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Both the volume and pattern of water flow play a role in maintaining the long-term ecological health of a river. Only a natural or close to natural flow regime can ensure that the biota and ecological processes in a river approximates a "natural" state of ecological health. However, as developments in catchments have increased the demand for water, few streams and rivers still experience completely natural flow regimes. In particular, large impoundments have been constructed to store water for later use for domestic or irrigation purposes, and the altered flow patterns can imoair river health. In such cases the Department of Water and Sanitation supports the determination of an In-stream Flow Requirement (IFR), to define the nature of the required releases that will at least maintain the river in a pre-determined state of health. The IFR specifies different flow conditions that are needed during different times of the year, and recognises that these flows may differ for wet, normal, and dry years.
The factors that are considered when setting an IFR for a river include:
The minimum depth required at rapids to maintain fish passage and habitat for flow-dependent species;
Seasonal flow variations to stimulate fish spawning and to provide refuges and nursery areas for juvenile fish;
Flows that would ensure the presence of a range of habitat types;
The frequency and size of floods required for inundating riparian vegetation and dispersing seeds of riparian plants as well as for maintaining necessary geomorphological processes;
Minimum flows for periods of drought that would provide survival conditions for river ecosystems;
Flows required for sediment movement down the river bed and for
maintaining required flows in estuaries downstream;
Frequency and size of floods required to flush sediment and
organic matter from the system.
The specified flow releases should be interpreted in
combination with climatic events in the catchment. Floods
should be simulated during rain events, and the first
high flow after a dry season should coincide with
the first rains.
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