers that have lost their living organisms because of canalisation (see later) or poisoning are unable to clean the water flowing in them.

Another surprising feature of rivers is their importance for the coastal zone, as well as for estuaries. The land is much richer in nutrients than the sea is, so the food chain in the shallow coastal zone often depends on nutrients that are brought down rivers, through their estuaries, and out to sea. What is more, the power of the water flowing in a river causes erosion of the bed and banks. The eroded particles of soil and sediment are washed downstream; if the flow becomes slower, then those particles are deposited within the bed of the river itself. Some of the material is washed out to sea, however, and contributes to the sand that forms beaches and dunes. If a river stops flowing, it becomes silted up. If a river stops flowing, it also stops providing sand to the beaches near its mouth, although of course currents in the sea continue to erode beaches. This is why beaches near some rivers are beginning to disappear.

Wetlands, particularly those with dense stands of emergent plants, provide further services for humans. They are effective at controlling floods because the plants slow down the force of floods, forcing the water to spread out and reducing its damaging effects. They also store and slowly release flood waters to river channels, reducing the worst of the flood peak, extending the time taken for the flood waters to drain away, reducing ‘flash flooding’, and supplying water during the dry season. Because the water is forced to move slowly, it is also likely to drain downwards into local aquifers, thus recharging them. Wetlands are also natural filters, removing sediments, nutrients and bacteria from the water flowing through them. As a result, water emerging from a wetland is often cleaner than the water entering it. The reason is that the plants cause water to flow slowly and thus to drop its silt; at the same time they, and microbes associated with them, extract the nutrients for their own growth. Artificial wetlands are sometimes cultivated for the final cleansing of sewage effluent and, in some areas, for flood control. Wetlands also form some of the most productive lands on Earth, providing habitat, food and shelter for an enormous variety of plants and animals, from minute to huge and from aquatic to terrestrial species. The biodiversity of some wetlands approaches that of tropical forests.

Rivers and wetlands can provide us with all these services only if they are reasonably ‘healthy’. So what does an aquatic ecosystem need in order to stay healthy? Firstly, any aquatic ecosystem is affected by its catchment, so a river or wetland is unlikely to be healthy if it receives more than a minimal amount of pollution from its catchment. Then it needs its bed and banks to be left alone (and not be canalised – lined in concrete) so that its water can interconnect with the water upstream and downstream, and below its bed. It is only in this way that the system will be able to acquire (or rid itself of) the sediments and nutrients on which its biota relies. Despite the many ‘goods and services’ supplied by healthy rivers and wetlands we have abused them in many ways. One of the major threats is by the construction of dams which, in addition to excessive abstraction (by means of pumping), leads to disruption of the natural flow regime and has an adverse effect on the living organisms downstream. For example, the migration and spawning of the large-mouthed yellow fish (Barbus kimberleyensis) in the summerrainfall region is triggered by high flows in spring. Due to the damming of many rivers the numbers of this fish are dwindling. Reduced or altered flows can have a detrimental effect on aquatic organisms, but can favour others. Flows in the Orange River are extremely regulated – meaning that because of upstream dams and weirs, very high flows and very low flows, which would be the natural regime in the river, no longer occur. These steady flows are considered to be the major cause of black fly (Simulium spp.) infestations, which can cause extensive damage to livestock. Dam walls also often pose insurmountable barriers to fish movement – preventing fish from moving upstream to spawn.

Not only do dams control movement of water, and of animals, but also of sediment. In catchments where there is extensive erosion (often due to over-grazing or inappropriate farming methods), much valuable topsoil ends up in dams, reducing the capacity for storing water. Sediments (and nutrients bound to them) are often important for downstream floodplains and estuaries. Cessation of the flow of sediments to these systems due to the construction of dams or weirs can lead to catastrophic degradation of these rich ecosystems. A classic example of this is the collapse of the prawn fishery at the mouth of the Zambezi, Mozambique, due to construction of the Cahora Bassa dam.

