Abstract: Effects of freshwater inflow on water environment and biological processes in the estuarine ecosystem are analyzed by considering the great diversity of influencing factors with a significant temporal and spatial variability under the interaction between river and ocean.The methods for quantifying environmental flows are divided into three types based on different ecological objectives,which include biological resources objectives,habitat objectives,and integrated multi-objectives.Research progresses are reviewed for different types of methods in this study.The main challenges are discussed according to the objective op tim izing,the relationship between objectives and freshwater in 2 flow,and the limited availability of different methods for the environmental flow assessment in estuaries.The results indicate that identifying an index of the ecosystem health could serve as a scientific foundation for determining environmental flows.Ecohydrological modeling based on the key elements of the estuary ecosystem could be an effective approach in determining environm ental flows in estuaries.