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It is recognised that environmental and riparian water requirements should be accommodated in designing dams. World Bank and other lenders suggest the volume of release for downstream uses should be as high as 5% of the inflow. It is shown, however, that in the case of arid catchments, the release could affect the reliable yield of the dam by up to 25%. This because the bigger the dam the more the evaporation loss and the more probable it is that the sustainable yield is affected. The safe yield in such an. environment is much less than the inflow. Environmental and social water requirements along rivers in deserts and arid environments are considerably less than in temperate and humid regions. Population densities are sparse and vegetation is thin or has been adapted to suit arid climates. Base flows often occur in the sandy bed of the river. Release rules are suggested whereby the flows released from a dam could be as low as 1% of the inflow, which may be up to 5% of the reliable yield of the dam. Releases do not necessarily have to follow the pattern of the natural flow of the river, as over-bank flows could be lost in sandy aquifers or catchments and in any case the ability to release large floods is highly dependant on the configuration of the release works. Many clams and hydraulic structures in rivers are designed to pass only average river flows or a proportion thereof, and often the gates are dual purpose i.e. for sediment discharge as well as downstream releases. The floods in arid areas are invariably intense and of short duration and this further aggravates the ability to assimilate the natural flows for downstream uses.