The aquatic environment should always be considered as a legitimate consumer of water, whose requirements must be met alongside basic human requirements, and ahead of any other demand. In the case of water projects involving impoundment, this translates to maintenance of flow in the reaches of the river downstream of the impounding structure, dam, or diversion. Environmental flows are required to:
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Maintain the riverine ecology
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Recharge riverine aquifers
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Maintain the river channel
For a detailed discussion of environmental flow requirements in the Kunene River basin and the relevant legislation see Environmental Flows in Resource Management.

Links between direct water use and ecosytems in a catchment.
Source: Falkenmark, M. and V. Galaz. Agriculture, Water and Ecosystems. Swedish Water House Policy Brief Nr. 6. SIWI, 2007
( click to enlarge )