The determination of a river reach’s environmental flow is a key point for the sustainable development of water resources and environmental restoration. The effective discharge is theoretically the discharge that forms and maintains in the long term the shape of the river section. It is linked with various ecological processes in streams that assure good ecological status. The effective discharge is calculated for the Evinos R. catchment in Central Greece and conclusions are drawn in respect to the management of Evinos Dam upstream of the gauging station.