River regulation has attracted considerable attention over the past 20 years. The effects of (i) changes in the seasonal flow regime below dams and reservoirs and (ii) reduction in flow caused by water abstraction and diversion, upon lotic and riparian ecosystems have been demonstrated for rivers in a range of geographical regions. This paper presents an approach to determining ecologically acceptable flow regimes and volumes. The approach is founded on a set of fundamental scientific principles concerning longitudinal connectivity, vertical exchanges, floodplain flows, channel maintenance flows, minimum flows and optimum flows. The need for a policy for allocating water to protect river ecosystems in England and Wales is discussed and the method is illustrated by a case-study of a chalk stream that has been affected by groundwater abstraction. Sixty per cent of the available resource is shown to be required to sustain the river as a trout stream. Several judgemental decisions are needed in setting an ecologically acceptable flow regime and further research is required to improve our capability for modelling the roles of different flows and patterns of flows in sustaining river ecosystems.