Evolution and concept of environmental flows (e-flows): meta-analysis

River ecosystems have become fragile due to river flow alterations in the wake of expending structural controls triggered by the ever-increasing demands of freshwater. The efforts to alleviate the effects of river flow alterations have paved the way for the emergence and expansion of the concept of e-flows. The role of e-flows in ecosystem protection and water resources management is not yet fully understood and practiced. This review summarizes the historical evolution of the concept of e-flows and recaps its assessment methods; analyzes the impacts of river flow alterations; indicates the challenges e-flows have been facing and identifies the gaps in the focus of e-flows research. A total of 303 articles published since 2010 were collected and systematically assessed. Most of the publications are focused on describing the effects and management of river flow alterations. According to the findings of this meta-analysis, less attention is given to basin scale, social and policy aspects of e-flows. Furthermore, little considerations are given to the part played by groundwater in e-flows along with the roles of e-flows in maintaining the health of floodplains and e-flows of intermittent rivers and ephemeral streams.