6.4 Discharges causing variations in flow rate
Variations in the flow of a river can result from excessive abstraction or from intermittent discharges of relatively large volumes of effluent, as when settling ponds (which are used to remove particulates from effluents in the ceramic industry, for example) are emptied. There are, however, maximum limits that must be adhered to.
Since the organisms that become established in a river will be those best suited to its conditions, sudden and repeated fluctuations in the rate of flow will mean that only those organisms that can withstand the changes will survive. Plants growing in silt deposits on the bed of a stream will be destroyed when the silt is washed away by a sudden increase in flow. When the flow falls, organisms that are dependent on a high dissolved oxygen concentration will die if the river reverts to a series of near-stagnant pools.