Because rivers are such convenient systems for the disposing of waste, there has been a tendency to pollute them. Sewage (treated and untreated), industrial waste, salts, nutrients and pesticides have all often ended up in water bodies. This is sometimes due to direct discharge of effluents into rivers and lakes through pipes. However, rainwater flows over the catchment surface soaks into the ground, gradually making its way into groundwater bodies and in so doing, carries pollutants present on the surface or in the soil into the water. Nutrients (phosphorus, nitrogen) and pesticides are often washed from agricultural lands in this way. Eutrophication is a form of pollution in rivers and lakes, which results from the excessive loads of nutrients being washed or dumped into these water bodies (originating largely from fertilisers, sewage and effluent from food-processing plants). Some species of plants thrive in a nutrient enriched environment leading to rapid and excessive growth. The blocking of waterways by reeds and thick smelly scums of phytoplankton on the surface of lakes are just some of the problems caused by eutrophication.

Natural rivers in lowland areas tend gradually to change their channel (banks eroding in some areas and being built up in others). Furthermore, during periods of high flow floodplain areas become inundated with water. This is an important ecological process as fish move into the shallow, floodplain waters to spawn and the protected nutrient-rich backwaters provide an excellent nursery for the young. However in urban situations where land is limited (and developers are greedy) development has been allowed to encroach into the floodplain areas. Then, because of the risk of flooding or erosion, structural measures need to be put in place to remove flood waters as rapidly as possible as well as to stabilise the banks. Thus many urban rivers are canalised (with the bottom and sides encased in concrete). Whilst this may be an efficient engineering solution, the result is not ecologically sensitive since there is no habitat within the smooth walls for aquatic organisms to live, and few organisms will be found in such systems. More environmentally friendly options are available – for example the use of gabions (metal baskets of stones) that can be arranged to prevent erosion and allow the river to meander). Such options allow plants to grow along the banks and offer some habitat for aquatic organisms (but not a lot of protection from flooding).

Despite the important role of wetlands in cleaning water and regulating water flow in South Africa, it is estimated that at least 50% of our wetlands have already been lost due to draining and filling for agriculture or housing development.

So what can be done about the abuse of our precious rivers and wetlands? South Africa is an arid country in which a large proportion of the population still does not have access to basic amenities such as sanitation and potable water. However, we need to have a balance between protection of aquatic resources on one hand, and development on the other.

The New South African Water Act (1998) is considered remarkable internationally in that it recognises that the entire aquatic ecosystem needs to be protected because it is the source of all goods and services provided to people. More than laws will be required, however, as there needs to be a greater understanding by all sectors of the community that whilst access to water is a right – that water is precious and our aquatic resources need to be preserved, protected and kept unpolluted. People need to start cutting down on consumption and ensuring that water is not wasted. Some ways in which this can be done include:

  • Planting indigenous gardens (these use much less water than exotic plants such as kikuyu lawns)
  • Recycling of ‘grey water’ (e.g. using water from baths, washing machines, etc. for irrigation)
  • Using dual-flush toilets
  • Using porous surfaces for driveways, parking areas, etc. (this allows rain to soak into the ground)
  • Watering gardens and agricultural fields early evening and mornings (to reduce evaporation).

Pollution can be reduced by using less fertiliser on gardens and disposing of pesticides and oil in a responsible manner (rather than emptying them down the drain). People can campaign against building too close to the flood line of rivers and the filling-in of wetlands.

Although dams have major impacts on rivers, they also have an extremely important role in ensuring that people have a reliable water supply. But before building any new dam, there needs to be a very clear understanding of the potential long-term costs and benefits, remembering that the impacts may only be felt many miles downstream and many years after completion of the dam wall.

Ensuring that we utilise our limited water resources in a way that will realise the most benefits for all people will be an increasingly difficult challenge in the future. It is however a challenge that needs to be faced if all the people of South Africa are to attain a dignified standard of living