11.4 METHODS FOR REHABILITATION

Restoration of the main channel?/ Restoration of the floodplain / Restoration of longitudinal connectivity

Restoration of Water Quality--
It is important that the quality of the water in the river is improved before corrections can be made to the physical structure of the river. There are two main sources of contamination:

Pollution by toxic materials that usually come from industry, mining and agriculture. Pollution usually involves substances that are not naturally present in the water that poison fish and other aquatic organisms on which fish and other aquatic organisms feed. This may result in death of the organisms, as in fish kills, or may interfere with some part of life cycle, such as spawning success or the development of eggs. Potential transfers of pollutants through the food chain, especially to fish, molluscs, birds, reptiles, amphibians and eventually to humans is of concern and needs to be carefully monitored.

Enrichment of river water by nutrients (phosphorus and nitrogen) that usually comes from domestic sewage and agriculture. Some degree of nutrient enrichment is natural in rivers and this is usually dealt with by the self-cleaning properties of the river. Excessive nutrient inputs can go beyond this self-cleaning capacity, and cause local lack of oxygen in the water. This suffocates or drives away species that have high oxygen demand, changing species composition of plankton, benthos, and fish.

Both pollution and eutrophication can make parts of the river inaccessible to aquatic organisms and particularly to fish, and can interrupt upstream and downstream migrations. The impacts are similar to those caused by dams.

Pollution and nutrient enrichment can be:

Point source, in other words injected into the river at a defined point through a single identifiable discharge; or

Diffuse (Non-Point source), reaching the river through numerous, unidentifiable discharges, such as run-off in from surrounding land or filtration through groundwater. (see Section 5)

It is comparatively easy to identify and control point source pollution from factories and urban discharges by the construction of treatment plants. Rigid application of the "polluter pays" principle, whereby those who cause the contamination are responsible for its correction, is now commonly accepted. Diffuse pollution is less easy to control. Routing wastes through sewers to treatment plants can control diffuse urban discharge, but the widespread seepage of agricultural fertilizers, pesticides and herbicides is more difficult to control.

The population living on the banks of a river contribute significant amounts of pollution and nutrient enrichment to the waters of the Mekong. It is difficult to control this pollution, because there are literally thousands of places where it enters the river. Education and awareness of villagers can help reduce impacts of non-point source pollution. As the population increases in future, this issue will become very important and therefore needs to be addressed by water managers.

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