4.4 THE CONCEPT OF ENVIRONMENTAL FLOWS

All differences in flow and hydrological regime affect the animals and plants living in water. Different species respond to differences in flood and dry season patterns in different ways, producing changes in the abundance and composition of the various living organisms. Analysis of information from all river associated disciplines and the structure of the river can be used to produce a series of scenarios that predict the responses of the living organisms to different flow conditions. Such scenarios can also predict livelihood components of the riparian peoples, as the abundance and availability of food organisms change according to flood type. The hydrological conditions used in these predictions are known as 'ecological flows' and can be used to select alternatives for management that correspond most closely to society's goals for the use of the river. An understanding of environmental flows is needed to plan and legislate for:

• The amount of water that can be controlled by dams;
• The quantity and timing of impounded water that must be released;
• The amount of water that can be removed through water transfers;
• The amount of water that can be taken for irrigation;
• The allocation of water among users.

A number of methods are used to calculate ecological flows. For example the Instream Flow Incremental Methodology (IFIM) is one of the generally accepted methods for smaller rivers that do not flood. This is a stepwise method for estimating the effects of different hydrological regimes by:

1. Estimating the areas of different habitat in a stream at different depths of water;
2. Estimating the suitability of the habitats for the fish or other aquatic organisms;
3. Combining the two estimates to predict the effects of alternative regimes.

A similar procedure known as the DRIFT methodology, is being developed for larger flood rivers and has already been applied to some arid rivers in South Africa. Attempts to establish environmental flows through DRIFT methods for large rivers such as the Mekong are very recent and are still being developed.